Diving behaviour of harbour seals (Phoca vitulina) from the Kattegat

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Diving behaviour of harbour seals (Phoca vitulina) from the Kattegat

Diving behaviour of harbour seals

(Phoca vitulina) from the Kattegat

A master thesis by

Magda Chudzinska

National Environmental Research Institute &

Department of Biological Sciences

University of Aarhus, Denmark

Supervisors:

Jonas Teilmann

National Environmental Research Institute

*

Tomas Cedhagen

Aarhus University

May 2009

Photo: Aqqalu Rosing Asvid


NATIONAL

ENVIRONMENTAL

RESEARCH INSTITUTE

AARHUS

UNIVERSITY

Diving behaviour of harbour seals (Phoca vitulina) in the

Kattegat, Southern Scandinavia

Master thesis

By

Magda Chudzińska

20070005

Department of Arctic Environment

National Environmental Research Institute

University of Aarhus, Denmark

Supervisors:

Jonas Teilmann

Department of Arctic Environment

National Environmental Research Institute

University of Aarhus, Denmark

Tomas Cedhagen

Marine biology

Department of Biological Sciences

University of Aarhus, Denmark

Date of defence: 26.05.2009


TABLE OF CONTENT

PREFACE.............................................................................................................................................. 3

ACKNOWLEDGEMENTS.................................................................................................................. 3

BACKGROUND.................................................................................................................................... 5

HARBOUR SEAL.................................................................................................................................... 5

HARBOUR SEALS FROM THE KATTEGAT AREA .................................................................................... 6

Haul-out sites and populations....................................................................................................... 6

Movement and reproduction of the seals ....................................................................................... 7

Food and foraging........................................................................................................................... 8

HISTORY AND POPULATION SIZE OF THE HARBOUR SEALS IN THE KATTEGAT.................................... 8

SEAL MANAGEMENT IN THE KATTEGAT .............................................................................................. 9

PHYSICAL PARAMETERS OF THE KATTEGAT...................................................................................... 11

DIVING AND FORAGING BEHAVIOUR OF THE HARBOUR SEALS.......................................................... 13

Possible biological meaning of the dive shapes ........................................................................... 14

Breeding diving and foraging behaviour ..................................................................................... 16

FORAGING AREAS OF HARBOUR SEALS.............................................................................................. 17

DIVING PHYSIOLOGY OF HARBOUR SEALS......................................................................................... 18

METHODS........................................................................................................................................... 18

Principles of satellite transmitters (PTTs).................................................................................... 18

Problems in satellite telemetry ...................................................................................................... 19

Equipment used in the study......................................................................................................... 20

Determination of feeding areas and evaluation of analytical methods used in the study .......... 21

REFERENCES ...................................................................................................................................... 22

MANUSCRIPT.................................................................................................................................... 28

INTRODUCTION .................................................................................................................................. 29

MATERIAL AND METHODS ................................................................................................................. 30

RESULTS............................................................................................................................................. 37

DISCUSSION ....................................................................................................................................... 48

REFERENCES ...................................................................................................................................... 56

APPENDIX .......................................................................................................................................... 62

IMPLICATIONS FOR THE CONSERVATION ............................................................................ 66

FUTURE DIRECTIONS .................................................................................................................... 67

REFERENCES ...................................................................................................................................... 67

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PREFACE

This thesis describes a study of diving behaviour of harbour seal from Anholt under the

supervision of Jonas Teilmann (Department of Arctic Environment, National Environmental

Research Institute, Aarhus University) and Tomas Cedhagen (Marine biology, Department of

Biological Sciences, Aarhus University). It constitutes a part of the larger project ‘Et Vindue

til sælerne’ (A window to the seals) aiming at monitoring the whereabouts and ecology of

harbour seals from Anholt in relation to human disturbance and to increase the public

awareness on seals and the problem of disturbance at the seals’ haul-out site. This study

includes a general introduction describing in detail the background of the studied animals,

study site and methods used to estimate foraging areas of the studied individuals.

Furthermore, it includes a draft manuscript showing the results and discussion of the collected

data, entitled ‘Diving behaviour of harbour seals (Phoca vitulina) in Kattegat, Southern

Scandinavia’.

ACKNOWLEDGEMENTS

So much helpful advice and support have been provided in the past years from family and

friends that mentioning all the persons will make a list too long. The study was financed by

Aage V. Jensens Fonde, the Danish Forest and Nature Agency (Skov- og Naturstyrelsen),

Gorenje and Friluftsrådet. Thanks for they support. For the realization of this work I have to

thank all the people at the National Environmental Research Institute for the amazingly

friendly environment that made the redaction of this thesis a very nice experience. In

particular, thanks for all supervision, corrections, suggestions and enthusiasm to Jonas

Teilmann and Tomas Cedhagen. I would also like to thank Jonas for his hospitality.

Additionally, thanks to: Signe May Andersen, Rune Dietz, Susi M.C. Edrén, Lee Miller,

Morten Tange Olsen, Morten Abildstrøm, Jonas Teilmann, Henrike Seibel, Niels Martin

Schmidt, Maja Faust Rasmussen, Lars Renvald, Nikolaj Ernst, Aqqalu Rosing Asvid,

Redningsstationen på Anholt and people that found the tags on the beach (Ingrid Hansen and

Stig Krogh-Lund) for their support in the field work and retrieval of the tags (it was rather me

helping them than the other way round) and much more afterwards; Aurore Aubail and

Guisella Gacitua for a warm atmosphere in the office and not keeping me homeless for a

couple of days, Frank Riget for a statistic support and Susi Edrén for GIS advice. I would also

like to thank Peter Teglberg Madsen, Maria Iversen and Anna Wojda for their useful

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suggestions and comments. A huge thanks to Danuta Wiśniewska for her comments, hours

spent on reading my thesis and keeping me up (and delicious cakes and dinners). And thanks

to Łukasz Ulbrych, the support of whom goes far beyond this work (especially for his

patience).

4


Background

BACKGROUND

Harbour seal

Harbour seal (Phoca vitulina): order Carnivora, superfamily Phocoidea, family Phocidae (Perrin et

al. 2002) is widely distributed in coastal habitats of both North Pacific and North Atlantic (Bonner

1989; Reeves et al. 2002) (Figure 1). This distribution extends across different habitats and

ecosystems from the high Arctic, oceanic shores in temperate zone, to inshore waters and estuaries

(Härkönen & Heide-Jørgensen 1990). There are several distinguished subspecies within the

distribution and harbour seals from the Kattegat belong to the European subspecies Phoca vitulina

vitulina (Bonner 1989; Härkönen et al. 2005) also called Northeast Atlantic harbour seal (Burg et

al. 1999) Then, the described subspecies comprises six genetically distinct populations: Iceland,

Ireland and Scotland, English east coast, the Wadden Sea, western Scandinavia (Norwegian west

coast, the Skagerrak-Kattegat, west Baltic) and the east Baltic (Härkönen et al. 2005). Härkönen

(2003) includes to this subspecies also populations from Svalbard, Greenland, Barents Sea and

Spain.

Figure 1. Worldwide distribution of harbour seal (Phoca vitulina), reproduced from Bonner (1989).

The harbour seal is one of the smallest pinniped, reaching up to 180 cm and 110 kg. The sexual

dimorphism in this species is barely visible with males slightly larger than females (Burg et al.


Background

1999; Härkönen & Heide-Jørgensen 1990). Males reach their sexual maturity after 4-5 years and

physical maturity after 7-9 years. Females are sexually mature at age 3-4 and physically mature at

age 6-7 (Härkönen & Heide-Jørgensen 1990; Perrin et al. 2002). Harbour seals occur in breeding

groups of one or two up to several hundred individuals and can be found breeding on a variety of

habitats including ice, inter-tidal sand bars and rocky beaches (Van Parijs et al. 1997). Females

give birth to one pup at a time during the breeding season from early February to September

depending on the area with northern populations giving birth later in the period (Bonner 1989). In

northeast Atlantic mating season lasts from June through August (Härkönen & Hårding 2001).

Lactation takes from 3 to 6 weeks (Bonner 1989). Females do not fast throughout lactation. Prior

to mating males are relatively dispersed, but decrease their range significantly at the onset of the

mating season (Van Parijs et al. 1997). Males seem to adopt their temporal and spatial behaviour

pattern to variations in females’ distribution and density (Van Parijs et al. 1999). The haul-out

frequency typically peaks during the pupping, mating and moulting season from May through

August in the Northeast Atlantic (Härkönen & Hårding 2001; Thompson et al. 1989).

Populations of harbour seals are considered non-migratory but, as a species, use a wide range of

habitats across their geographical distribution (Tollit et al. 1998).

Harbour seals feed on fish and marine invertebrates depending on prey availability (Härkönen

1987a; Olsen 2006; Reeves et al. 2002; Tollit et al. 1998). This species is known to show high site

fidelity and feed close to its haul-out sites usually within 50 km (Frost et al. 2001; Härkönen

1987a; Härkönen & Hårding 2001; Thompson et al. 1998; Tollit et al. 1998); however, 500 km

ranges were also observed (Bjorge et al. 2002).

Harbour seals from the Kattegat area

Haul-out sites and populations

The harbour seal (Phoca vitulina) is the most abundant and one of the two seal species occurring

in the Kattegat area (the other is the grey seal Halichoerus grypus) (Olsen et al. 2009). Danish and

Swedish populations are divided into seven subregions (Figure 2) which serve as units for research

and management. These regions were defined based on geographical features, behavioural and

telemetry studies and genetic analysis (Olsen 2006). There are several haul-out sites within Danish

waters (Figure 2). Harbour seals from the Kattegat haul-out on rocky and sandy shores, scattered

rocks and sand banks (Härkönen 1987a; Olsen et al. 2009).

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Background

Figure 2. Regions and haul-out sites of harbour seals in the Kattegat – Skagerrak (modified from Olsen 2006).

According to satellite tracking, harbour seals from Wadden Sea, Anholt and Rødsand are not

interconnected (Dietz et al. 2006) and genetic analysis support a very clear division between 3

populations: Skagerrak (I in figure 2) – Kattegat/Belt (II+III) - western Baltic (IV), the central

Limfjord (V) and the Wadden Sea (VII) (Härkönen & Hårding 2001; Olsen 2006). Distance alone

cannot explain the observed structure, rather life characteristics (like the fact that dispersal might

be biased towards the juvenile stage), habitat composition and past demographic processes (Olsen

2006). Although harbour seals are presumed to be relatively sedentary, few long-range and more

frequent short stepping-stone-like migrations made per generation suffice to increase connectivity

among haul-out sites in the long term (Olsen 2006).

Movement and reproduction of the seals

Of the 19 harbour seals tagged on Anholt between 2005 and 2006 40 % frequented Læsø, 30 % -

visited Hesselø, 20 % - northern Sealand, 20 % - Swedish coast and a single animal (2.5 %) visited

Samsø (Dietz et al. 2006), all areas are part of the Kattegat area (II+III in figure 2). The furthest

7


Background

recorded migration of harbour seal from Anholt extended less than 200 km (Dietz et al. 2006).

Males tend to disperse more than females and are widely dispersed during most of the season

returning more often to the natal place during mating and moulting season (Härkönen & Hårding

2001; Olsen 2006). Females’ site fidelity tends to increase with age (Härkönen & Hårding 2001).

Anholt harbour seals reproduce late May- early June and mating take place usually in July –

August (Härkönen & Heide-Jørgensen 1990).

