Fisheries Research Diel variation in gillnet catches and vertical ...

Fisheries Research 96 (2009) 64–69
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Fisheries Research
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Diel variation in gillnet catches and vertical distribution of pelagic fishes in a
stratified European reservoir
M. Vašek a,∗ ,J.Kubečka a ,M.Čech a ,V.Draštík a ,J.Matěna a ,T.Mrkvička b ,J.Peterka a , M. Prchalová a
a Biology Centre of the Academy of Sciences of the Czech Republic, Institute of Hydrobiology, Na Sádkách 7, 370 05 České Budějovice, Czech Republic
b University of South Bohemia, Faculty of Science, Branišovská 31, 370 05 České Budějovice, Czech Republic
article
Keywords:
Gillnet
Diel catchability
Distribution
Epilimnetic fishes
CPUE
Man-made lake
info
abstract
The diel changes in gillnet catch rates and vertical distributions of pelagic fishes were determined in a deep,
moderately eutrophic European reservoir. In late summer, when steep vertical gradients in temperature
and dissolved oxygen developed in the reservoir, monofilament gillnets (12.5–70 mm bar mesh) covering
the upper 7.5 m of the water column were set at two open-water locations over 44- and 46-h periods,
respectively. Gillnets divided the sampled strata (i.e., the epilimnion and metalimnion) into five horizontal
layers each 1.5 m thick. Captured fish were removed from the nets at regular 2-h intervals. The catches
were numerically dominated by bleak Alburnus alburnus, roach Rutilus rutilus, perch Perca fluviatilis, bream
Abramis brama and asp Aspius aspius. Catch rates varied considerably with the time of day. The total gillnet
catch per 2-h was on average 3–4 times higher at twilight than during the day or at night. Decreased net
visibility and increased fish activity were likely responsible for the high gillnet catches at dusk and dawn
periods. The catches of individual species were not distributed uniformly across the five sampled depth
layers. The majority of bleak and asp were captured in the surface layer (0–1.5 m), irrespective of day,
twilight or night. During the daytime, roach were most abundant in the layer just above the thermocline
(3–4.5 m), but at night they occupied the upper part of the epilimnion (0–3 m). Perch inhabited the lower
epilimnion and the upper metalimnion (1.5–6 m), and were caught only during daytime and twilight. For
bleak and roach, greater proportions of larger individuals were caught during the day, while proportions
of smaller fish increased in twilight and night catches. This implies that individuals of the two species
made diel migrations between pelagic and littoral habitats.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
During the last 10 years, gillnets have become extensively
used for fish monitoring in European inland waters (Appelberg
et al., 1995; Appelberg, 2000; CEN, 2005). Many research surveys
have employed standardised multi-mesh gillnets to assess
fish assemblages in a large number of lakes differing in morphology,
productivity and other environmental parameters (Holmgren
and Appelberg, 2000; Jeppesen et al., 2000; Olin et al., 2002;
Mehner et al., 2005; Diekmann et al., 2005). Gillnets have been
commonly used for fish stock assessments because they represent
cost-effective and easily operated fishing gear when compared with
other options such as trawls, purse seines and beach seines. However,
gillnets have the limitation of being a passive sampling gear.
This implies that the catchability of gillnets is directly related to the
movement activity of fishes.
∗ Corresponding author. Tel.: +420 387775831; fax: +420 385310248.
E-mail address: mojmir.vasek@seznam.cz (M. Vašek).
Fish activity and spatial distribution change greatly during a
diel cycle (Bohl, 1980; Helfman, 1981). Hence, diel changes in fish
activity and habitat use may cause considerable variation in gillnet
catches. In fish monitoring programs, gillnets are usually set before
dusk and lifted the next day after dawn. It is assumed that this setting
time covers maximum activity periods for all catchable species.
