Download An in situ study of predator aggregations on scallop (Pecten

Journal of Experimental Marine Biology and Ecology
255 (2000) 111–129
www.elsevier.nl / locate / jembe
An in situ study of predator aggregations on scallop (Pecten
maximus (L.)) dredge discards using a static time-lapse
camera system
L.O. Veale a , *, A.S. Hill b , A.R. Brand a
a
University of Liverpool Port Erin Marine Laboratory, Port Erin, Isle of Man IM9 6 JA, UK
b
SEPA, Clearwater House, Heriot-Watt Research Park, Edinburgh EH14 4 AP, UK
Received 25 May 2000; received in revised form 19 September 2000; accepted 21 September 2000
Abstract
The impact of demersal fishing gears on benthic habitats and species has been the subject of
much attention recently, and suggestions have been made that scavenging epifaunal species may
benefit at the population level from the additional food source provided by discards. This paper
investigates some aspects of this process, including the relative attractiveness to predators of
different discard species, and the role of damage in scavenger attraction. A time-lapse video
system with a 1000 m long cable was positioned in an area closed to fishing, adjacent to the most
heavily fished scallop (Pecten maximus) ground in the Irish Sea. A variety of undamaged and
damaged by-catch animals were positioned in front of the camera, and the subsequent predator
aggregations investigated. Densities of scavenger species up to 200 times that of the background
population were observed, and aggregations of some species persisted for up to 3 days. The most
frequently recorded scavengers, and therefore presumably those species most likely to benefit from
discards as a food source, were: Asterias rubens L., Astropecten irregularis (Pennant), Liocarcinus spp Stimpson, Pagurus spp Fabricius and Callionymus lyra L. Predator attraction to
apparently undamaged queen scallops, Aequipecten opercularis (L.), was almost as high as to
damaged A. opercularis. Of all the prey species studied, queen scallops were the most attractive to
scavengers. A directional relationship was found between the ambient water current and the arrival
of the starfish, Asterias rubens.  2000 Elsevier Science B.V. All rights reserved.
Keywords: Fisheries by-catch and discards; Fishing impact; Pecten maximus; Predator aggregation; Time-lapse
video
*Corresponding author. Tel.: 144-1624-831-017; fax: 144-1624-831-001.
E-mail address: [email protected] (L.O. Veale).
0022-0981 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved.
PII: S0022-0981( 00 )00295-1
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1. Introduction
Mobile demersal fishing gears are known to have a detrimental effect on benthic
epifaunal and infaunal communities (Dayton et al., 1995; Thrush et al., 1995; Jennings
and Kaiser, 1998), often dramatically increasing local mortality in the wake of the gear
(Kaiser and Spencer, 1994b); this may have long-term implications for the structure of
the community (Thrush et al., 1998). Both those animals disturbed or damaged by the
passage of the gear and left on the seabed, and those retained in the catch and
subsequently discarded, are known to attract mobile predators (Kaiser and Spencer,
1994a, 1996), which act as facultative scavengers (Britton and Morton, 1994). This
ability to feed opportunistically on dead and dying animals, coupled with robustness to
capture and damage in the fishing gear, may confer an increased survivorship on certain
species that, in turn, may lead to an enhanced population size (Polis et al., 1996; Ramsay
et al., 1997b), e.g. the flatfish, Limanda limanda (L.) (Kaiser and Ramsay, 1997). In
intensively fished areas, such as the North Sea, this carrion will inevitably subsidize
some marine food webs (Furness, 1996; Ramsay et al., 1997b). The levels of fishing
effort presently exerted by scallop (Pecten maximus) and queen scallop (Aequipecten
opercularis) dredging in the northern Irish Sea (approx. 177,000 metre hours for the
1994–95 fishing season) will potentially introduce a large amount of by-catch material
[approximately 231 tonnes for the 1994–95 season (L. Veale, unpublished data)] into the
food web; a proportion of this will become available to benthic predators and
scavengers.
Aggregations of scavenging species after the passage of towed demersal fishing gears
have been recorded in several previous studies, mainly by submersible, video or diver
observations made along a fished track (e.g., Medcof and Bourne, 1964; Caddy, 1973;
Chapman et al., 1977; Murawski and Serchuk, 1989; Kaiser and Spencer, 1994b). These
have all noted increased densities of several fish and invertebrate scavenging species in
response to fishing activities, but details such as the relative attractiveness of the
different prey species, the importance of prey damage, and the relationship between
direction of attraction and water movement, have not been addressed. It is of particular
importance to ascertain whether apparently undamaged animals are equally likely to be
preyed upon after discard, as this will have implications for the potential benefits of gear
modifications designed to reduce animal damage.