Food and foraging

The harbour seal in the Kattegat and western Baltic areas are primarily bottom feeders with a diet

consisting mainly of bottom fish (common sole (Solea vulgaris), lemon sole (Microstomus kitt),

lesser sandeel (Ammodytes tobianus), dab (Limanda limanda), flounder (Platichtus flesus), plaice

(Pleuronectes platessa)) and gadoids (cod (Gadus morhua), Norwegian pout (Trisopterus

esmarkii), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus)) (Andersen

et al. 2007; Härkönen 1987a; Härkönen 1987b). Still, some pelagic or benthopelagic species like

herring (Clupea harengus) and garpike (Belone belone) have also been recorded to be components

of their diets (Andersen et al. 2007; Härkönen 1987a; Härkönen 1987b). Although some

commercially important species such as mackerel (Scomber scombrus) are abundant in the

Skagerrak – Kattegat area, have been know to only exceptionally being preyed upon by harbour

seals what may indicate that seals avoid or are not able to catch such fast moving species

(Härkönen 1987b; Härkönen & Heide-Jørgensen 1991). Annual, seasonal and individual variations

in prey preferences are observed for harbour seals within the whole distribution area (Brown &

Pierce 1998; Härkönen 1987b)

History and population size of the harbour seals in the Kattegat

First harbour seals in the Baltic and the Kattegat appeared around 8000-8500 years ago after

retrieving of the last ice age (Härkönen et al. 2005). Harbour seals recolonized the Kattegat and

went extinct several times during the past 8000 years while a stable colonization has been noted

since 18 th -19 th century (Härkönen et al. 2005). The harbour seal from Denmark and Sweden have

had a turbulent history, with dramatic fluctuations in population size, primarily caused by habitat

fragmentation, extensive hunting, organochlorine pollution and two outbreaks of the Phocine

Distemper Virus (PDV) (in 1988 and 2002) (Hårding et al. 2002; Härkönen et al. 2005). The

maximum number of seals was observed prior to 1905, followed by a severe decline due to

hunting and poaching up to 1977 when the harbour seal was protected in Denmark (in 1967 in

8


Background

Sweden). Since then the population has rapidly recovered, although the two epidemics had a

severe impact on population size in some years (Olsen et al. 2009). In total, approximately 35 000

harbour seals were hunted before 1977 (Heide-Jørgensen & Härkönen 1988). At present around

12000 harbour seals is reported in all Danish waters (Søgaard et al. 2006),

Figure 3. Trends in estimated number of seals in the Kattegat and Belt region (region II and III) based on aerial survey

from 1979 to 2008 and corrected for the proportion of seals in the water. Lowess smoother curves were applied using a

25 % smoothing factor, after Olsen et al. (2009).

Both PDV outbreaks started on Anholt (Härkönen et al. 2006). In region II and III the epidemic in

1988 resulted in the population decline of 51.1 % and in 2002 – 17.6 % (Hårding et al. 2002;

Olsen et al. 2009). After the outbreak in 2002, the populations in the Kattegat and Belt region II

and III increased and seem to reach its carrying capacity around 1998 – 2000 at approximately

10000 individuals (Figure 3) (Olsen et al. 2009). At present 5000 seals inhabit the Kattegat (region

II) and up to 1000 use to haul – out on Anholt (NERI, unpublished data).

Seal management in the Kattegat

The Danish Forest and Nature Agency (under the Danish Ministry of Environment) and National

Environmental Research Institute (Aarhus University) are responsible for the seal monitoring

program in Denmark (Olsen 2006). In 1979 Denmark signed the Bern Convention (The

Convention on the Conservation of European Wildlife and Natural Habitat) aiming to conserve

wild plants and animals including their habitat and protect them against threats. Recommendation

9


Background

no 43 (1995) of this convention on the conservation of threatened mammals in Europe includes

harbour seal as one of the species requiring special conservation.

The Helsinki Convention (Convention on the Protection of the Marine Environment of the Baltic

Sea Area) was signed in 1992 by all the states bordering the Baltic Sea (including Denmark), and

the European Community. The main aim of the Convention is to protect the marine environment

of the Baltic Sea which also includes the Kattegat. Harbour seals are also covered by EU Habitat

Directive (1992) under Annex II (Heide-Jørgensen & Härkönen 1988; Thompson et al. 2001). This

obligates all the countries to carry out monitoring of the seal population and to establish Special

Areas of Conservation (SAC). The convention for the Protection of the Marine Environment of the

North-East Atlantic (the OSPAR convention) has been ratified by all North Sea countries in 2002

and identifies the Ecological Quality Objectives (EcoQOs) which are defined as the level where

anthropogenic influence on the ecosystem is minimal and could be developed also for marine

mammals. In 2001 HELCOM established “Seal Groups under HELCOM” and Denmark is part of

it. This group focuses on the increasing problems regarding seal/fisheries interactions and the

difficulties in mitigating and managing damages on fishing gear and catches. At present, 300

harbour seals are by-caught every year in south western Baltic (including the Kattegat) (ICES

2003). HELCOM recommendation 27-28/2 (2006) stated that the seal population decline, is now

reversed, and that the Kattegat harbour seals is assumed to be over the theoretically calculated

population levels if compared to the beginning of the 20 th century (HELCOM 2006). At present

seal hunting is still banned, but recently the Seal Project Group has proposed changes in the

recommendations to permit the controlled hunting of seals in certain areas (HELCOM 2001). In

2002, licences for a total of 6 animals in Sweden and 14 in Denmark were issued to kill harbour

seals (ICES 2003). There are three marine protected areas (Baltic Sea Protected Areas) for seals

under NATURA 2000 within the Kattegat (Figure 4) (HELCOM 2008).

10


Background

Figure 4. NATURA 2000 reserves for seals within the Kattegat (HELCOM, 2008).

Totten Reserve (Anholt) is not a NATURA 2000 site but instead a national seal sanctuary that was

established in 1981 (Figure 5). This is an important haul-out and breeding site for harbour seals. It

covers approximately 1800 ha and it is not allowed to enter the reserve at any time of the year.

Harbour and grey seals are also observed hauling out outside the reserve outside the tourist season

when the beaches are undisturbed.

Figure 5. Totten seal sanctuary, Anholt (after Teilmann (2008)).

Physical parameters of the Kattegat

To fully understand the foraging ecology of predators also the information about the conditions

under which predators forage is needed. Prey availability is often correlated with physical and

11


Background

biological properties of the ocean, such as depth, temperature, and substrate type (Austin et al.

2006; Tollit et al. 1998). Therefore detailed knowledge about the physical parameters of the study

area is required. The Baltic and North Sea are joined through the Kattegat and the Skagerrak. The

average water depth of the Kattegat is about 23 m (Härkönen 1987a). The shallow waters (up to 20

m) dominate the north-western part of the Kattegat especially along the north-western Danish

coastline and around Læsø (Figure 6a). The deeper waters are located primarily along the Swedish

coast (max 106 m). This is also the area of high boat traffic and highest fishing activity (Nilsson &

Ziegler 2007). Anholt is located in the centre of the Kattegat (Figure 6). It is surrounded primarily

by shallow waters (up to 10m) except for its south-eastern part where depth increases steeply

beneath 25 m (Figure 6b).

Coast line

Depth [m]

0 - 5

11 - 15 31 - 35 51 - 55 71 - 75 91 - 95

16 - 20 36 - 40 56 - 60 76 - 80 96 - 100

21 - 25 41 - 45 61 - 65 81 - 85 100 - 110

5.1 - 10 26 - 30 46 - 50 66 - 70 86 - 90

a) b)

Figure 6. a) Bathymetry of the Katttegat b) Close up of Anholt and surroundings.

Five sediment types dominate in the Kattegat (Figure 7a). Sandy bottom occurs mainly in shallow

areas and mud dominates in deeper areas. Therefore, the western and central regions of the

Kattegat are dominated by sandy habitats and by muddy habitats in the eastern part. Anholt is

surrounded mainly by sandy bottom. Clay and bedrock are limited to the western part of the

Kattegat.

12


Background

a) b)

Figure 7. a) Sediment types within the Kattegat b) Annual average surface salinity in the Kattegat (Geological Survey

of Sweden - Baltic Data BALANCE).

Tides are barely detectable in the Kattegat – Skagerrak area where water level is mainly wind

driven (Härkönen 1987a). Salinity varies in the Kattegat between 30 and 18 PSU (Figure 7b) and

is strongly influenced by a flow of salt water from the Skagerrak. The highest salinity occurs in the

deepest waters of the Kattegat. Such significant salinity gradient makes the environment very

changeable and makes it impossible for some marine organism to live in this area (Härkönen

1987a).

Diving and foraging behaviour of the harbour seals

Diving behaviour of seals include several functions like foraging, reproduction, communication,

travelling, resting and predator avoidance (Madden et al. 2008). Some of these categories can be

performed simultaneously. This study focuses mainly on travelling and foraging dives; however,

other function should also be taken under consideration. Foraging behaviour itself can be further

subdivided into search, pursuit and handling phase (Madden et al. 2008). Identification of the

13


Background

foraging dives is particularly important because of the potential benefits that correct conclusions

could have on better understanding of foraging strategies, influence of Anholt seals on sea

environment, predator-prey interactions and proper management and protection of the animals.

Possible biological meaning of the dive shapes

There is some evidence that a dive shape is related to foraging behaviour of diving animals

(Hindell et al. 1991; Lesage et al. 1999). Most of the dive classification is based on 2D data set

with depth on the y-axis and time on the x-axis. Several parameters may be used for classification

depending on the objectives of the study and available data set. Maximum depth and dive duration

is most commonly used.

U-shaped dives

U–shaped dives are considered to be mainly associated with feeding since more time is spent at

depth where prey is more likely to be encountered and the ascent and descent rates are minimized

to maximize the time spent at the bottom of a dive. Animals may swim directly or even vertically

to and from a certain depth where a food patch is assumed to be located (Lesage et al. 1999;

Simpkins et al. 2001b). Lesage et al. (1999) distinguished a number of U-shaped categories

depending on the rate of a descent and an ascent, a bottom speed, duration and depth. Very

shallow U-shaped or V-shaped dives (e.g., up to 5 m) are thought to be associated with movement

and fast travelling (Boyd 1996; Lesage et al. 1999). It is energetically more efficient for seals to

swim fully submerged rather than at a surface due to the surface drag effect (Hindell et al. 1991;

William et al. 1991). Harbour seals are usually slightly negative in buoyancy (Ramasco 2008) and

therefore tend to sink in water what may save some energy during descent but require additional

power during ascent. Therefore shallow dives may also play a role in energy saving. Based on

stomach temperature sensors, Lesage et al. (1999) reported that 40 % of feeding events were

associated with dives < 4 m. Therefore, shallow dives cannot be exclusively associated with

travelling and exclusion of shallow dives from foraging dives may underestimate foraging effort

by the seals. Any dive shape may also resemble bottom shape of an area and therefore take any

shape no matter the diving activity. However, looking for food may not be an exclusive way of

describing U-shaped dives. Hanggi & Schusterman (1994) reported a vocal reproductive behaviour

while performing U-shaped dives.

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Background

V-shaped dives

V-shaped dives characterized short bottom phase, short duration and relatively steep descent and

ascent (Lesage et al. 1999). V-shaped dive are mainly regarded as exploratory and travel dives

(Austin et al. 2006; Baechler et al. 2002; Eguchi & Harvey 2005; Hindell et al. 1991; Lesage et al.

1999). It may also indicate vertical passing of food patch (Lesage et al. 1999) or fast diving to a

previously explored prey patch. Due to negative buoyancy of a harbour seal, V-shaped dives may

be associated with resting, sleeping or food processing (so called drifting dives) (Austin et al.

2006; Biuw et al. 2003).

W-shaped dives

Dive data recorded with high temporal resolution identified also W-shaped dives where a series of

rapid vertical movement during the bottom phase of a dive is performed (Baechler et al. 2002;

Lesage et al. 1999). These dives may both be present in pelagic and benthic feeding (Simpkins et

al. 2001c). Based on video observations, Bowen et al. (2002) distinguished two types of benthic

feeding of harbour seals which both can be recorded as W-shaped by a time-depth recorder:

‘cruising’ – when a seal swam about 2 m from the bottom and captured individual fish by quickly

thrusting its head towards the bottom and ‘digging’ when a seal descended to the bottom and dag

in the sand with front flippers and disturbed fish hidden in the substrate. W-shaped dives are also

reported during pelagic feeding when a seal dart toward the edge of the fish school attempting to

separate a small number of fish from the school or to break the school into smaller units and chase

the individual prey (Bowen et al. 2002).

Two-phased dives

Two-phased dive is when a seal spent considerable amount of time on two distinct depths. An

example of this could be a dataset from an animal-borne video system showing a harbour seal

approaching a school of fish from behind swimming horizontally at the level of the school and

than by separating it into smaller units, chasing a fish down to the bottom and than returning back

to the school (Bowen et al. 2002). Another possible explanation of such dive shape may be a

‘scanning’ behaviour when a seal swims at an intermediate depth and search for prey below and

once spotted descending to that depth to catch the prey. Similar dive shape was recorded by

Ramasco (2008) and in that study such dives were classified as dives corresponding to the bottom

shape.