In European lakes, however, few attempts have been made to examine
the effect of different times of day on gillnet catch rates (Olin
and Malinen, 2003). That is why more information on diel catchability
patterns of different species in different habitats is useful to
both fisheries managers and scientists, helping them to ascertain
when and where the sampling of fish population parameters by
gillnets is most appropriate.
In the present study, we aimed to explore whether the gillnet
catch rates of epipelagic fishes in the deep, moderately eutrophic
Římov Reservoir (Czech Republic) change markedly during a diel
cycle. Concurrently, we also examined small-scale vertical distributions
of fishes within epilimnetic/metalimnetic waters of the
reservoir on a diel basis. The study was conducted in late summer
when the Římov Reservoir was sharply thermally stratified
0165-7836/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.fishres.2008.09.010

M. Vašek et al. / Fisheries Research 96 (2009) 64–69 65
and the majority of pelagic fishes were concentrated in the upper
few meters of the water column (Kubečka and Wittingerová, 1998;
Vašek et al., 2004). We hypothesized that catch rates and vertical
distributions may change with time of day. We also expected that
these parameters might differ among species.
2. Material and methods
The research was conducted in the Římov Reservoir, a deep
steep-sided and moderately eutrophic water body situated in the
valley of the Malše River in South Bohemia. The reservoir was
filled in 1978 to be used primarily as a source of drinking water.
It has a maximum surface elevation of 471 m above sea level, an
area of 210 ha and a volume of 33 × 10 6 m 3 . The mean depth is
16 m, the maximum depth is 45 m and the mean theoretical retention
time is approximately 100 days. The reservoir is dimictic with
sharp thermal stratification developing through summer. There is
no commercial fishery in the reservoir and sport fishing is prohibited.
Sampling was carried out on 14–20 August 1999 at two open
water localities, situated in the dam (locality 1: 48 ◦ 50 ′ 53 ′′ N,
14 ◦ 29 ′ 12 ′′ E) and middle (locality 2: 48 ◦ 49’56 ′′ N, 14 ◦ 28 ′ 32 ′′ E) parts
of the reservoir (for map see Vašek et al., 2004). Sampling in the
late summer was chosen because fish distribution and habitat utilization
were not affected by spawning, and feeding activity was
expected to be high. During the study, sunrise was at 05:55 and
sunset at 20:15 h. The weather was calm and mostly sunny, but
sometimes also cloudy with occasional light rain. Water temperature
in the epilimnion attained 19–21 ◦ C and the thermocline was
located around 5 m depth. The concentration of dissolved oxygen
decreased sharply under the thermocline (0–6.3 mg O 2 l −1 in
hypolimnion, 7.6–12.6 mg O 2 l −1 in epilimnion).
At both localities, two gillnet series of slightly different construction
were used to sample fish within epilimnetic and metalimnetic
waters. Each gillnet series was in total 225 m long and contained
9 nets of different mesh sizes (12.5, 16, 19.5, 24, 29, 35, 43, 55
and 70 mm knot-to-knot; each net was 25 m long and made of
polyamide monofilament). One series had nets 3 m high and was
set with the headlines at the surface. The second series had nets
4.5 m high and was set with the headlines at 3 m below the water
surface. The gillnet series thus altogether sampled the upper 7.5 m
of the water column.