Some studies have used baited time-lapse stills cameras deployed from research
vessels to investigate the scavengers attracted to damaged animals typical of material
discarded from fishing gears (e.g., Kaiser and Spencer, 1996; Ramsay et al., 1997b).
Here, the maximum scavenger activity occurred within 24 h. This approach allows a
detailed investigation of the arrival times of different scavenging species and the rate of
bait consumption. However, a stills camera will inevitably return fewer frames than a
video camera, and its deployment from a ship will either limit the duration of the
investigation, or incur high ship-time costs. Invertebrate scavenger species are generally
slow moving, and slow at ingesting and digesting food, so it is important that such
investigations are of sufficient duration to adequately record the arrival of all species
attracted to the prey, and to monitor their subsequent departure.
A static time-lapse video system was deployed in this study to identify the major
L.O. Veale et al. / J. Exp. Mar. Biol. Ecol. 255 (2000) 111 – 129
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species involved in scavenging dead and damaged benthos typical of that either
discarded from scallop dredges, or damaged and left on the seabed. A 1000 m long cable
allowed the camera to be positioned offshore, on an area of seabed typical of that
supporting the local scallop fishery, but within an area closed to commercial fishing
(Bradshaw et al., 2000). Recordings totalling over 2000 h were made, which would have
been impossible to achieve using SCUBA divers or cameras deployed from a ship.
These extended time periods ensured that the aggregation and dispersion of slow-moving
invertebrate scavengers were adequately monitored.
Firstly, the relative merits of using white and red light to record nighttime footage
were compared. Next, the aggregation of scavengers on mixed damaged benthos was
examined, and then the attractiveness of different by-catch species was investigated
using four mono-specific baits: damaged Aequipecten opercularis, Pecten maximus,
Asterias rubens and Buccinum undatum L. These species were chosen as they
represented some of the most abundant members of the catch assemblage of the north
Irish Sea scallop fishery (Veale et al., 2000). The composition and size of scavenger
aggregations attracted to both damaged and undamaged A. opercularis were then
compared. An exploration of the interaction of water current direction and distribution of
olfactory stimuli was conducted for Asterias rubens.
2. Materials and methods
From the 14th June to 5th October 1996, a static video camera system was deployed
in the area closed to fishing off Port Erin, Isle of Man. This was during the closed season
for the great scallop, which runs from 1st June to 31st October inclusive: no commercial
dredging occurred in the vicinity during the study. A Rovtech Systems low-light colour
camera with two 250 W lights was used. The signal was transmitted to a terrestrial
control box via a booster unit and 1000 m of cable. The image was recorded on a
Panasonic SVHS time-lapse video recorder, and real time video was viewed on a JVC
colour monitor. The camera was situated approximately 600 m offshore in 25 m depth,
due west of the Marine Laboratory (Fig. 1), mounted on a galvanised Dexian frame at an
angle of 458, 1 m above the seabed. The camera was orientated facing east, perpendicular to the prevailing currents in the area, which travel north or south, depending on
tidal state.
Throughout the study period divers positioned a variety of baits loose on the seabed in
front of the camera. Mixed bait comprised a variety of dredge-caught epifaunal species,
including Aequipecten opercularis, Pecten maximus, Neptunea antiqua (L.), Buccinum
undatum, Echinus esculentus Lamarck, and Asterias rubens, which were damaged just
prior to baiting: damage was applied with stones and metal bars to simulate that
observed in damaged by-catch animals. Baited periods were interspersed with unbaited
(control) periods, and the camera was repositioned three times to reduce the number of
animals permanently associating themselves with the frame, although no differences in
background (unbaited) abundance were noted after repositioning.
Initially, the relative merits of using white and red light to record nighttime footage
were compared in single 3-day unbaited trials with each type of light. Then the
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Fig. 1. Approximate positions of the fixed camera within the area closed to fishing. Diamonds indicate the positions of navigation buoys. The camera was relocated
over three positions throughout the study.
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aggregations on mixed damaged benthos were investigated, with two unbaited / baited
comparisons. Mono-specific baits of Aequipecten opercularis, Pecten maximus, Asterias
rubens and Buccinum undatum were also studied in single unbaited / baited trials, and
then finally the effect of damage incurred by Aequipecten opercularis on the ensuing
aggregation was investigated by a single undamaged / damaged comparison. Generally,
3-day unbaited periods were followed by 3-day baited periods; previous studies in the
Irish Sea had noted that peak numbers occurred less than 24 h after baiting (Kaiser and
Spencer, 1996). The extended time period used here allowed the dispersion of the
aggregation to be monitored. On one of the mixed baiting occasions, the recording was
continued for 5 days after baiting, to further assess the dispersal of the aggregation.