15


Background

A recent study by Madden et al. (2008) identified foraging dives with a use of video records where

the presence of prey was used to determine which dives were used for feeding. They concluded

that several factors like dive duration, depth, distance, speed variability and energetic costs should

be taken into account in order to properly identify foraging dives. Repeated dives to a similar

depth during a diving bout may also indicate foraging (unless this is a series of travelling, shallow

dives) (Eguchi & Harvey 2005). Seals may change tactics (thus performing different dive shapes)

when within a foraging area, when prey is both diffused and dispersed at different depths or

uniformly or patchy distributed (Austin et al. 2006; Hassrick et al. 2007). Nevertheless, it is

unlikely that any dive shape will generally represent exclusive behaviour (Baechler et al. 2002;

Lesage et al. 1999; Simpkins et al. 2001b).

Dive bouts

Some studies identify foraging behaviour by looking at series of dives, bouts rather than each dive

separately (Austin et al. 2006; Boyd 1996; Coltman et al. 1997; Mori et al. 2001). Series of

relatively shallow dives along a straight, directional route may be determined as travelling bout,

whereas foraging bouts should constitute of U-, V- and W-shaped dives and/or repeated dives to

the similar depth (Boyd et al. 1994; Coltman et al. 1997; Mori et al. 2001). Bouts of directional

movements could represent travel and directional search behaviour between prey patches

(Simpkins et al. 2001a). Based on simultaneous deployment of time-depth recorders and stomach

temperature loggers, Austin et al. (2006) found out that diving bouts in which feeding occurred

were three times longer, deeper and had greater bottom time than those without evidence of

feeding. Harbour seals probably use different tactics for different prey types (i.e. cryptic and

conspicuous) and for different species within each group (Bowen et al. 2002) and for prey patchy

distributed in horizontal and vertical direction (Simpkins et al. 2001b). Dive classification usually

categorizes all dives to a simple shape. However, it should be kept in mind that a certain dive

category may contain dives with slightly different shape but possibly of similar biological

meaning.

Breeding diving and foraging behaviour

Data for this study were collected at the beginning of the breeding season. Therefore some adult

individuals might have been involved in breeding behaviour during that time. Preparation for

reproduction may cause adult individuals to increase diving frequency and energy storage (Austin

et al. 2006). During breeding season, distribution of females while feeding (since females forage

16


Background

during late lactation) is influenced by the vicinity of breeding site when suckle their pups and

feeding distribution of adult and possibly subadult males is additionally influenced by the access to

females (both on land and in the water, since mating usually takes place in the water) (Van Parijs

et al. 1997). Males tend to make shorter feeding trips during the breeding season, reduce their

home range and make shorter dives during that time, and some dives could be associated with

underwater vocalization and displays during the mating period (Thompson et al. 1989; Thompson

et al. 1998; Van Parijs et al. 1997). Therefore males face a trade off between staying close to land

where probability of encountering females is higher but opportunity for foraging may be limited

due to high competition, and areas farther offshore where situation may be opposite (Van Parijs et

al. 1997). Since during breeding season, breeding individuals are concentrated around the breeding

site, leading possibly to local depletion of prey, the non-breeding animals should be compensated

for their travel costs if they encounter less depleted prey resources at the greater distances (Robson

et al. 2004). Males tend to forage in relatively deep waters offshore prior to mating to maintain

their body mass (Coltman et al. 1997). During the mating season, males move to shallower areas,

closer to haul-out sites and reduce feeding to be engaged in slow patrolling, agonistic behaviour

with other males and visual and acoustic display towards females (Coltman et al. 1997).

Foraging areas of harbour seals

It has been shown that seals using the same haul-out sites may forage in very different water

depths and habitats within a short period (Frost et al. 2001).Therefore, the chosen feeding area

could affect the foraging behaviour of a seal (Madden et al. 2008). The sizes of the feeding

grounds available to seals depend on the local bathymetrical and physical conditions (Härkönen

1987a). Additionally, oceanographic features may provide a means of navigation for animals

foraging and travelling within the three-dimensional marine environment (Robson et al. 2004).

Individuals can operate at unique spatial scale due to preferred prey type, learned behaviour and/or

opportunistic events (Frost et al. 2001; Robinson et al. 2007). Härkönen (1987a) concludes that

low prey density in shallow waters and the increased energetic costs of diving in deeper waters

result in maximum energy gains being obtained by foraging at intermediate water depth. The

choice of an optimal dive depth should depend on several factors like: local bathymetry, the ability

to maximize the proportion of time spent foraging, prey availability, cost and benefits of feeding

on different prey species and physiological constraints (Eguchi & Harvey 2005; Tollit et al. 1998).

Since many pelagic predators (like harbour seals) rely on resources that are patchy distributed both

in space and time, they must invest time and energy into the exploitation of multiple prey

resources during each foraging trip (Robinson et al. 2007). Differences in length of a foraging trip

17


Background

may result from prey abundance and\or travel distance required to locate the prey (Thompson et al.

1998). If prey is of limited distribution, then partitioning of foraging area among individuals

should be expected (Eguchi & Harvey 2005).

Diving physiology of harbour seals

All diving pinnipeds show a special adaptation for prolonged breath hold and withstanding high

hydrostatic pressure. Harbour seals are relatively shallow divers; however, dives exceeding 450-

500 m and lasting more than 30 min have also been recorded (Eguchi & Harvey 2005; Ries et al.

1997; Schreer & Kovacs 1997; Tollit et al. 1998). An aerobic dive limit (ADL) is the maximum

breath hold time without an increase of lactic acid concentration in the blood during or after a dive

(Kooyman 1981). ADL depends on inter alia basic metabolic rate, body mass and oxygen store in

the blood (Thompson & Fedak 2001; William et al. 1991). Juvenile seals have a lower predicted

ADL due to higher metabolic rate, less efficient heart rate regulation, respiration rate,

vasoconstriction, body temperature and higher blubber rate than in the case of adults (Hassrick et

al. 2007). ADL for a harbour seal pup is around 3 min (Jørgensen et al. 2001), whereas for an 80

kg female it is almost 9 min (Bowen et al. 1999). Since the studied animal are of different age, sex

and body mass, differences in diving behaviour may also be due to physiological constraints.

Methods

Principles of satellite transmitters (PTTs)

Satellite transmitters deployed on seals are programmed to send signals to satellites at periodic

intervals. Five polar orbiting satellites flying at an orbit of 850 km above the earth and being in

‘view’ of PTTs for 9-12 minutes per passage, pick up the signals and store them on-board and

relay them in real-time back to earth. The accuracy of the position is dependent on the number of

consecutive transmissions, received within a satellite pass and the time between them. Over 40

antennas located at all points of the globe collect the data from satellites. There are two global

Argos processing centers, one located just outside of Toulouse in Southwestern France, and the

other near Washington, DC, USA. Once the data arrive at a processing center, locations are

automatically calculated and information made available to users. ARGOS users around the world

receive data directly in their office or on-site (ARGOS 2009; Dietz et al. 2003).

18


Background

Problems in satellite telemetry

The studied seals spent considerably amount of time underwater and locations of seals while

foraging are of particular importance for the scientist. Since each PTT is ‘seen’ only for 9-12

minutes by the satellites during a single passage, there is a high probability that a seal will be

underwater for most of this time where transmitting is not possible. Therefore, in order to get a

location with a high accuracy, more than two uplinks from the transmitters during a single satellite

passage are necessary and a tagged seal should be at the surface for a certain time. Moreover, over

the North Sea one or more satellites will be visible on the sky approximately 30 % of the time

(Tougaard et al. 2006). Hence, there are usually few locations per day provided by the ARGOS

system. Additionally, the satellite coverage is not uniform throughout the day, with usually better

coverage during the daytime (Tougaard et al. 2006). Based on the number of uplinks received by a

single satellite during a single passage and on the algorithm calculated by the ARGOS system,

positions are assigned to one of the six precision location classes (Tougaard et al. 2008). The

system specifies the precision of the three best location classes (3-1). Some positions may display

differences in the actual position and the position specified by the ARGOS. Vincent (2002) and

Robson et al. (2004) empirically determined the errors of each location class as shown in Table 1.

Table 1. 68% percentiles for difference between true and calculated position for each of the 6 location classes;

Nominal values provided by System Argos, and empirically determined errors from Vincent et al. (2002) and Robson

et al. (2004) are shown. – : no data available; long.: longitude; lat.: latitude.

Service ARGOS (Vincent 2002) (Robson et al. 2004)

Location

class

Long and lat [m] Long. [m] Lat. [m] Long. and lat [m]

3 150 295 157 278

2 350 485 259 903

1 1000 1021 427 1496

0 - 3029 1851 4483

A - 909 678 4131

B - 4815 3193 9057

The results of the field studies differ from those presented by the ARGOS, in that they usually

have higher values than suggested by the system. They also specify the error for the three other

location classes (0-B) and show that class A is more accurate than class 0. Vincent (2002)

compared the errors in latitude and longitude showing that distortion for longitude is larger than

19


Background

that for latitude. Therefore the ARGOS system may be too inaccurate to carry out the small scale

and\or short-term studies.

Equipment used in the study

Three equipment types were deployed for each studied animal: satellite transmitter, Time-Depth-

Recorder (TDR) package and VHF transmitter (Figure 8 and 9).

TDRs

Release

mechanism

VHF transmitters to retrieve

the package

Figure 8. TDR packages: the Mk9 (left) and Mk6 (right) encased in the floatation foam.

TDR – package

(to get the dive data)

Satellite transmitter (PTT)

(to know the location)

VHF transmitter

(to know the

haul-out behaviour)

Figure 9. The studied seal with deployed equipment.

20


Background

Two different satellite transmitters were used in this study, all manufactures by Wildlife

Computers Inc., Seattle, USA: SPOT 4 and SPOT 5. All types transmit radio signal to the ARGOS

system every 45 s at sea and every 90 s when hauled out. Two types of TDRs were deployed, also

manufactured by Wildlife Computers: Mk6 and Mk9 (Figure 8). Mk9 measured water temperature

and depth and the Mk6 was additionally equipped with a velocity sensor, which unfortunately did

not work.

Determination of feeding areas and evaluation of analytical methods used in the study

To fully understand the foraging ecology of predators, information about foraging behaviour and

the conditions under which predators forage are needed. Given that it was not possible to directly

measure the quantity, quality and presence of a prey in this study, characteristic of the habitat and

dive shapes were used as proxies. Prey availability is often correlated with physical and biological

properties of the ocean, such as depth, temperature, and substrate type (Austin et al. 2006; Tollit et

al. 1998). Optimal foraging movements usually consist of low speed, variable directional changes

in high recourse density areas and prolonged time spent in these areas, a strategy known as area

restricted search (ARS) (Austin et al. 2006). Several methods are used to calculate ARS, based on

satellite locations, like First Passage Time (FPT) (Fauchald & Tveraa 2003; Pinaud 2008), Focal

Foraging Areas (FFA) (Simmons et al. 2007) or Fractal Dimension (Nams 1996; Tremblay et al.

2007). However, it is unclear whether the spatial accuracy and temporal resolution of ARGOS data

are sufficient to extract the location of ARSs at the resolution of individual feeding bouts

(Robinson et al. 2007). Since it is difficult to assess the minimum scale at which a track should or

can be analyzed (Robinson et al. 2007), the relatively small and restricted area of the Kattegat

which is surrounded by Swedish and Danish coast (the farthest distance to the land from Anholt is

84 km) may make it impossible to distinguish the individual foraging areas based on commonly

used methods. Most of the studies based on ARS were analysed over a large spatial and temporal

scale like elephant seals (Mirounga angustirostris) in eastern North Pacific Ocean (Robinson et al.

2007; Simmons et al. 2007), Antarctic Petrel (Thalassoica antarctica) in the Southern Ocean

(Fauchald & Tveraa 2003) or yellow-nosed albatross (Thalassarche carteri) in the Indian Ocean

(Pinaud & Weimerskirch 2005). Uplink frequency produced by pinnipeds may be reduced during

feeding bouts, because time spent at the surface may be minimized (Robinson et al. 2007). Also

number of uplinks may differ over shallow and deeper areas due to more frequent surfacing over

the shallow depths what may underestimate potential foraging areas over the deeper areas. An

indirect method of estimating potential foraging areas for seals based on dive duration, bottom

21


Background

time and post-dive surface time was described by Mori et al. (2005). They made a hypothesis that

dive shapes should reflect not only the depth of a prey patch, but also the richness of the patch and

that there is a relationship between the bottom time and prey richness and between dive duration

and post-dive surface time. This so called ‘index of patch quality’ is suitable for pelagic dives

deeper than 50m, since such depths were more likely correlated with foraging (Mori et al. 2005).

