P. Ugarte et al. - periwinkle

A preliminary study to monitor periwinkle by-catch and incidence of holdfasts in
harvested rockweed, Ascophyllum nodosum, from Cobscook Bay, Maine
Raul Ugarte
Acadia Seaplants Limited
30 Brown Avenue, Dartmouth, NS B3B 1X8
Christopher Bartlett
Maine Sea Grant/University of Maine Cooperative Extension
16 Deep Cove Road, Eastport, ME 04631
Lonna Perry
contracted by Maine Department of Marine Resources
P.O. Box 8, West Boothbay Harbor, ME 04575
Submitted to
Maine Department of Marine Resources
P.O. Box 8, West Boothbay Harbor, ME 04575
February 9, 2010
1

Introduction
Commercial-scale harvest of the brown seaweed Ascophyllum nodosum (rockweed) in
Cobscook Bay began in the mid 1990s, with intermittent annual landings not exceeding
363 MT (400 short tons). In recent years, a more organized operation has increased
landings, with 2722 MT harvested in 2008. The 2009 landings decreased to about 1090
MT, in part due to the division of the bay between two harvesting companies and
conserved lands that were prohibited for harvest. Acadia Sea Plants harvested 997 MT
and Butch Harris of Eastport harvested 93.5 MT in 2009, respectively. The biomass of
rockweed in Cobscook Bay is estimated to be 50,000 tons without the exclusion of
conserved lands.
Questions exist about the effect of rockweed harvesting on other commercial and noncommercial
species. Fishermen in Cobscook Bay raised concerns about the quantities of
periwinkles that were caught incidental to the rockweed harvest. The common
periwinkle, Littorina littorea, is an abundant commercially exploited snail found among
the intertidal rockweed beds. Statewide landings of the common periwinkle totaled
720,004 pounds in 2008 with an estimated value of $601,410 (Maine DMR). To address
this concern, we implemented a preliminary monitoring study to evaluate the abundance
of three periwinkle species found as by-catch in harvested rockweed.
In addition, the incidence of plants harvested with and without holdfasts was evaluated to
provide information on the proportion of plants dislodged during the harvest. Sexual
reproduction has been claimed to be a poor mechanism to maintain rockweed beds
(Vadas 1990), with the holdfast being the portion of the plants that plays the important
role of regenerating new shoots (fig. 1). Thus the portion of plants harvested with
holdfast tissue is an important parameter to evaluate potential changes in the resource
dynamic and habitat architecture.
This report presents the results of a monitoring program carried out between September
17 and October 13, 2009 with harvesters contracted by Acadia Sea Plants Limited (ASL)
during the final weeks of their seasonal operations. We also intended to monitor the
rockweed harvests of Butch Harris, an affiliate of North American Kelp. Unfortunately,
Mr. Harris experienced equipment problems during our sampling period and we were not
able to collect meaningful information from his operations.
Methods
Sampling procedure
ASL contractors use a cutter rake to hand-harvest rockweed and load it into open boats
for transfer to a storage site. Harvesters operated two to five boats on a daily basis during
the study timeframe. After the daily harvest was completed, three ~6 kg samples of
rockweed per boat were randomly collected in plastic bags. Care was taken to include all
periwinkles and not break or tear off any rockweed clumps with holdfasts in the sample.
In addition, once the boat was completely unloaded, all periwinkles remaining in the
bottom of the boat were collected by draining the water and sieving the sample through a
2.0 mm mesh screen, and all animals placed inside a plastic bag with label. Each label
contained the following information: boat #, harvester’s name, harvesting sector, date and
2