The direction of arrival of Asterias rubens at all the baits described above (where
possible) was compared with simultaneous estimates of water current direction, using
circular statistics (Batschelet, 1981). The current direction was noted by observing the
movement of particles in the water, or pieces of seaweed across the seabed, and the
direction of arrival of A. rubens was estimated as it entered the field of view of the
camera.
The background densities of the major scavenging species were estimated by sixteen
200 m 2 diver transect surveys. Pairs of divers followed a marked 50 m bottom line
(repositioned for each survey), recording animals within 2 m of the line on either side.
The width of the survey was delimited by a 4 m pole, on which were mounted slates for
recording data.
2.1. Data analysis
The video tapes were played back on the same Panasonic SVHS time-lapse recorder,
which was equipped with an on-screen date and time display. Instantaneous counts of all
the species present were made every 10 min of real time. It was impossible to determine
whether or not some species (e.g., starfish) were feeding on the bait, so counts include all
animals in the vicinity of the bait, but not necessarily feeding on it.
For graphical presentation, hourly mean abundance of each species at the bait was
calculated and plotted against time. Mean abundance was used in favour of the
maximum because it included elements of animal abundance and residence time at the
bait.
Due to lack of independence between consecutive observations in a time series, it was
decided to use daily (24 h) means for statistical analysis. This time period was large
enough to ameliorate the independence problem, and also eliminated any diel differences. However, it meant that there were only three replicates for each treatment. A
square-root transformation provided an approximation to normality so that parametric
ANOVA could be used to compare daily mean numbers of each species: before and after
baiting, under the different light regimes, or between damaged and undamaged A.
opercularis. After the second mixed baiting recording was continued for 5 days. On this
occasion, three groups were used in the ANOVA: days 1–3 before baiting; days 1–3
after baiting; days 4–5 after baiting.
Multivariate analysis was used to assess any differences between the assemblages
observed daily in front of the camera, before and after baiting with mixed damaged
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benthos. The data were standardised and square-root transformed; no species were
excluded. These decisions were based on the fact that only a small number of species
were visible in front of the camera, and no species greatly dominated the assemblage.
Dendrograms and non-metric multidimensional scaling (MDS) plots of the sample
relationships defined by the Bray–Curtis similarity index were calculated and plotted
using the PRIMER software package. The statistical significance of any difference
observed before and after baiting was tested using ANOSIM (Clarke and Warwick,
1994).
The relationship between the direction of arrival of Asterias rubens at the bait, and the
direction of tidal water flow, was examined using the Jupp–Mardia circular–circular
correlation coefficient (Batschelet, 1981). The test statistic nr 2 is compared with
tabulated values for x 2 with four degrees of freedom. However, there is no way of
distinguishing a positive from a negative correlation using this technique, so the
differences between the directions from which the current and the A. rubens were
coming were calculated. It was postulated that if A. rubens were moving into the water
current, possibly in response to olfactory stimuli, then this mean angle (6confidence
interval) would fall between 135 and 2258 (i.e. 1806458). The confidence interval for
the population mean angle (u ) was derived from graphed values presented in Batschelet
(1981).
3. Results
A total of 22 taxa were identified throughout the study period (Table 1). They ranged
in percentage occurrence (number of frames in which the animal was observed / total
number of frames examined*100) from 0.01 to 45.2%. The starfish, Asterias rubens,
was the most frequently observed species, appearing in three times as many frames as
any other species. The next most frequently observed species were: the crustaceans,
Liocarcinus spp and Pagurus spp; the starfish, Astropecten irregularis; and the demersal
fish, Callionymus lyra. All other species were observed at much lower frequencies.
Initially, the relative effects of red and white light were assessed; 3-day periods of
each light regime, without bait, were filmed. Statistical analysis of the daily means
showed significantly elevated counts for Pagurus spp, Asterias rubens, Liocarcinus spp
and Astropecten irregularis under white light conditions compared to red light. A.
irregularis was also significantly more abundant at night than during the daylight hours
(Table 2, Fig. 2). Additionally, Cancer pagurus L. and A. irregularis both exhibited a
significant interaction term between type of light and time of day (Table 2), i.e. activity
over the day / night cycle was related to the type of light used; at night they were more
frequently observed under white light. Given the significance of these results, red light
was used for all subsequent studies, and the data collected under white light was not
used.