Relatively shallow divers, like harbour seal, are able to dive to the bottom while remaining within

their physiological depth limit because of the limited travel time to and from the bottom. This may

‘mask’ the relationships stated above (Austin et al. 2006). Also areas deeper or equal to 50 m

constitutes only 4 % of the central Kattegat and 75 % of all dives performed by the studied seals

were shallower than 17 m. Since harbour seals from the study area are mainly believed to be

bottom feeders (Härkönen 1987a; Härkönen 1987b), the relationship between bottom time of a

dive and prey richness may differ between foraging for pelagic and benthic prey. Austin et al.

(2006) observed almost as many feeding events for grey seals along relatively straight sections of a

track as in more tortuous sections which may indicate feeding while travelling. All methods stated

above take the animals’ location or dive shapes into account. In this thesis both locations and dive

data are available and therefore animal foraging may be estimated both in the horizontal and

vertical dimension.

Due to the fact that the studied animals were foraging within a relatively small and shallow area

and that there was a resolution discrepancy between dive and location data which additionally

were not collected simultaneously , an alternative method of locating foraging areas for harbour

seals was presented in this study.

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Manuscript

MANUSCRIPT

Diving behaviour of harbour seals (Phoca vitulina) in the Kattegat, Southern Scandinavia

Abstract

Harbour seals are top predators in the Kattegat and are considered shallow and benthic divers. To

understand the foraging ecology of these predators, this study focuses on diving behaviour and the

conditions under which harbour seals forage. Since quality and quantity of their prey was not

obtained, the characteristics of the habitat and diving behaviour of the seals were used as proxies.

Therefore, the results were discussed in the light of oceanographic features of the Kattegat,

potential prey of the harbour seals and inter- and intraspecific competition. The study was

conducted in April-May 2008 on Anholt where four harbour seals (three males, one female) were

equipped with time-depth recorders (TDRs) and satellite transmitters. The TDRs recorded dive

data for 10h, 14, 20 and 21 days respectively. Since only four individuals were tagged, emphasis

was put on individual behaviour and preferences. A diurnal dive pattern was found for all dive

parameters for each individual with low number but long and deep dives during midday and more

but shallower dives during the night, probably due to change in foraging strategy in response to

prey availability. The haul-out time and duration was influenced by human disturbance at the haulout

site. Harbour seals from Anholt fed within the whole Kattegat but had individually preferred

feeding areas. Differences in the size and location of the foraging areas are probably due to habitat

differentiation, prey distribution and preferences and inter- and intraspecific competition. This

study gives a first inside into diving and foraging behaviour of the harbour seals from Kattegat and

shows individual behaviour and preferences of the tagged seals. An alternative method for

defining foraging areas is also shown.

Keywords: harbour seal, Phoca vitulina, diving and foraging behaviour

In agreement with my supervisors, I have imbedded figures and tables in the text to improve

readability.

28


Manuscript

Introduction

Many of the processes in an ecosystem are shaped by predator–prey interactions and interactions

between individuals and their habitat (Odum et al. 1971). Each species or population occupies a

certain niche in this ecosystem. Such niche has to satisfy species or population needs like access to

food, resting places and reproduction (Odum et al. 1971). Therefore, to get an overview about the

processes influencing the studied ecosystem, information on several levels is required, such as

predator–prey interactions, inter- and intraspecific interactions, number and the energetic of the

prey, the physical parameters of the habitat shaping such interactions and niche differentiation.

Therefore to understand the foraging ecology of predators, this study focuses on foraging

behaviour and the conditions under which predators forage. Competition and prey distribution are

probably one of the main factors shaping the predators foraging area. The high concentration of

central place foragers can lead to local depletion of resources (Dolman & Sutherland 1997);

therefore individuals may develop specific foraging routes and tactics that minimize competition

(Staniland et al. 2004). Also if prey is of limited distribution, partitioning of the foraging area

among individuals is expected (Eguchi & Harvey 2005).

Harbour seals are top predators in many ecosystems (Bonner 1989). This species is known to show

high site fidelity and feed close to their haul-out sites usually within 50 km (Frost et al. 2001;

Härkönen 1987a; Härkönen & Hårding 2001; Thompson et al. 1998; Tollit et al. 1998). Harbour

seals are opportunistic predators and their food preferences are largely dependent on the habitat

they live in and relative abundance of the prey (Brown & Pierce 1998; Eguchi & Harvey 2005;

Härkönen 1987b; Härkönen & Heide-Jørgensen 1991; Tollit et al. 1998).

Harbour seals are also top predators in the Kattegat. There are nine main hauling out sites of

harbour seals in the Kattegat and Anholt, the island in middle of the Kattegat, is on of them (Olsen

2006). Approximately 5000 harbour seals inhabit the Kattegat and up to 1000 use to haul–out on

Anholt (NERI, unpublished data). The harbour seals in the Kattegat and western Baltic area are

primarily bottom feeders with a diet consisting mainly of bottom fish and gadoids; however, still,

some pelagic or benthopelagic species have also been recorded to be components of their diets

(Andersen et al. 2007; Härkönen 1987a; Härkönen 1987b). Very little in known about diving

behaviour of the harbour seals in the Kattegat, and such an information is necessary to understand

the ecological processes in the Kattegat marine ecosystem.

Usually, direct measurements of the quantity, quality and presence of a prey are impossible and

therefore characteristic of the habitat and diving behaviour of harbour seals are used as proxies.

The use of time–depth recorders (TDRs), together with satellite telemetry shows how harbour

29


Manuscript

seals utilize their environment (e.g., Ramasco 2008; Robinson et al. 2007). TDRs enable to record

dive data with a very good time resolution what make it possible to distinguish between a certain

dive shapes, which are believed to be related to certain behaviour of harbour seals (Hindell et al.

1991; Lesage et al. 1999). Specific animal behaviours frequently occur in bouts, i.e. similar events

occur successively in clusters (Mori et al. 2001). Therefore analyses of bouts may offer a better

understanding of the behaviour. Satellite telemetry provides information about horizontal

movement of the tagged animals and therefore allows relating diving behaviour of these animals to

the environmental parameters of the habitat they live in.

Prey availability is often correlated with physical and biological properties of the sea, such as

depth, temperature, and substrate type (Austin et al. 2006; Tollit et al. 1998). Therefore, the results

were discussed in the light of oceanographic features of the Kattegat, potential prey of the harbour

seals and inter- and intraspecific competition. The present study hopefully leads to a significant

improvement in understanding of the behaviour of harbour seals around Anholt. The recorded data

allowed investigating the following aspects i) diving behaviour of the tagged seals in relation to

environmental factors and diurnal rhythm ii) their habitat use and foraging areas. We also make a

hypothesis that the studied seals do have the individual variability in diving and foraging

behaviour and that they feed within the whole Kattegat but have individually preferred feeding

areas.

Material and methods

The study area

The study was conducted in April-May 2008 on the island Anholt (Denmark) located in the central

part of the Kattegat (Figure 1). The maximum depth of the area is 106 m, and deeper waters are

mainly located along the Swedish coastline. Anholt is surrounded primarily by shallow waters (up

to 10 m) except for its south-eastern part where depth increases steeply beneath 25 m (Figure 1).

The haul out site is located on the eastern tip of Anholt in the Totten seal sanctuary; however, seals

hauling-out outside the reserve were also observed. Sandy bottom occurs mainly in shallow areas

and mud dominates in deeper areas of the Kattegat. Tides are barely detectable in the Kattegat

Skagerrak (Härkönen 1987a). Salinity varies between 30 and 18 PSU and is strongly influenced by

a flow of saltier water from the North Sea and Skagerrak.

30


Manuscript

Coast line

Depth [m]

0 - 5

5.1 - 10

11 - 15

16 - 20

21 - 25

26 - 30

31 - 35

36 - 40

41 - 45

46 - 50

51 - 55

56 - 60

61 - 65

66 - 70

71 - 75

76 - 80

81 - 85

86 - 90

91 - 95

96 - 100

100 - 110

Figure 1. Kattegat and the study area in a dashed square.

In the Kattegat a strong halocline at a depth 2-20 m occurs throughout the year with the highest

salinity in the deepest waters flowing in from the north and with low saline water flowing out of

the Baltic at the surface (Geological Survey of Sweden - Baltic Data BALANCE).

Equipment and instrument deployment

Three male and one female harbour seals (Table 1) were captured in nylon surface gillnets in the

vicinity of the Anholt haul-out site. Diving depths of the harbour seals were recorded using one

Mk6 and three Mk9 time-depth recorders (TDRs) (Wildlife Computers, Redmond, USA) (Table

1).

31


Manuscript

Table 1. Details of the tagged harbour seals.

Sex (abbreviations

used in the text)

Estimated maturity and

age (year)*

Weight

(kg)

Length

(cm)

Deployed

TDR

model

Satellite

transmitter

Male (M1) Adult (4-5) 62 140 25-04-2008 MK9 Spot 5

Female (F1)

Subadult/

Adult (3-4)

46 120 26-04-2008 MK9 Spot 4

Male (M2) Subadult (2-3) 38 118 26-04-2008 MK9 Spot 5

Male (M3) Subadult (2-3) 46 118 28-04-2008 MK6 Spot 4

* Age and maturity estimated in accordance with: Frost et al. 2001; Härkönen & Heide-Jørgensen 1990; Suryan &

Harvey 1998; Thompson et al. 1998.

The period when TDRs were in use is called TDR period and the period when all satellite

transmitters were operating will be from here on called PTT period (Figure 2).

Figure 2. Duration of TDRs and PTTs deployments for the four individuals. Numbers to the right of the bars indicate

days of deployment.

The Mk9 TDRs recorded data every second and the Mk6 every 5 seconds. Depth was measured

with 0.5 m resolution for all TDRs and temperature with 0.05 ºC resolution in the case of the Mk9.

The Mk6 was additionally equipped with a velocity sensor, which unfortunately did not work. A

TDR together with a release mechanism (Little Leonardo, Japan) and a VHF transmitter

(Advanced Telemetry Systems - ATS M110), which enables to locate the TDRs after having been

released, were encased in a floating material and glued to the fur on the mid-back of the seal using

fast hardening epoxy. The TDR packages weighed between 120 and 160g corresponding to < 0.5

% of the seal’s body mass. Additionally each seal was deployed with a satellite transmitter

(Platform Transmitter Terminal - PTT) (SPOT 4 or SPOT 5, Wildlife Computers, Redmond, WA,

32


Manuscript

USA) glued on top of its head. An additional VHF transmitter (Sirtrack, New Zealand), which

aimed to indicate whether the seal was on land by means of an automatic receiver (ATS R4500S,

USA) was also glued onto the back of the seal behind the TDR.

Visual observations

During the TDR period, movement and behaviour of all the tagged seals were monitored when

they were close to land both through binoculars (10x42) and a telescope. The observations were

made both from a 40 m lighthouse situated 500 m from the haul-out site and from the dunes along

the haul-out site, to observe a haul-out group outside the reserve. Movements, behaviour and

number of dives were noted. To better elucidate the possible function of different dive types, the

visual results were compared afterwards with the data from the TDRs.

Data analysis

For the Mk9 recordings, dive parameters were analysed in purpose built Wildlife Computers

software Instrument Helper 1.0.53 (Appendix A) whereas the recordings obtained with Mk6 were

processed using and in Matlab 6.5.1. (Mathworks Inc., using a custom written routine by courtesy

of K. Beedholm, Aarhus University) and in Dive Analyses Package (Wildlife Computers). A dive

was defined as any vertical movement equal or exceeding 3 m. After a prolonged haul-out period,

a marked negative drift of the zero offset may occur, resulting in unreliable measurements.