The nets of both series were divided with a marker strand into
1.5 m vertical sections in order to classify the vertical position of
captured fish (i.e., catches were affiliated to 0–1.5, 1.5–3, 3–4.5,
4.5–6 and 6–7.5 m depths, respectively). The gillnet series were set
in a straight line approximately parallel to the shore over maximum
depths (maximum depths attained were 30–40 m at locality
1 and 20–25 m at locality 2). The two gillnet series fished simultaneously
for a total of 46 h at locality 1 (starting on 14 August 10:30
a.m. and ending on 16 August 08:30 a.m.) and for a total of 44 h
at locality 2 (starting on 18 August 12:30 p.m. and ending on 20
August 08:30 a.m.). Catches were removed from the nets at regular
2-h intervals using a boat rowing along the gillnet series. Captured
fish were classified as to depth in the water column and measured
to the nearest 0.5 cm standard length. In a small number of cases
(

66 M. Vašek et al. / Fisheries Research 96 (2009) 64–69
Table 1
Mean catch per unit effort (CPUE; number per 2 h per 1687.5 m 2 of gillnets) and percentage catch composition for dominant species in the water stratum 0–7.5 m at two
reservoir localities during day, twilight and night
Species Mean CPUE % catch composition
Day Twilight Night Day Twilight Night
Locality 1
Bleak 5.5 24.3 2.3 26.8 36.1 13.9
Roach 7.6 26.5 13.0 36.8 39.4 77.2
Perch 6.2 9.5 – 30.1 14.1 –
Bream 1.2 6.3 1.3 5.9 9.3 7.9
Asp 0.1 – 0.2 0.4 – 1.0
All species 20.7 67.3 16.8 100 100 100
N 13 4 6
Locality 2
Bleak 9.8 54.5 15.2 41.3 58.3 50.8
Roach 4.8 26.5 9.5 20.5 28.3 31.8
Perch 5.8 1.3 – 24.7 1.3 –
Bream 1.8 3.0 2.5 7.4 3.2 8.4
Asp 0.8 3.8 1.8 3.2 4.0 6.1
All species 23.6 93.5 29.8 100 100 100
N 12 4 6
Number of 2-h intervals (N) from which the mean CPUE was calculated is given.
gillnet survey revealed pronounced differences in the vertical distributions
of abundant species (Figs. 2 and 3). The majority of bleak
at both localities were found in the 0–1.5 m depth layer, irrespective
of whether the sampling was during daytime, twilight or night.
Roach in daytime were most frequently caught in the deep epilimnion
(3–4.5 m depth), while at twilight and at night they were
netted mainly in the upper epilimnetic layers (0–3 m depth). The
difference between the daytime and night vertical distributions of
roach catches was statistically significant (pooled data from the two
localities: 2 = 130.2, d.f. = 4, P < 0.0001).
Vertical distribution patterns of bream during daytime seemed
to be less consistent between the localities, as bream at locality
1(Fig. 2) were captured in the lower portion of the epilimnion
and below the thermocline (3–7.5 m depth) while bream at locality2(Fig.
3) were netted in the upper part of the water column
(0–3 m depth). At night, however, most bream were found in the
surface water layer at both localities. Specimens of asp were caught
predominantly in the uppermost water layer of 0–1.5 m depth.
By contrast, catches of perch were concentrated in the deeper
epilimnion and upper metalimnion (1.5–6 m) during daytime and
twilight. No perch were caught at night at either locality (Table 1),
and perch catches were also totally absent in the surface layer
(0–1.5 m depth) throughout the entire period of gillnet samplings.
The size structure of the two most abundant species, roach
and bleak, varied with different periods of the diel cycle. Greater
proportions of larger individuals were captured during day, while
proportions of smaller fish increased in gillnet catches at twilight
and at night (Fig. 4). For both species, the length-frequency distributions
differed significantly between the daytime and night (pooled
data from the two localities for bleak: 2 = 47.2, d.f. = 2, P < 0.0001;
and roach: 2 = 35.1, d.f. = 2, P < 0.0001).
4. Discussion
Total CPUE of pelagic gillnets varied markedly with time of day.
In general, total CPUE was much higher at twilight than during day
and night. Gillnet catches were numerically dominated by a few
cyprinid species (bleak, roach, bream and asp) and perch. Perch
were caught only during daytime and twilight. Cyprinids were
caught during daytime, twilight and night, but their catch rates
were highest at twilight. The dusk and dawn peak catches presumably
reflected both an increase in the movement activity of fishes
and a decrease in gillnet visibility during the periods of twilight.