time. These two methods of sampling provided an estimate of the numbers and biomass
of periwinkles in the harvested material.
By-catch analysis
In the lab, total weight of each rockweed sample was taken to the nearest gram. The
sample was then washed with freshwater over a container to receive the periwinkles. The
container’s contents were then sieved through 2.0 mm screen. The snail species contained
in the samples were identified using a taxonomic key (A Preliminary Guide to the Littoral
and Sublittoral Marine Invertebrates of Passamaquoddy Bay, Brinkhurst, D.S., et al.).
Snails from each of three species were separated, counted, and the total weight measured
to the nearest 0.01 g (see photos). The same procedure was followed with snail samples
recovered from the boat at the end of the unloading process.
Holdfast analysis
Once washed, the rockweed sample was spread out and clumps containing holdfasts were
separated, counted and weighed individually to the nearest 0.1g. The total mass of the
clumps with holdfasts was then divided by the total mass of the sample to estimate a
proportion or incidence. Four parameters were measured per clump: total length (to the
nearest 0.1 cm), wet weight (to the nearest 0.1g), number of shoots emerging from the
holdfast in the clump, and holdfast area (to the nearest 1.0 mm). The latter was
determined by making an ink impression of the holdfast base on a piece of white paper.
This impression was then scanned, saved in JPEG format, and the area measured with an
image analysis program (ImageJ).
Results
The company’s total harvest of rockweed in Cobscook Bay during the entire 2009 season
was 997 MT (1,099 tons). Of the 11 sectors ASL harvested, 4 sectors were harvested
during the monitoring period ( fig. 2) and provided nearly 60% of ASL’s overall 2009
landed biomass. A total of 31 boats containing harvested rockweed were sampled
between September 17 and October 13, 2009 with 93 samples (3 per boat) collected.
Samples averaged 6.4 kg, and the sampled boats contained an average of 3.4 MT of
rockweed.
Snail by-catch
Three species of snail were identified and measured; the rough periwinkle Littorina
saxatilis, the smooth periwinkle Littorina obtusata, and the common periwinkle Littorina
littorea.
Rockweed samples
L. obtusata was the most abundant by-catch component in the rockweed samples with
0.84 g /kg of harvested rockweed. The least abundant by-catch in the harvested material
was the commercially important L. littorea with 0.18 g/kg . If we assume this by-catch
rate to be consistent throughout the entire 2009 rockweed harvest in Cobscook Bay, then
3

L. obtusata, represented 64.9 % of the total by-catch, L. saxatilis (21.9%) and L. littorea
(13.3%) (Table I). The average weight of individual periwinkles in our samples was 0.09
grams for L. obtusata, 0.10 grams for L. saxatilis, and 2.2 grams for L. littorea.
Other species including blue mussel, Mytilus edulis, and dog whelk, Nucella lapillus,
were found in small numbers and not included in this preliminary analysis.
Boat deck samples
Contrary to the rockweed samples, the most abundant by-catch component on the boat
deck samples was the common periwinkle L. littorea. However, its total weight as bycatch
for the season was much lower than in the seaweed samples (72.1 kg). L. obtusata
followed with 44.9 kg and L. saxatilis with 5.5kg (Table II).
Table I. Snail by-catch estimates from harvested rockweed samples
Sample analysis
Snails in sample Total by-catch
Snails species (g/kg) (MT/997MT)
Smooth 0.861 0.858 (64.9%)
Rough 0.129 0.289 (21.9%)
Common 0.078 0.175 (13.3%)
Total
1.32 MT
Table II. Snail by-catch estimates from the decks of boats after unloading.
Snails on boat after harvest
Snails per boat Total snails
Snails species (g/boat) (kg/997 MT)
Smooth 149.80 44.5
Rough 18.40 5.3
Common 240.70 71.5
Total
* average boatload was 3.4 MT
121.3 kg
4

Holdfast analysis
The incidence of harvested plants containing holdfast tissue in Cobscook Bay during our
sampling period was 7.78%. The average weight of these plants was 147.5 g, with an
average length of 95.7 cm and containing an average of 3.3 attached shoots. The average
size of the detached holdfast tissue was 15.3 mm 2 . Table III shows the details of this
analysis.
Table III. Incidence of holdfasts and their morphology in the rockweed harvest in
Cobscook Bay in 2009
Incidence Weight Length No shoots Holdfast area
(%) (g) (cm) (mm2)
X 7.78 147.5 95.7 3.3 15.3
SD 129.6 30.2 1.7 12.6
n 93 151 151 151 151
Most of the plants (73.5%) with holdfast tissue weighed less than 200 g, and 47% were
less than 100 g (Fig 1).
Figure 1. Weight distribution of plants bearing holdfast tissue
The length distribution of the plants bearing holdfast tissue varied from 27 cm to 182 cm,
with 75% of the plants being below 115 cm (Fig. 2).
5