Therefore Zero Offset Correction was calculated for each individual, amounted to 1 m for M1, 1.5

m for F1 and M2 and varied from +5 to -10 m for M3. The following parameters were calculated

for each dive:

- total dive duration,

- maximum depth,

- bottom time defined as a time interval between the end of a descent till the beginning of an

ascent,

- bottom time (80 %) defined as a time interval between the first and last recorded depths

equal or exceeding 80 % of the dive’s maximum depth,

- time spent at the maximum depth,

- presence of wiggles defined as any vertical movement from the bottom phase of the dive,

larger or equal to 2 m and shorter than 30 s,

- duration of intervals between dives.

33


Manuscript

All statistical analyses were performed in JMP 7.0 (SAS) statistical software. Foraging areas were

defined with the use of Arc GIS 9.2, ESRI, USA. Physical parameters of the Kattegat were based

on the data provided by the Geological Survey of Sweden - Baltic Data BALANCE.

Dive classification

Six dive shapes were classified (Figure 3). Any dive including at least one wiggle was classified as

a W-shaped dive. A two-phased dive was defined as any dive during which a seal had spent

considerable amount of time on two distinct depths. W-shaped and two phased dives were

removed from the cluster analysis since they were already assigned to the appropriate shape. Any

vertical movement that involved less than 10 s at the maximum depth and less than 20 s at the

bottom (80 %), was defined a V-shaped dive. All other dives were classified as U-shaped dives.

Shallow-V-shaped dives Deep-V- shaped dives Shallow-U – shaped dives

Deep-U – shaped dives W-dives Two-phased dives

Figure 3. Classified dive shapes.

Due to a large number of data, the non-hierarchical K – means cluster analysis was run to classify

U– and V-shaped dives based on the maximum depth and a dive duration (McGarigal et al. 2000;

Schreer et al. 1998). All dives were standardized to the mean 0 and SD of 1 before applying the

clustering procedure. Since K-means clustering requires the number of clusters to be defined a

priori and the exact number of clusters was unknown, several cluster analyses were performed

specifying a different number of clusters in each, ranging from 2 to 5 and with the use of several

dive parameters. Random subsample of 100 dives of each individual per each cluster analysis was

selected in order to verify if the chosen method was correct. Finally, both U and V-shaped dives

were classified with the use of 2 clusters each and the classification was based on dive depth and

duration.

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Manuscript

Identification of foraging areas

Each foraging trip was defined as time between one haul-out period and the next one. A diving

bout was described as the time of continuous diving followed by at least a 15 - minute break in

diving until the beginning of the next bout (Boyd et al. 1994; Boyd 1996; Mori et al. 2005). To

identify a potential foraging area for each individual, a comparison of the maximum depth of each

dive, the locations of seals while diving and bathymetry of the Kattegat was conducted.

Based on the number of uplinks received by a single satellite during a single passage and on the

algorithm calculated by the ARGOS system, satellite positions are assigned to one of the six

precision location classes (Tougaard et al. 2008). Based on studies by Vincent (2002) and Robson

et al. (2004) who empirically determined the errors of each location class, an average error was

assigned to each location class as follow: location class (LC) 3=0.3 km; LC 2=0.5 km; LC 1=1

km; LC 0=3 km; LC A=1 km and LC B=5 km. By assuming a mean speed of a harbour seal to be

about 5 km/h (Markussen & Øritsland 1994; William et al. 1991) and by including the possible

location error of ARGOS data, the potential foraging area for each individual was determined as

follows:

1) Each foraging trip was analysed independently

2) A buffer zone covering 1 hour of seal’s movement for each location class was defined for

each seal. This buffer included average seal speed (5 km/h) and a potential ARGOS error

and therefore was defined with the following radiuses: LC 3 = 5.3 km (average speed of a

harbour seal 5 km/h + 0.3 km of the possible location error); LC 2 = 5.5 km; LC 1 = 6 km;

LC 0 = 8 km; LC A = 6 km; LC B = 10 km.

3) Dives were grouped into bouts and the maximum depth of each dive within a particular

bout performed up to 1h after the satellite position had been taken, was compared with

bathymetry of the area within the defined buffer zone in order to verify if dives were

performed to the bottom of the basin and to distinguish areas of predominantly pelagic or

bottom foraging.

4) Each diving bout was then classified into one of the following classes:

a - when majority of dives (> 90 %) was to the bottom and more than 80 % of dives were

deep U-shaped;

b – when more than 75 % of dives was to the bottom and 10 % of dives were two-phased

and 10 % were wiggle dives;

c – when less than 50 % of dives was to the bottom with no dominance of any dive shape

35


Manuscript

d – when less than 50 % of dives was to the bottom with the majority (> 60 %) of foraging

dives (deep U-shaped, two-phased and w-shaped dives);

e – when more than 90 % of dives was pelagic;

f – when there were series of dives to the shallow depths (< 5 m) – preassembly travelling

dives;

Diving bouts with dominance of U-shaped, W-shaped and two-phased dives (class a, b, d)

as well as pelagic dives (class e) were classified as foraging bouts.

5) The buffer zones around classes a, b, d and e were combined for each foraging trip to

identify the overall foraging area for each individual.

Satellite positions and kernel home range analyses

To remove unrealistic Argos locations, the satellite positions were filtered by a SAS-routine,

Argos_Filter v7.03 (Douglas 2006). This so called DAR (Distance-Angle-Rate) filter attempts to

identify implausible locations based on the fact that most suspicious Argos locations cause an

animal to incorrectly move a substantial distance and then return, resulting in a tracking-path that

goes 'out-and-back' (and further validated by unrealistic movement rates (we choose 10 km/h),

depending on the temporal frequency of the locations) (Sveegaard et al. 2009). ARGOS location

class 3 was not filtered, since high accuracy of the position was already expected. Number of

locations and proportion of each location class in the overall number of positions after filtering is

given in Table 2. Location class B occurred most often for all individuals.

Table 2 Percentage of satellite positions being specified by the ARGOS system to one of the six location classes [%]

during the TDR period.

M1 F1 M2 M3

# of positions (# days) 125 (21) 80 (22) 3 (0.4) 152 (15)

Location class % % % %

3 15 0 0 7

2 11 13 0 10

1 6 11 0 5

0 3 1 0 3

A 26 30 33 18

B 38 45 66 57

Separate kernel home ranges were calculated for the TDR and PTT periods for each individual.

The calculations of kernel home ranges were based only on the locations at the time when seals

36


Manuscript

were in the water. Therefore locations, for which the VHF transmitters signal indicated that the

seal was on land, were removed. Hawth’s Analysis Tools extension for ArcGIS 9.2 was used for

these analyses. Quartic fixed kernel density with scaling factor 1000000 and smoothing factor

10000 was used. To avoid data skewness due to unequal satellite coverage during the day, a

maximum of 2 positions per every 4-hour period (00:00-3:59; 4:00-7:59; 8:00-11:59; 12:00-15:59,

16:00-19:59; 20:00-23:59) were taken into account.

Results

Dive parameters

The basic dive parameters for all individuals are given in Table 3. None of the parameters were

either log-normally or normally distributed (KSL test, p


Manuscript

15%

13%

M1

800

700

14%

13%

F1

900

800

11%

9%

7%

5%

4%

600

500

400

300

200

Number of dives

11%

10%

8%

6%

5%

3%

700

600

500

400

300

200

Number of dives

2%

100

2%

100

0%

25/04/08

26/04/08

28/04/08

30/04/08

02/05/08

04/05/08

06/05/08

08/05/08

10/05/08

12/05/08

14/05/08

0

0%

26/04/08

27/04/08

29/04/08

01/05/08

03/05/08

05/05/08

07/05/08

09/05/08

11/05/08

13/05/08

15/05/08

17/05/08

0

Date

Date

14%

13%

11%

9%

7%

5%

4%

2%

M3

640

560

480

400

320

240

160

80

Number of dives

0%

28/04/08

29/04/08

01/05/08

03/05/08

05/05/08

07/05/08

09/05/08

11/05/08

13/05/08

0

Figure 4. Number and percentage of dives within the TDR

period.

Date

The duration of dive had unimodal and bimodal distribution for M1 and M3 respectively (Figure

5). Seventy five % of all dives of M1 were shorter than 3 min. The longest dive performed was

10.5 min. Seventy five % of all dives of F1 were shorter than 2 min 40 s and the longest recorded

dive was 11 min. The longest dive performed by M3 was 14.9 min yet 75 % of all dives were

shorter than 2 min 40 s.

All individuals showed unimodal distribution of dive depths (Figure 6). The maximum depth

reached by seal M1 was 29 m, yet 75 % of its dives were below 17 m. Seal F1 dove to the

maximal depth of 31 m, but the majority of its dives (75 %) were below 11 m. Seal M3 dove down

to 60 m with 75 % of all dives below 11 m.

38


Manuscript

Dive duration [min:s]

6:30

6:00

5:30

5:00

4:30

4:00

3:30

3:00

2:30

2:00

1:30

1:00

0:30

0:00

0.4

0.6

1.0

Figure 5. Distribution of dive durations within TDR period. Numbers beside each column indicate the percentage

count for each histogram’s interval. The outlier box plot shows the median together with 25 th and 75 th percentiles, and

outliers.

2.7

3.7

5.0

5.8

5.5

7.3

10.3

12.3

21.0

250 750 1250

Number of dives

M1

24.4

Dive duration [min:s]

6:30

6:00

5:30

5:00

4:30

4:00

3:30

3:00

2:30

2:00

1:30

1:00

0:30

0:00

0.0

0.2

0.5

0.8

1.2

1.9

4.1

8.2

12.3

13.0

12.3

13.8

F1

15.7

15.8

200 600 1000

Number of dives

Dive duration [min:s]

6:30

6:00

5:30

5:00

4:30

4:00

3:30

3:00

2:30

2:00

1:30

1:00

0:30

0:00

0.1

0.2

0.3

0.9

2.2

3.1

7.0

9.4

9.6

13.0

11.1

14.9

M3

28.1

250 750 1250

Number fo dives

Dive depth [m]

30

25

20

15

10

5

0

0.9 M1

5.3

49.9

15.8

20.9

7.2

500 1500 2500

Number of dives

Dive depth [m]

30

25

20

15

10

5

0

0.9

F1

8.8

6.5

11.5

52.2

20.1

500 1500 3000

Number of dives

Dive depth [m]

60

55

50

45

40

35

30

25

20

15

10

5

0

0.1

M3

0.3

0.5

0.9

1.6

1.8

2.7

4.9

6.4

11.5

27.0

42.2

500 1500

Number of dives

Figure 6. Distribution of dive depths within TDR period. Numbers beside each column indicate the percentage count

for each histogram’s interval. The outlier box plot shows values as in Figure 5.

Diurnal distribution of dive parameters

A 6 th -order polynomial regression curve best described the diurnal trend for all individuals except

for the three cases described in Table 4.

39


Manuscript

M1 performed few dives which were of long duration and varying depth during the midday

(Figure 7). For this seal, the twilight was a transition zone during which the dive parameters

changed rapidly. A similar midday trend is observed in the case of F1; however, no changes

related to the twilight are observed. A high number of relatively short and shallow dives were

performed by this individual at night. M3 also made few dives during midday but each long and

deep. Sudden changes in dive parameters occurred before sunrise and at sunset with many short

and shallow dives. M1 and M3 dove to varying depths within the same area during days and

nights. F1 dove in shallower areas at night and in deeper ones during day.

Mean number of dives

30

25

20

15

10

30

30

M1 25 F1 25 M3

20

20

15

15

10

10

5

0 2 4 6 8 10 12 14 16 18 20 22

5

0 2 4 6 8 10 12 14 16 18 20 22

5

0 2 4 6 8 10 12 14 16 18 20 22

3:30

3:30

3:30

Mean dive duration [min:s]

3:00

2:30

2:00

1:30

1:00

3:00

2:30

2:00

1:30

1:00

3:00

2:30

2:00

1:30

1:00

0:30

0 2 4 6 8 10 12 14 16 18 20 22

0:30

0 2 4 6 8 10 12 14 16 18 20 22

0:30

0 2 4 6 8 10 12 14 16 18 20 22

Mean dive depth [m]

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18 20 22

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18 20 22

Day time

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18 20 22

Figure 7. The diurnal distribution of dive parameters for all individuals with time of twilights as a shaded bar.