The general pattern of elevated activity at dusk and dawn with
lowest activity levels at night has been documented by telemetric
studies of roach (Jacobsen et al., 2004), bream (Schulz and
Berg, 1987) and perch (Jacobsen et al., 2002; Zamora and Moreno-
Amich, 2002), and it corresponded well with our catch data. Rask
(1986) used traps in his 24-h sampling programmes and found
increased catches of perch at twilight. In two shallow lakes in Finland
(dominated by roach, bleak, white bream Abramis bjoerkna
and perch), catches in benthic gillnets were also usually highest in
evening and morning periods and lowest during midday or after
midnight (Olin et al., 2004; Olin and Malinen, 2003). Nevertheless,
the telemetry-based studies cited above have further revealed that
the investigated species, in some conditions, showed relatively high
activity levels around midday.
Table 2
Results of Chi-square tests on differences in catch frequencies of particular species in five horizontal layers (each 1.5 m thick) within the upper 7.5 m of the water column,
given separately for different diel periods
Species Day Twilight Night
d.f. 2 P d.f. 2 P d.f. 2 P
Bleak 4 275.7

M. Vašek et al. / Fisheries Research 96 (2009) 64–69 67
Fig. 2. Day, twilight and night vertical distributions of gillnet catches for the five most abundant fish species at locality 1. Mean vertical distribution, calculated from vertical
distributions in 2-h intervals belonging to the specific diel period, is presented. Variance bars indicate 95% confidence limits. Total number of captured fish (N) per species
per diel period is given.
Based solely on our gillnet data, we cannot exclude the possibility
that the fishes in the pelagic habitat of the Římov Reservoir
were active during daytime and that the catches were relatively
low because the fishes could visually detect and avoid the nets
under sufficient light penetration conditions. Such visual avoidance
of gillnets has been demonstrated for active rainbow trout
(Oncorhyncus mykiss) in daytime water-tank experiments (Fujimori
et al., 1994). On the other hand, the low gillnet catches obtained at
night seemed to reflect well the probable decreased fish activity at
that time, since visual avoidance in darkness would be unlikely.
Comparisons of the length-frequency distributions of fish
caught in the pelagic zone on a diel basis showed that the high
catches at twilight were apparently connected with fish migrations
from littoral to pelagic habitats and vice versa. That is,
the greater proportions of smaller roach and bleak in the twilight/night
catches compared to the daytime catches indicated
that during the dusk, juvenile roach and bleak moved from the
littoral habitat to open water, stayed there through the night
and returned to the littoral zone at dawn. Such offshore migrations
of juvenile cyprinids to open water during dusk have been

68 M. Vašek et al. / Fisheries Research 96 (2009) 64–69
Fig. 3. Day, twilight and night vertical distributions of gillnet catches for the five most abundant fish species at locality 2. Mean vertical distribution, calculated from vertical
distributions in 2-h intervals belonging to the specific diel period, is presented. Variance bars indicate 95% confidence limits. Total number of captured fish (N) per species
per diel period is given.
demonstrated by Bohl (1980) and Gliwicz et al. (2006) and are
explained by the behaviour of foraging on abundant zooplankton
prey and the simultaneous avoidance of predation threat from
visual piscivores. In contrast, part of the adult fish population occupying
the pelagic habitat of the Římov Reservoir through daytime
might shift to the littoral area at night, as indicated by the results
from shore seining (Kubečka, 1993) that yielded much higher
catches at night than during the day. Also, the fact that no perch
were caught in pelagic gillnets at night, suggests they might have
moved to the littoral zone. The importance of the littoral zone as
a nighttime resting habitat for perch has been demonstrated in
studies by Imbrock et al. (1996) and Zamora and Moreno-Amich
(2002).
We found pronounced differences in the vertical distributions
of species within epilimnetic/metalimnetic waters of the Římov
Reservoir. Bleak and asp were caught predominantly in the surface
stratum during the whole diel cycle. Roach in daytime were
concentrated just above the thermocline but, at night, the majority
of roach occupied the upper part of the epilimnion. Perch inhabited
the lower epilimnion and upper metalimnion.