Figure 2. Length distribution of plants bearing holdfast tissue.
The number of shoots contained in the plants bearing holdfast tissue varied from 1 to 10
in the analyzed samples. 80% of the plants contained 4 shoots or less (Fig. 3).
Figure 3. Number of shoot in plants bearing holdfast tissue.
6

The size of the holdfast tissue (measured as area) contained in the plants bearing holdfast
varied from 2.3 mm2 to 77.2 mm2, with 80% of them being below 20 mm2 (Fig. 4)
Figure 4. Size distribution of the holdfasts tissue in plants bearing holdfast.
Discussion
By-catch
The analysis of harvested rockweed samples collected from Cobscook Bay in 2009
showed that the dominant by-catch component was the smooth periwinkle, Littorina
obtusata, with an estimated total of 0.86 MT removed with the rockweed during the
entire harvest season. We estimated that an additional 44.5 kg was left on the deck of the
boats that harvesters would return to the sea after the rockweed is transferred. L. obtusata
feeds predominantly on rockweed and it was found in all of our study’s samples.
Removal of the rough periwinkle, Littorina saxatilis, was estimated to be 0.289 MT with
the harvested rockweed and an additional 5.47 kg found on the deck of the boats. It was
identified 72% of the time in rockweed samples and 100% in boat samples. L. saxatilis is
most commonly found in the upper intertidal zone.
The commercially harvested common periwinkle, Littorina littorea, was the least
abundant by-catch snail species with 0.175 MT removed with the rockweed and 71.7 kg
left on the boats, for a total estimated loss of 243.4 kg during 2009. The average size of L.
littorea was 2.2 g in our samples, making the bulk of these snails below market size (>
5.0 g in NB). This species was identified in 93% of boat samples but only 20% of
rockweed samples, suggesting that it may be dislodged more readily than the other snails
when the rockweed is cut and loaded into the boats. The relatively small size of
individual snails in our samples also raises the question of whether larger snails fell off in
7

the water or whether smaller (i.e. younger) snails were more prevalent in the upper
canopy.
Total weight of snails removed from the intertidal of Cobscook Bay in 2009 by Acadia
Seaplants was estimated to be 1.44 MT, which includes boat loss. While we have
documented the amount of periwinkles taken in the rockweed harvest, further research is
needed to evaluate the direct impact on the snail population. Studies to assess the biomass
of these species along the intertidal zone of Cobscook Bay are required to estimate the
rate of by-catch mortality. Average densities of L. littorea on the rockweed canopy in a
harvest experiment carried out at Shackford Head in Cobscook Bay in 2008 (Trott and
Larsen, 2009) showed L. littorea densities in the canopy of approx 45 individuals per m-2
and 140 individuals per m-2 in the substrate. If similar densities exist along all rockweed
beds of Cobscook Bay, the total biomass of L. littorea on rockweed covered areas would
be around 2,327 MT. If this were the case, then approximately 0.008% of the
commercial periwinkle population would be taken by the harvest from the rockweed beds
in 2009. We know that L. littorea is also found in the open intertidal, thus the impact on
the total population could be much lower. Documenting the common periwinkle
landings in Cobscook Bay and rates of natural mortality would further inform the
potential impact of by-catch mortality on this fishery.
Using Trott and Larsen (2009) density estimations for the other snail species, then
approximately 0.044% of the total snail population found on rockweed beds was removed
during the 2009 harvest.
Holdfast analysis
Holdfast incidence in the harvested material can be the result of a friable substrate
holding the rockweed clumps, a naturally damaged clump, or the condition of the
harvesting tool. The 7.78 % holdfast incidence estimated for Cobscook Bay in 2009
corresponded to 77.7 MT of biomass being harvested with holdfast tissue. This incidence
was slightly higher than the incidence found in the Canadian harvest (6.6%).
The impact of the harvest can be compared to natural events. For example, the total
rockweed biomass for Cobscook Bay is approximately 46,000 MT (conservative
estimation made by ASL ). From growth and production data for this seaweed in the
region (Vadas et al. 2004), a minimum of approximately 18,000 t of biomass is released
as storm-cast material each year. Average holdfast incidence in storm cast material has
been evaluated around 27% along the Bay of Fundy.
Considering that rockweed is a perennial macroalgae, and that recruitment is considered a
poor mechanism to maintain these populations (Vadas et al., 1990; Viejo et al., 1999;
Dudgeon & Petratis, 2005), then alternative mechanisms must exist that prevent the
decline of these populations by the cumulative effect of storms or other natural events.
One explanatory mechanism could be that only a small portion of the holdfast surface is
dislodged when impacted by both the cutter rake and storms. Unfortunately we did not
collect data on the population structure of rockweed from Cobscook Bay to confirm this
mechanism. However, some comparisons could be made with rockweed populations from
8