40


Manuscript

Table 4. Statistical parameters

of the polynomial fit degrees (p


Manuscript

% of time spent on each activity

% of time spent on each activity

% of time spent on each activity

100%

M1

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

0 2 4 6 8 10 12 14 16 18 20 22

Day time

100%

F1

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

0 2 4 6 8 10 12 14 16 18 20 22

Day time

100%

M3

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

0 2 4 6 8 10 12 14 16 18 20 22

Day time

% of time spent on each activity

% of time spent on each activity

% of time spent on each activity

100%

M1

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

0 2 4 6 8 10 12 14 16 18 20 22

Day time

100%

90%

F1

80%

70%

60%

50%

40%

30%

20%

10%

0%

0 2 4 6 8 10 12 14 16 18 20 22

Day time

100%

M3

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

0 2 4 6 8 10 12 14 16 18 20 22

Day time

Diving

g

Haul-out Resting close to land Resting

Figure 8. Diurnal variations in seal activity. Resting: haul-out + resting close to land; Resting close to land: slow

swimming around the houl-out site with no clearly distinguishing dives.

42


Manuscript

Analyses of dive shapes

The median values of dive duration and depth for each dive shape did not differ between

individuals (Kruskal-Wallis test, p> 0.69). Deep U-shaped, W-shaped and two-phased dives were

the deepest and longest for all seals (Table 5).

Table 5. Median (MED) maximum dive depth and duration for all the identified dive shapes (SD in brackets).

Shallow-V Deep-V Shallow-U Deep-U w-dives Two-phased

MED

max

depth

[m]

MED

duration

[min]

MED

max

depth

[m]

MED

duration

[min]

MED

max

depth

[m]

MED

duration

[min]

MED

max

depth

[m]

MED

duration

[min]

MED

max

depth

[m]

MED

duration

[min]

MED

max

depth

[m]

MED

duration

[min]

M1

4.8

(1.7)

0:17

(0:09)

12.2

(3.8)

0:57

(0:23)

4.4

(0.7)

1:45

(1:04)

14.5

(4.3)

2:40

(1:01)

15.7

(4.1)

2:31

(0:51)

17.4

(3.2)

2:31

(0:28)

F1

4.1

(1.0)

0:11

(0:04)

7.2

(2.6)

0:26

(0:14)

4.8

(1.1)

1:40

(1:08)

12.7

(5.6)

2:18

(1:07)

12.5

(5.1)

2:23

(0:55)

13.1

(4.6)

2:21

(0:42)

M2

4.5

(2.1)

0:37

(0:01)

16.5

(0.7)

1:11

(0:01)

4.9

(1.5)

2:19

(1:22)

31.1

(3.0)

3:54

(1:21)

28.2

(8.5)

3:42

(1:30)

- -

M3

3.3

(0.6)

0:06

(0:05)

13.1

(14.6)

0:46

(0:58)

4.6

(2.0)

0:56

(0:42)

17.3

(10.9)

2:56

(0:38)

13.7

(8.6)

2:55

(1:27)

- -

Two-phased dives were only found in the case of M1 and F1. For all individuals, U-shaped dives

constituted the majority of dives (Table 6).

Table 6. Percentage share in the total number of all dives performed by a given individual [%].

Shallow Deep V- Shallow U- Deep U-

twophased

W-shaped

V-shaped shaped shaped shaped

M1 5.0 1.3 9.5 70.2 7.0 6.9

F1 6.9 5.2 37.8 47.4 2.3 0.4

M2 1.5 1.5 47.0 30.3 19.7 -

M3 9.9 0.6 48.6 32.0 8.8 -

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Foraging trips and diving bouts

Number and duration of foraging trips and diving bouts within each foraging trip did not differ

among individuals (Kruskal-Wallis test, p> 0.5) (Appendix B). Duration of diving bouts and

feeding trips ranged from 7 minutes to more than 69 hours and from 2 h 48 min to 3 days 16 h,

respectively (Table 7).

Table 7. Range of diving bouts and feeding trips duration for three individuals.

Feeding trips

Diving bouts

Mean Range Mean Range

M1 36 h 17 min 4 h 12 min – 3 d 16 h 10 h 26 min 7 min – 2 d 35 min

F1 24 h 58 min 2 h 55 min -3 d 9 h 7 h 32 min 7 min – 23 h 1 min

M3 35 h 30 min 2 h 48 min – 3 d 8 h 24 h 5 min 2 h 48 min – 2 d 21 h 21 min

For each individual, duration and number of dives in each foraging trip varied with no general

trend observed. A longer foraging trip did not imply more diving bouts or deeper and longer dives.

Nor did shorter foraging trips result in an increase in overall amount of time spent underwater.

None of the feeding trips started with a directional heading towards the feeding area. Foraging

dives and foraging diving bouts were also observed close to the haul-out site.

Foraging areas

Foraging areas varied in terms of location and size among the tagged seals (Figure 9, Table 8). M1

foraged within the largest area and swam the farthest distance from the haul-out site (Table 8).

Table 8. The distance travelled from the haul-out site and the area of the foraging ground of each individual.

Furthest distance from

the haul-out site [km]

The area of the foraging

ground [km 2 ]

M1 56 2048

F1 30 1286

M2 15 253

M4 42 1011

Frequently visited areas did not always correspond to the highest density of satellite positions.

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Depth [m] 20.01 - 25 45.01 - 50 70.01 - 75

- overall area where diving took place

25.01 - 30 50.01 - 55 75.01 - 80

0 - 5 - areas with dominance of pelagic dives

5.01 - 10

10.01 - 15

15.01 - 20

30.01 - 35 55.01 - 60 80.01 - 85

35.01 - 40 60.01 - 65 85.01 - 95

40.01 - 45 65.01 - 70 95.01 - 110

- frequently visited areas

• - satellite positions

- land

Figure 9. Potential foraging areas for all individuals.

There was no single area represented only by one class of diving bouts except for pelagic diving

bouts (class e) performed by F1 east of Anholt (Figure 9). Each class of the diving bouts was

performed in a variety of depths and at different distances from the haul-out site. 16 % (4 diving

bouts), 22 % (n=9) and 11 % (n=1) of all diving bouts could not be located due to the lack of

satellite positions for M1, F1 and M3 respectively. Classes d and e were present only in the case of

F1 (Table 9), whereas class a constituted most of the diving bouts for three seals (M1, F1 and M2),

while class b dominated for the fourth one (M3). Class f was equally frequent for all individuals,

except for M2 (Table 9).

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Table 9. Overall percentage share of each class of diving bout for each individual [%].

The class of

diving bouts a b c d e f

M1 37.5 37.5 0 0 0 25

F1 55 0 5 5 15 20

M2 100 0 0 0 0 0

M3 0 62.5 12.5 0 0 25

Classes a and b were mainly performed in slopes and trenches, whereas class f in the shallow

areas. Sixty three % of the potential foraging ground for M1 consisted of sand, 22 % rocky bottom

and 15 % muddy bed. F1 foraged mainly on sandy bed (43 % of the potential foraging area) but

also visited muddy (37 %) and rocky bottom habitats (17 %). The only foraging trip of M2 was to

an area with muddy (70 %) and sandy (30 %) sediments. M3 dove mainly on muddy (59 %) and

sandy (21 %) sediments. The rest of the visited area consisted of rocky bottom (10 %) and hard

clay (9 %) sediment types.

Kernel home range

Kernel home ranges differed between individuals in terms of shape, location and size (Figure 10,

Table 10). For individual F1 and M2 these ranges were located around Anholt, whereas for seal

M3 home range was focused around two centres. Ninety five % kernel home range of M1 covered

the largest area (1794 km 2 ) and of F1 – the smallest (605 km 2 ) (Table 10). In order to verify if the

movement of the studied seals has changed by the time, an additional kernel home range was

calculated based on the PTT period (Appendix C). For M1, M2 and M3 the area of 95 % kernel

home range probability increased by 23 to 31 %, while in the case of F1 it decreased by 8 %

(Table I and II, Appendix C).

Table 10. The area of kernel home ranges with 25 to 95 % probability for all individuals [km 2 ].

Individual\Kernel home range

probability [%]

95 75 50 25

M1 1794 709 285 84

F1 605 280 126 50

M2 669 249 111 45

M3 754 351 181 76

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Figure 10. Kernel home range probabilities for the TDR period. For M2, the satellite positions taken between 26/4 –

15/5 were used for calculations.

Visual observations

29 observations were taken within 2 km from the shore where depth of water did not exceed 8 m.

Most of the observations were related to slow swimming around the haul-out site or fast, shallow

swimming after human disturbance from land. Feeding was observed once in a non - studied

animal over a 5 m deep area. At least 5 pups were born during the observation period indicating

the onset of the breeding season. However, mating was not observed during that time.

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Discussion

Evaluation of the methods

One has to be careful when interpreting data collected from just four individuals of various age

and sex. The power to make general observations about the seals at Anholt was hampered by a low

sample size, but it has nevertheless allowed us to gain insight into the individual behaviour.

This study combines two dimensional (2D) dive data from TDRs and 2D satellite positions, which

in combination gives a 3D impression of the habitat utilization. These two data sets were not

collected simultaneously since satellite locations were only provided when the seal was at the

surface and when satellite coverage was sufficient, whereas dive data were collected continuously

when underwater every 1 or 5 seconds depending on the TDR model employed. This led to a high

resolution in dive data being combined with positioning data of lower resolution. It should also be

noted that these data sets, although both recording in 2D, comprised data collected in horizontal

and vertical dimensions respectively. Due to the resolution discrepancy between the two data sets,

the estimated potential foraging areas should serve as proxies. In the present study, it was not

possible to measure the quantity, quality and presence of the prey as well foraging success of the

studied animals, therefore characteristic of the habitat and dive shapes were used as proxies for

identification of the potential foraging areas of the studied seals.

Harbour seals are often reported as relatively shallow divers; however dives exceeding 450 - 500

m and lasting more than 30 min have also been recorded (Eguchi & Harvey 2005; Ries et al. 1997;

Schreer & Kovacs 1997; Tollit et al. 1998). Exclusion of very shallow dives may greatly

underestimate diving and foraging effort by harbour seals (Frost et al. 2001). A study of harbour

seals by Lesage et al. (1999) showed that 40 % of feeding occurred at depth less than 4 m. The

reason for choosing 3 m as the dive limit in this study was that feeding was observed in shallow

waters close to the haul-out site. However, setting the upper dive limit to 3 m instead of e.g., 2 m,

allowed avoiding bias due to the zero offset drift of the TDRs.

Diurnal pattern of dive parameters and haul–out

There was a diurnal pattern of diving found for the three seals with few long and deep dives

during the day and numerous shallow and short dives during the night or twilight (Figure 7).

Diurnal changes in a harbour seal diving behaviour were also observed in other studies (Coltman

et al. 1997; Frost et al. 2001; Thompson et al. 1989); however, studies where such pattern was not

observed can also be found (Suryan & Harvey 1998). Differences between nocturnal and diurnal

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patterns of dive have been interpreted as a change in foraging strategy in response to prey

availability (Suryan & Harvey 1998) or as a part of breeding behaviour (Coltman et al. 1997). If

such pattern is explained by a change in foraging tactic, two strategies are possible: to follow a

certain prey species which changes behaviour between day and night and/or to switch to a prey

type which is more abundant and/or easily detectable during day or night (or twilight). Herring

(Clupea harengus), which is a part of the diet of harbour seals from the Kattegat (Andersen et al.

2007; Härkönen 1987a; Härkönen 1987b), changes their schooling behaviour and moves to the

surface waters during the night and other species like lesser sandeels (Ammodytes tobianus), also

preyed by these seals (Andersen et al. 2007; Härkönen 1987a; Härkönen 1987b), are believed to

be particularly vulnerable to predators in the twilight hours, when they start to forage and leave the

sediments (Adlerstein & Ehrich 2003; Thompson et al. 1989). It has been suggested that benthic

species are higher in the water column during the night, while pelagic species are mainly found

closer to the bottom during the day (Adlerstein & Ehrich 2000). Therefore, species like Norway

pout (Trisopterus esmarkii), haddock (Melanogrammus aeglefinus), herring and sprat (Sprattus

sprattus) were found at the bottom during daytime (Adlerstein & Ehrich 2000; Adlerstein &

Ehrich 2003; Engas & Soldal 1992). Bottom fish like dab (Limanda limanda) were recorder closer

to the surface during the night (Adlerstein & Ehrich 2000). In the North Sea, cod (Gadus morhua)

was found close to the bottom in the early morning and around midday (Adlerstein & Ehrich

2003). At night, prey may be less sensitive to the approaching predator due to the reduced light

penetration into the water column (Boyd et al. 1994). Harbour seals from the Kattegat are usually

considered to be bottom feeders with a diet consisting mainly of bottom fish (Andersen et al.