M. Vašek et al. / Fisheries Research 96 (2009) 64–69 69
Acknowledgements
This study was financially supported by the Academy of Sciences
of the Czech Republic (projects 1QS600170504, AVOZ60170517
and IAA600170502) and the Czech Science Foundation (project
206/06/P418).
References
Fig. 4. Diel patterns in length-frequency distributions of bleak and roach caught in
the pelagic zone. Data from the two reservoir localities were pooled. Note different
scale of x-axis for the two species.
Vertical spatial segregation of perch and roach has been reported
from several lakes and reservoirs, where roach usually preferred the
upper water layers while perch were found frequently in deeper
layers (Persson, 1986; Horppila et al., 2000; Kahl and Radke, 2006).
Such separation was explained by avoidance of inter-species competition
and by temperature-dependent species-specific foraging
efficiency and growth (Persson, 1986; Kahl and Radke, 2006). In
the Římov Reservoir, however, the option of vertical segregation
for the two species seems to be rather limited in the late summer
period, as the hypoxic conditions under the thermocline were
certainly unfavourable for fish.
Bleak and asp are generally considered to be surface-oriented
species. Nevertheless, there is no quantitative information on depth
preferences of the two species within pelagic habitats of lakes and
reservoirs in the scientific literature. Our results clearly showed that
bleak and asp preferred the surface stratum in the Římov Reservoir,
since despite the epilimnion extending vertically through 4–5 m,
the two species were predominantly caught in the uppermost
0–1.5 m of the water column.
In general, gillnet catch rates in the pelagic habitat of the sharply
stratified Římov Reservoir were highest at twilight. In fish monitoring
programmes, the standard approach is to set gillnets before
dusk and lift them the next day after dawn (CEN, 2005). Our detailed
data on the catching ability of gillnets throughout the whole diel
cycle indicated that the standard net-setting time is suitable for
fish stock monitoring. It covers periods of twilight when most fish
species are usually active. Additionally, in relatively clear waters,
fish are probably more efficiently caught at this time due to the
reduced possibility of visual avoidance of the gear.
Appelberg, M., 2000. Swedish standard methods for sampling freshwater fish with
multi-mesh gillnets. Fiskeriverket Informerar 1, 3–32.
Appelberg, M., Berger, H.M., Hesthagen, T., Kleiven, E., Kurkilahti, M., Raitaniemi, J.,
Rask, M., 1995. Development and intercalibration of methods in Nordic freshwater
fish monitoring. Water Air Soil Pollut. 85, 401–406.
Bohl, E., 1980. Diel pattern of pelagic distribution and feeding in planktivorous fish.
Oecologia 44, 368–375.
CEN, 2005. Water quality—Sampling of fish with multi-mesh gillnets. European
Committee for Standardization, EN 14757 Brussels.
Diekmann, M., Brämick, U., Lemcke, R., Mehner, T., 2005. Habitat-specific fishing
revealed distinct indicator species in German lowland lake fish communities. J.
Appl. Ecol. 42, 901–909.
Fujimori, Y., Tokai, T., Matuda, K., 1994. Effect of diurnal activity of rainbow trout
and light intensity on gillnet catching in water tank experiments. Nippon Suisan
Gakkaishi 60, 577–583.
Gliwicz, Z.M., Slon, J., Szynkarczyk, I., 2006. Trading safety for food: evidence from
gut contents in roach and bleak captured at different distances offshore from
their daytime littoral refuge. Freshwater Biol. 51, 823–839.
Helfman, G.S., 1981. Twilight activities and temporal structure in a freshwater fish
community. Can. J. Fish. Aquat. Sci. 38, 1405–1420.
Holmgren, K., Appelberg, M., 2000. Size structure of benthic freshwater fish
communities in relation to environmental gradients. J. Fish Biol. 57, 1312–
1330.