Canada. For example, the average holdfast size of rockweed plants in New Brunswick is
124 mm 2 with a total of 41.6 shoots per plant. If this morphological structure is similar to
the New Brunswick population, then the portion of the clump that is most affected are the
largest and heaviest shoots conforming the canopy, leaving behind a large amount of
meristematic tissue in the form of suppressed shoots and laterals. This would seem to
suggest that the removal of limited portions of the algal canopy does not have a
significant impact on the population structure of this macroalgae.
Acknowledgements
Maine Department of Marine Resources and Acadia Seaplants Limited funded this
preliminary monitoring study. Thanks to Jeffrey Brown, Acadia Seaplants, for long hours
of assistance in the field and laboratory. We are grateful to John Sowles and Peter
Thayer, Maine DMR, for guidance in developing and implementing this effort. We
appreciate the council of Dr. Brian Beal, University of Maine at Machias, in the
preliminary identification of organisms. Brian Beal, UMM, and Catherine Schmitt, Maine
Sea Grant, generously provided editorial suggestions for this report. A special thanks is
extended to Chouan Strongin and her crew of rockweed harvesters for graciously
accommodating our sampling efforts.
References
Brinkhurst, D.S., et al. 1976. A Preliminary Guide to the Littoral and Sublittoral Marine
Invertebrates of Passamaquoddy Bay. Huntsman Marine Laboratory (St. Andrews, N.B.)
Dudgeon S, Petraitis P (2005) First year demography of the foundation species,
Ascophyllum nodosum, and its community implications. Oikos 109 (2), 405–415.
Maine DMR . 2009. Historical Maine Periwinkle Landings, 1950 - 2008. DMR Report.
Trott, T. J. and P. F. Larsen. 2009. Evaluation of short-term changes in rockweed
(Ascophyllum nodosum) and associated epifaunal communities following cutter rake
harvesting in Maine. Report to DMR.
Vadas LV, Wright A, Miller SL (1990) Recruitment of Ascophyllum nodosum: wave
action as source of mortality. Mar Ecol Prog Ser 61: 263-272.
Vadas RL, Wright WA, Beal BF. 2004. Biomass and Productivity of Intertidal
Rockweeds (Ascophyllum nodosum LeJolis) in Cobscook Bay. Ecosystem Modeling in
Cobscook Bay, Maine: A Boreal, Macrotidal Estuary. Northeastern Naturalist 11(Special
Issue 2):123–142.
9

Viejo RS, Aberg P, Cervin G, Lindegarth M (1999) The interactive effects of adult
canopy, germling density and grazing on germling survival of the rockweed Ascophyllum
nodosum. Mar Ecol Prog Ser 187: 113-120.
10

Figure 1. Ascophyllum nodosum frond and associated terminology: A, apical tip; B, basal
or suppressed shot; H, Holdfast; I, internode; L, lateral shoot or branch; S, main shoot; R,
receptacle (reproductive organ); V, vesicle.
11

Figure 2: Map of rockweed sectors in Cobscook Bay. Colored sectors were harvested by ASL in 2009.
Pembroke
A-18
A-27
A-28 A-26
Dennysville
A-29 A-25
A-30
A-22
A-23
A-21
A-24
A-20
A-12
A-17
A-19
A-16
A-6
A-5
A-7
A-11
A-15
Eastport
A-4
A-14
A-3
A-2 A-13
A-31 A-36
A-10
A-9
Lubec
A-1
State Park A-35
A-8
A-32 A-34
A-33
Whiting
Harvested sectors
Sampled sectors
12

Photos of sor)ng rockweed plants and by-­‐catch
Rockweed washed and ready for sor0ng
Rockweed plants with holdfast material
Rockweed plant with holdfast a7ached
to substrate
Sor0ng of periwinkle species
Smooth periwinkle, L. obtusata, to be
counted and weighed
A closer view of L. obtusata during sor0ng
13