2007; Härkönen 1987a; Härkönen 1987b). However, due to lack of the direct observations of

feeding, it is hard to conclude if both diurnal and/or nocturnal dives were connected to prey

foraging. The adult male (M1) tagged in this study may have been involved in the breeding

behaviour. Coltman et al. (1997) found that male breeding diving behaviour show a strong diurnal

pattern. Deeper foraging dives were more common during daytime, when females also tended to

forage, and shallow dives during the twilight and dark hours were associated with reproductive

behaviour. Coltman et al. (1997) reported that females left the island in the evening and returned

from foraging in the morning so males may have made many shallow displays near the haul-out

site to maximize encounter rate during twilight hours. This might suggest that foraging and

reproductive behaviour occurs at different times of the day. At least 5 pups were born during the

TDR period, suggesting that breeding had already started and some animals may have been

involved in breeding behaviour. Only F1 dove in shallower areas at night and in deeper during

daytime, whereas such relationship was not found for M1 and M3. If diurnal prey behaviour has

been a main factor driving diurnal dive pattern of the studied seals, this may indicate that the

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female may have a different foraging tactic than the two males. However, low spatial resolution of

ARGOS data may have contributed to the individual differences recorded. The diurnal pattern of

resting differed between individuals (Figure 8), with M1 resting primarily during midday, whereas

F1 and M3 resting during night and early hours. When seals make long feeding trips, returning

inshore to rest for a short period is probably non - profitable and they may rest at sea between

feeding bouts. The timing of haul-out periods is therefore less likely to reflect feeding patterns

when trip duration is long because when seals return inshore primarily to rest, they probably haulout

irrespectively of time of the day (Thompson et al. 1989). For the three seals the least time

spent hauling out was between 10:00 and 20:00. When haul out decreased between 10:00 and

20:00, resting close to the land increased. This is in contrary to generally observed hauling out

behaviour of harbour seals from the Kattegat and Skagerrak at the similar time for which the

maximum number of seals hauling out on land is during the morning hours, between 9.00 and

12.00 (Edren et al. 2004; Helander & Bignert 1992). In this study, most of the human disturbance

occurred before midday (low flying airplanes, human entering the reserve, boats passing by close

to the reserve borders). These disturbances caused the seals to flee to the water and not come back

until the evening. During these hours, many seals stayed close to the haul-out site. Tollit et al.

(1998) proposed that any locations obtained within 2 km of a haul-out site may be associated with

haul-out activity. This may indicate that human activity on Anholt had a significant influence on

the haul–out and resting patterns of the seals.

Dive shapes

The grouping of deep U-, W- and two-phased dive shapes in to ‘foraging dives’ was made based

on the behavioural interpretation (feeding or searching for food) of similar classes found in the

literature (Hindell et al. 1991; Lesage et al. 1999). However, it is always difficult to assign a

function to any particular dive without independently collected data. Such methods like

deployment of stomach temperature loggers (Lesage et al. 1999; Wilson et al. 1992), or

underwater cameras (Bowen et al. 2002), have previously been used to identify feeding events. A

more clear interpretation of the different dive shapes could also be achieved by deploying 3Daccelerometers

in association with time-depth recorders and/or cameras (pers. comm. Yasuhiko

Naito, National Institute of Polar Research, Tokyo, Japan). This would register not only movement

related to depth, but also acceleration bursts associated with prey capture (Ramasco 2008).

U-shaped dives were the most common for all the studied individuals. This is in accordance with

other studies that have classified dives of harbour seals (Baechler et al. 2002; Eguchi & Harvey

2005; Lesage et al. 1999; Ramasco 2008). Relatively shallow divers, like harbour seals, are able to

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dive to the bottom while remaining within their aerobic dive limit (ADL) because of the limited

travel time to and from the bottom (Austin et al. 2006). Seventy five % of all dives of the studied

animals were shorter than 2.5 min what is much shorter than estimated 7-8 min ADL for this

species (Lydersen et al. 1992). Thus, if these animals are foraging optimally, they should spend

more time at the bottom phase of the dive, what can explain the relatively high proportion of U-

shaped dives recorded for all individuals in the present study. Hanggi & Schusterman (1994) also

reported a vocal reproductive behaviour while performing U-shaped dives. 3D measurements of

dive shapes revealed additional activities, not previously described in the case of 2D data, such as

convoluted movements during ascent and descent (Simpkins et al. 2001a; Simpkins et al. 2001b;

Simpkins et al. 2001c). Such 3D analysis of ringed seal dives suggests that they spent most of their

time either searching for prey patches, or are engaged in non-foraging behaviour. Therefore, dives

could not be accurately grouped into simple behavioural categories based on their time-depth

profiles (Simpkins et al. 2001a). In the present study, shallow U and V- shaped dives were

classified as travelling dives as had been commonly described (Boyd 1996; Lesage et al. 1999).

Visual observations conducted during the TDR period confirmed the described function of shallow

dives; however, other activities, like foraging, cannot be rejected. The relatively high proportion of

shallow V-shaped dives in the case of M3 could result from an artefact arising from longer

sampling intervals (5 s) than in the case of the other studied animals (Wilson et al. 1995).

Mori et al. (2005) hypothesized that dive shapes should reflect not only the depth of a prey patch,

but also its richness. They further argued that there was a positive relationship between bottom

time and prey richness, as well as between dive duration and post-dive surface time. The latter

relationship was tested for the animals studied here; however, no statistical significance was found

(p=0.32; F=0.98). The relationship between dive duration and prey patch quality may have been

undetectable due to the majority of shallow dive, and therefore a seal may stay longer at the

bottom, still within its aerobic metabolism, irrespectively of the prey quality encountered.

Diving bouts and foraging trips.

Foraging behaviour of the studied harbour seals were organized on two temporal scales; (i)

foraging trips at sea lasting from 3 h up to 4 days, between visits ashore to haul-out, (ii) distinct

diving bouts lasting from 7 min up to 3 days within the foraging trips. Harbour seals usually

surface only to replenish their oxygen supply, therefore many other activities take place while

submerged and the time they spend diving is unlikely to clearly equate with foraging (Boyd et al.

1994; Suryan & Harvey 1998; Thompson et al. 1998). Due to short inter bout intervals (the mean

for the three individuals equals 23 min), resting, digestion and assimilation were probably

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combined with diving activity and not put off until breaks in activity like in the case of elephant

seals (Mcconnell et al. 1992). Using dive recorders and stomach temperature sensors deployed on

grey seals, Austin et al. (2006) showed that in only 58 % of foraging trips feeding occurred. The

maximum duration of diving bouts found in the present study was longer than reported in most

studies. The maximum duration of diving bout described for a mother and a pup of the harbour

seal was 9.2 h (Bowen et al. 1999). Metabolism used during the dive and quality of the patch that

the diver uses affect the optimal time budget of a diving bout and the optimal number of dives

during the diving bouts (Mori 1998). The model predicts that when the diving bout is composed of

many dives, the dive duration and surface interval of the diving bout cycle are short, whereas

longer dive durations with longer surface intervals results in low number of dives during the bout

(Mori 1998). Since the studied seals were diving to a shallow depth and usually within ADL,

prolonged diving bouts may not lead to a significant increase in their energy budget. Boyd et al.

(1994) suggested that diving animals feeding on patchily distributed prey, such as the animals

studied here, repeat dives with relatively short surface intervals once foraging begins. In other

studies, two types of diving bouts have been described: bouts of directional, travelling dives and

bouts with the majority of foraging dives (Coltman et al. 1997; Simpkins et al. 2001a). In the

present study, most bouts were classified as foraging bouts, suggesting that the suitable foraging

areas for harbour seals were both found close to the island and further inshore. The dominance of

classes a and b in the case of all individuals, confirms the widely described benthic feeding

behaviour of harbour seals (Eguchi & Harvey 2005; Härkönen 1987a; Härkönen 1987b; Lesage et

al. 1999; Tollit et al. 1998). There was no significant relationship for any individual between the

length of a feeding trip and the length of a post–feeding trip haul-out duration indicating that there

must have been some additional reasons for the seals to haul-out beside fatigue or there is a

minimum time required by a harbour seal to recover, independent of the duration of the proceeding

foraging trip. Human disturbance at the haul-out site may have had an influence on such

relationship.

Foraging areas

A separate foraging area was found for each individual. This individual and opportunistic foraging

strategy has been described as a general feature for harbour seals (e.g., Boyd 1996; Call et al.

2008; Ramasco 2008; Staniland et al. 2004; Suryan & Harvey 1998). Harbour seals are known to

show high site fidelity and feed close to their haul-out sites usually within 50 km (Frost et al.

2001; Härkönen 1987a; Härkönen & Hårding 2001; Thompson et al. 1998; Tollit et al. 1998),

which fairly well corresponds with the data collected in this study. Although the areas of highest

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residency, as estimated with kernel home range analysis, do not necessarily correspond to feeding

areas (Robinson et al. 2007), it was the only way to test whether movement of the studied seals

had changed from the TDR to the PTT periods. Therefore, the potential foraging area of each

individual may change if based on the longer studies.

The estimated foraging areas for the studied animals differed in terms of bathymetry, topography

and sediment types. The variation among individuals can be attributed to differences in foraging

strategies, as also described for Antarctic fur seals (Arctocephalus gazella) (Boyd et al. 1994) and

northern elephant seals (Mirounga angustirostris) (Robson et al. 2004). Differences in prey

capture and harvesting may be due to individual tactic or differences in age, sex, size, intra- and

inter-specific competition, experience or habitat selection (Staniland et al. 2004). Harbour seals are

known to be opportunistic predators and their food preferences are largely dependent on the

habitat they live in and the relative abundance of their prey (Brown & Pierce 1998; Eguchi &

Harvey 2005; Härkönen 1987b; Härkönen & Heide-Jørgensen 1991; Tollit et al. 1998). Since the

prey of harbour seals is patchily distributed, both horizontally and vertically (Simpkins et al.

2001b), the differences in seal foraging tactics and diet, may also be due to exploitation of

different patches by different individuals. The differences observed in the travelled distances, trip

duration, and habitat use are likely related to the rate at which seals encounter their prey, as well as

the transit time between the preferred patches (Call et al. 2008). M3 showed more directional

movement than the other seals. This may suggest the existence of individually preferred profitable

foraging grounds that were repeatedly used. Seals that learn to exploit productive foraging habitats

are likely to increase their foraging efficiency by returning repeatedly to the same site (Call et al.

2008). Foraging routes of individuals are likely influenced by a combination of local habitat

structure (i.e. bathymetry or oceanographic fronts) and previous foraging experience (Robson et al.

2004).

Cod, dab, plaice, flounder and sandeels are the most common prey species for Anholt colony

between May and September (Härkönen 1988). The repeatedly use of the shallow sandy area north

east from Anholt by M3 could have been due to a source of sandeels (pers. comm. Peter Grønkjær,

Marine Biology, Aarhus University). The foraging area of M3 was situated in the deeper area of

the Kattegat, which acts as a ‘corridor’ between Skagerrak and the Baltic (Figure 1). The more

saline water from the Skagerrak enters the Kattegat via a deep current (ICES 2008). This may

allow some stenohaline prey like blue whiting (Micromesistius poutassous) or cephalopods

(Härkönen 1987a; Härkönen 1988) to occur in this area and enrich the diet of M3. By foraging in

trenches and along the slopes, seals could have increased the variety of prey species encountered.

In this study, competition is suggested to be the most probable reason of the partitioning of the

Kattegat into distinct foraging areas. Around 5000 harbour seals inhabit the Kattegat and up to

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1000 use to haul-out on Anholt (NERI, unpublished data). Since the Kattegat is a restricted area,

and the studied seals used Anholt as the only haul-out site and there is uneven access to deeper

waters within the area, the competition may be a significant factor. When large numbers of

animals overlap in range, prey patches are likely to be subject to high levels of use, and therefore

potential sites of competition (Hindell et al. 2002). There is therefore a risk of prey limitation if

seals were feeding in groups or within the same area. Additionally the Kattegat is a foraging

ground for approximately 7000 black cormorants (Phalacrocorax carbo) (as estimated for 2005

and including the population from east Limpfjorden) (Eskildsen 2005), 10 – 20 grey seals

(Halichoerus grypus), approximately 17000 harbour porpoises (Phocoena phocoena) (estimation

based on SCAN II 2005 survey for the Kattegat – Skagerrak and southern Baltic area, S.