Horppila, J., Ruuhijärvi, J., Rask, M., Karppinen, C., Nyberg, K., Olin, M., 2000. Seasonal
changes in the diets and relative abundances of perch and roach in the littoral
and pelagic zones of a large lake. J. Fish Biol. 56, 51–72.
Imbrock, F., Appenzeller, A., Eckmann, R., 1996. Diel and seasonal distribution of
perch in Lake Constance: a hydroacoustic study and in situ observations. J. Fish
Biol. 49, 1–13.
Jacobsen, L., Berg, S., Broberg, M., Jepsen, N., Skov, C., 2002. Activity and food choice of
piscivorous perch (Perca fluviatilis) in a eutrophic shallow lake: a radio-telemetry
study. Freshwater Biol. 47, 2370–2379.
Jacobsen, L., Berg, S., Jepsen, N., Skov, C., 2004. Does roach behaviour differ
between shallow lakes of different environmental state? J. Fish Biol. 65, 135–
147.
Jeppesen, E., Jensen, J.P., Søndergaard, M., Lauridsen, T., Landkildehus, F., 2000.
Trophic structure, species richness and biodiversity in Danish lakes: changes
along a phosphorus gradient. Freshwater Biol. 45, 201–218.
Kahl, U., Radke, R.J., 2006. Habitat and food resource use of perch and roach in a deep
mesotrophic reservoir: enough space to avoid competition? Ecol. Freshwater
Fish 15, 48–56.
Kubečka, J., 1993. Night inshore migration and capture of adult fish by shore seining.
Aquacult. Fish. Manage. 24, 685–689.
Kubečka, J., Wittingerová, M., 1998. Horizontal beaming as a crucial component
of acoustic fish stock assessment in freshwater reservoirs. Fish. Res. 35, 99–
106.
Mehner, T., Diekmann, M., Brämick, U., Lemcke, R., 2005. Composition of fish communities
in German lakes as related to lake morphology, trophic state, shore
structure and human-use intensity. Freshwater Biol. 50, 70–85.
Olin, M., Malinen, T., 2003. Comparison of gillnet and trawl in diurnal fish community
sampling. Hydrobiologia 506–509, 443–449.
Olin, M., Rask, M., Ruuhijärvi, J., Kurkilahti, M., Ala-Opas, P., Ylönen, O., 2002. Fish
community structure in mesotrophic and eutrophic lakes of southern Finland:
the relative abundances of percids and cyprinids along a trophic gradient. J. Fish
Biol. 60, 593–612.
Olin, M., Kurkilahti, M., Peitola, P., Ruuhijärvi, J., 2004. The effects of fish accumulation
on the catchability of multimesh gillnet. Fish. Res. 68, 135–147.
Persson, L., 1986. Temperature-induced shift in foraging ability in two fish species,
roach (Rutilus rutilus) and perch (Perca fluviatilis): implications for coexistence
between poikilotherms. J. Animal. Ecol. 55, 829–839.
Rask, M., 1986. The diet and diel feeding activity of perch, Perca fluviatilis L., in a
small lake in southern Finland. Ann. Zool. Fennici 23, 49–56.
Schulz, U., Berg, R., 1987. The migration of ultrasonic-tagged bream, Abramis brama
(L.), in Lake Constance (Bodensee-Untersee). J. Fish Biol. 31, 409–414.
Vašek, M., Kubečka, J., Peterka, J., Čech, M., Draštík, V., Hladík, M., Prchalová, M.,
Frouzová, J., 2004. Longitudinal and vertical spatial gradients in the distribution
of fish within a canyon-shaped reservoir. Int. Rev. Hydrobiol. 89, 352–362.
Zamora, L., Moreno-Amich, R., 2002. Quantifying the activity and movement of
perch in a temperate lake by integrating acoustic telemetry and a geographic
information system. Hydrobiologia 483, 209–218.