Sveegaard, NERI, unpublished data) and is an intensive area of fishery (Härkönen 1987b; Nilsson

& Ziegler 2007). Harbour porpoises have been known to consume cod, herring and sprat – prey

species also recorded in the diet of harbour seals (Börjesson et al. 2003; Härkönen & Heide-

Jørgensen 1991). High density areas for harbour porpoises were located south east of Anholt

(Teilmann et al. 2008). Black cormorants may compete with harbour seals for flatfish and cod

(Andersen et al. 2007). Fishery has been suggested to have a minimal overlap with the diet of

harbour seals. Many fish consumed by harbour seals are either not targeted by fishery or are under

the legal minimum landing sizes (Andersen et al. 2007; Brown & Pierce 1998; Olsen et al. 2009).

Although each competitor may have had a different influence on the competition and on the

resources, partitioning of the foraging area may be a solution to mitigate these relationships

(Dolman & Sutherland 1997). It should also be noted that harbour seals from Anholt fed in 4D

environment (including time as one dimension). A broad overlap between the foraging areas used

by harbour seals from the same site has been reported in the other studies (Ramasco 2008; Tollit et

al. 1998). Although harbour seals in general are considered opportunistic feeders (Brown & Pierce

1998; Eguchi & Harvey 2005; Härkönen 1987b; Härkönen & Heide-Jørgensen 1991), some

harbour seals are believed to specialise on specific prey or foraging techniques (Tollit et al. 1998).

Such prey specialization may be the result of high competition.

F1 had a relatively small estimated foraging area and kernel home range and showed decrease in

kernel home range size between the PTT and TDR periods, spent the largest (23 %) amount of

time on haul-out and performed low number of dives during the six last days of deployment, This

may be an indication of moulting or individual behaviour of this seal since blood analysis ruled out

pregnancy for this individual (progesterone level 2.01 ng/ml, Research and Technology Center

Westcoast, Büsum, Germany). The moulting season usually peaks in August on Anholt (Härkönen

et al. 1999).

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Conclusions

This study combines vertical and horizontal movement of the harbour seals tagged on Anholt. The

recorded data were discussed in the light of diving behaviour of the tagged seals in relation to

environmental factors and diurnal rhythm. Diurnal dive pattern was found for all dive parameters for

each individual. Differences between nocturnal and diurnal patterns of dive have been interpreted

as a change in foraging strategy in response to prey availability. The haul-out time and duration

was strongly influenced by human disturbance at the haul-out site where almost all daily

disturbances caused the seals to flee into the water and not returning to land until the evening.

The study is also the evidence for the individual behaviour and preferences of the tagged seals.

Foraging activity was estimated based on dive shapes and dive bouts which are believed to be

related to certain behaviour of the seal. However, once has to be careful to assign a function to any

particular dive without independently collected data. Diving bouts of the tagged seals were mostly

classified as foraging, however, due to short inter bouts intervals, resting, digesting and

assimilation were probably combined with the foraging activity.

This study confirms the widely described benthic and relatively shallow feeding behaviour of

harbour seals; however, numerous pelagic dives were also observed.

The results confirmed the stated hypothesis that the harbour seals from Anholt fed within the

whole Kattegat but had individually preferred feeding areas. Differences in the size and location of

the foraging areas are probably due to habitat differentiation, prey distribution and preferences and

inter- and intraspecific competition.

This study gives the first insights into the feeding behaviour and important foraging places for

harbour seals in the Kattegat. While the presented data are from a limited period and from only

four seals, the overall diving and foraging behaviour is most likely not uncommon in this habitat.

However, given the small temporal and spatial resolution of the current dataset, the collection of

additional data over longer periods and covering a larger portion of the population is needed in

order to give general conclusions on diving and foraging behaviour of the studied population. The

information may be valuable during planning of the worlds largest offshore wind farm in the

Kattegat. The wind farm is going to be built 20 km west off Anholt, at the area of the feeding

ground of the studied seals. Although there is little reason to believe that a wind farm in normal

operation has a significant impact on harbour seals in general, there is a probability that the

construction phase togather with human disturbance at the haul-out site may have a negative

influance on the haul-out and foraging behaviour of the studied seals.

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Appendix A

All dive data of M1 displayed in Instrument Helper 1.0.53, Wildlife Computers. The upper light grey horizontal bar

shows 1 m zero offset correction. X-axis – date, y-axis – depth [m]

An example of 1 h dive records of M1 displayed in Instrument Helper 1.0.53, Wildlife Computers

62


Background

Appendix B

The parameters of the foraging trips for individual M1

Foraging trip

Duration [h]

# of

dives

# of

diving

bouts

% of time

spend underwater

% of foraging dives

(deep-U + w-shaped

+ two-phased)

% of travelling

dives (shallow V +

shallow U-shaped)

1

2

3

4

5

6

7

8

9

10

11

42.5 800 4 56.9 80.9 17.6

49.3 536 3 88.4 85.5 12.5

15.2 120 1 31.1 15.0 85.0

15.1 55 1 45.8 32.7 67.3

43.2 659 1 77.4 86.3 12.6

8.3 40 1 56.5 70.0 30.0

74.5 1332 2 76.0 88.4 9.6

4.2 41 1 87.8 92.7 4.9

81.2 1279 6 72.4 86.6 12.4

38.5 209 2 89.8 83.3 16.3

27.3 428 1 81.4 92.1 7.5

The parameters of the foraging trips for individual F1

Foraging

trip

Duration [h]

# of

dives

# of

diving

bouts

% of time

spend underwater

% of foraging dives

(deep-U + w-shaped

+ two-phased)

% of travelling

dives (shallow V +

shallow U-shaped)

1 16.2 120 2 47.0 0.0 94.2

2 46.9 802 6 45.0 39.8 57.6

3 2.9 51 1 26.0 0.0 100.0

4 7.6 138 1 16.0 73.2 26.1

5 19.8 286 3 48.0 50.0 47.6

6 15.8 304 2 22.0 37.5 57.6

7 16.7 231 2 43.0 69.3 24.7

8 81.3 1549 5 33.0 54.6 40.0

9 17.8 179 2 56.0 43.0 54.8

10 88.1 1974 6 35.0 61.4 30.2

11 12.4 45 3 18.0 24.4 75.6

12 14.4 91 3 16.0 7.7 90.1

13 10.8 62 1 8.0 75.8 22.6

14 15.7 168 1 27.0 15.5 84.5

15 18.5 115 1 67.0 11.3 87.0

16 14.7 93 1 38.0 53.8 45.2


Manuscript

The parameters of the foraging trips for individual M3

# of

% of time

% of foraging

% of travelling dives

Foraging trip Duration [h] # of dives

diving

spend under-

dives (deep-U

(shallow V + shallow

bouts

water

+ w-shaped)

U-shaped)

1 41.08 542 2 75.23 74.35 22.69

2 55.22 854 2 65.12 64.75 34.66

3 80.23 1486 1 50.12 35.13 64.60

4 69.35 1229 1 36.20 19.61 80.23

5 2.80 33 1 32.89 66.67 33.33

6 5.93 31 0-1 31.86 3.23 96.77

7 9.98 38 0-1 69.26 2.63 97.37

8 19.37 247 1 60.51 31.98 68.02

Appendix C

Kernel home range calculated based on the PTT period

64


Manuscript

Table I The area [km 2 ] of each kernel home range.

TDR period

PTT period

Kernel home range

Kernel home range

probability [%]

probability [%]

Length of the

period [d]

95 75 50 25

Length of the

period [d]

95 75 50 25

M1 20 1794 709 285 84 74 2595 873 332 95

F1 21 605 280 126 50 57 558 205 97 40

M2 19 669 249 111 45 46 871 285 121 49

M3 15 754 351 181 76 40 1067 398 198 83

Table II Differences between the kernel home range of TDR and PTT period [%].

Differences between the kernel home

range of TDR and PTT period [%]

95 75 50 25

M1 30.9 18.8 14.2 11.6

F1 -8.4 -36.6 -29.9 -25.0

M2 23.2 12.6 8.3 8.2

M3 29.3 11.8 8.6 8.4

65


Implications for the conservation and future directions

Implications for the conservation

In order to properly manage and protect the seals, information about population size, haul-out

sites, individual movements and foraging behaviour is necessary. This study gives an insight in the

latter issue and along with the simultaneous and long term studies of harbour seals from Anholt

can give an overview picture of the studied colony.

Although competition between harbour seals and fisherman is believed to be of minor importance

in some areas (Andersen et al. 2007), changes in the distribution of prey due to fisheries

management actions (or overfishing) or environmental changes may alter the foraging patterns of

seals (Robson et al. 2004). The previous outbreaks of PDV in 1988 and 2002 both started on

Anholt (Härkönen et al. 2006). Since seals are infected with this virus by the contact with other

individuals, also between haul-out sites, understanding of interactions between individuals is of

particular importance in understanding the mechanism of spreading of this disease.

There is an increase in worldwide demand for renewable energy and therefore an increasing

interest in construction of offshore wind farms. One of such wind farm is proposed to be built west

from Anholt. There are several studies evaluating potential impact of wind farms on harbour seals

(Edren et al. 2004; Koschinski et al. 2003; Madsen et al. 2006). According to Edren et al. (2004),

activities during construction phase - ramming/vibration in particular, caused a decrease in number

of seals on land in Rødsand seal sanctuary 4 km from the wind farm by 10-60 % in comparison to

the time before constructions. Although this was probably a short-term effect, construction during

moulting or pupping season may significantly affect the haul-out behaviour of the studied seals. If

hauling – out is both affected by construction and direct human disturbance at the site; it may have

severe impact on the studied seals. However, on Anholt the wind farm will be around 20 km away

and therefore it is unlikely to have an effect on the haul-out behaviour. Offshore wind farms are

usually built in shallow waters (< 20 m) (Madsen et al. 2006). As shown by this study, shallow

areas are of particular importance for seals hauling- out on Anholt. Noise both from the

construction activities as well as noise produced by the turbines in operation is considered the most

disturbing factor of the wind farms (Edren et al. 2004; Koschinski et al. 2003; Madsen et al.

2006). Such noise could potentially affect fish behaviour at ranges of several kilometres

(Wahlberg & Westerberg 2005). Prey distribution is a factor strongly influencing foraging

behaviour of the seals. Although there is little reason to believe that a wind farm in normal

operation has a significant impact on harbour seals in general (Madsen et al. 2006), harbour seals

can hear sound in the frequency range typical for this operation (Kastak & Schusterman 1998).

Study by Koschinski at al (2003) showed that seals do avoid the sound by leaving the area, around

66


Implications for the conservation and future directions

350 m from the turbines. If a wind farm construction and\or operation does have any impact on the

seals (both foraging and haul-out behaviour) or their prey, it may influence the size and the quality

of the potential foraging areas of the individuals from the Anholt colony.

Future directions

The present study should lead to significant improvement in understanding of the behaviour of

harbour seals from Anholt; however, appropriate management will still require bringing of

knowledge gaps with further data. In this respect, future directions could involve: i) increasing

number of tagged animals; ii) studies on seasonal and inter annual variations in diving and

foraging behaviour; iii) studies on harbour seals’ diet, including variations between individuals; iv)

mapping of prey distribution and their diurnal patterns, like acoustic surveys (Ramasco 2008) or

direct samplings (Adlerstein & Ehrich 2003; Engas & Soldal 1992); v) identification of feeding

events like simultaneous use of TDRs, accelerometers and\or cameras (pers. comm. Yasuhiko

Naito, National Institute of Polar Research, Tokyo, Japan) (Bowen et al. 2002); vi) improving the

resolution of animals’ tracking by the use of fast-lock GPS (e.g., Ramasco 2008; Trathan et al.

2008); vii) developing a better method for describing foraging areas and combining dive and

movement data sets, by the use of purpose - built models or software, like MAMVIS (Fedak et al.

1996); viii) understanding diving behaviour and movements of harbour seals from the other areas

of the Kattegat, like Hesselø and Læsø.

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68

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