Increases in abundance of Asterias rubens, Astropecten irregularis, Pagurus spp,
Liocarcinus spp and the other species pooled, were apparent after baiting with mixed
damaged benthos (Figs. 3 and 4). Aggregations of up to 17 individual animals were
visible at any one time. The maximum abundance of several species aggregating on the
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Table 1
Taxa observed throughout the duration of the study, and their percentage and rank occurrence in all video
frames examined
Phylum
Species / taxa
%
occurrence
Rank
occurrence
Crustacea
Atelecyclus rotundatus
Cancer pagurus
Inachus spp
Liocarcinus spp
Pagurus spp
0.2
4.1
0.3
15.5
17.1
15.5
7
12
3
2
Mollusca
Eledone cirrhosa
Nudibranch 1
Nudibranch 2
Nudibranch 3
Echinodermata
Asterias rubens
Astropecten irregularis
Crossaster papposus
Porania pulvillus
Ophiocomina nigra
Ophiura spp
Ophiothrix fragilis
45.2
12.3
0.02
1.9
0.2
0.3
0.3
1
4
21
8
15.5
12
12
Pisces
Agonus cataphractus
Callionymus lyra
Flatfish
Synagnathus sp
Scyliorhinus caniculata
Triglidae
0.5
10.9
4.5
0.1
0.01
0.05
9
5
6
18
22
20
0.1
0.1
0.3
0.3
18
18
12
12
bait within the 2 m 2 field of view was many times greater than the background densities
recorded by diver surveys in the same area at the same time (Table 3). Some species
dispersed more quickly than others. For example, A. rubens declined slowly from the
initial mean of six individuals present, in the days following baiting, reaching the
background level by day 4, but aggregations of Liocarcinus spp were much shorter
lived, lasting only 12 h (Fig. 3). This is inevitably linked to the time required for
Table 2
Comparison between the daily mean numbers of the six most common species observed under full and red
light conditions (no bait used) (two-way ANOVA). *Significant at 5% level, **significant at 1% level. Results
in bold type had a power of the performed test of .0.8
Species
Light used
(white or red)
Time of day
(day or night)
Interaction
(Light3Time of day)
Asterias rubens
Astropecten irregularis
Cancer pagurus
Liocarcinus spp
Callionymus lyra
Pagurus spp
F 5 9.5, P , 0.05*
F 5 89.0, P , 0.01**
F 5 1.1, P 5 0.33
F 5 6.3, P , 0.05*
F 5 0.1, P 5 0.78
F 5 22.0, P , 0.01**
F 5 0.6, P 5 0.47
F 5 49.7, P , 0.01**
F 5 1.1, P 5 0.33
F 5 4.8, P 5 0.07
F 5 1.3, P 5 0.29
F 5 0.0, P 5 0.95
F 5 0.1, P 5 0.71
F 5 40.0, P , 0.01**
F 5 17.9, P , 0.01**
F 5 0.6, P 5 0.46
F 5 0.3, P 5 0.60
F 5 0.0, P 5 0.95
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Fig. 2. Hourly mean numbers of animals observed over unbaited 3-day periods with full light (top) and red
light (bottom). Night and day are indicated by the black and white bar. *No data recorded during this night.
feeding, and their relative mobilities. It is also clear that A. irregularis is a night-active
species.
Statistical analysis of the data from the first baiting occasion revealed significant
differences between daily mean abundance before and after baiting, for Asterias rubens
(ANOVA, F1,4 5 18.61, P , 0.05), Astropecten irregularis (ANOVA, F1,4 5 8.24, P ,
0.05), Cancer pagurus (ANOVA, F1,4 5 22.47, P , 0.01) and the data for Pagurus spp
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119
Fig. 3. Mean numbers of animals observed per hour before and after the first baiting event. The solid vertical
line indicates the baiting event, and the dotted lines enclose areas of the graphs where no data were recorded.
Night and day are indicated by the black and white bars.
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Fig. 4. Mean numbers of animals observed per hour before and after the second baiting event. The solid
vertical line indicates the baiting event, and the dotted lines enclose areas of the graphs where no data were
recorded. Night and day are indicated by the black and white bars.
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Table 3
Maximum densities of some species observed at the bait within the 2 m 2 field of view, compared to
background density estimates obtained from 16 diver surveys in the area at the time
Species
Maximum density
observed at bait
(Nos. per 2 m 2
field of view)
Mean background density
(Nos. per 2 m 2 )
Size of
aggregation
(3 increase)
Asterias rubens
Astropecten irregularis
Pagurus spp
Liocarcinus spp
Callionymus lyra
8
8
7
4
5
0.18
0.04
0.09
0.10
0.07
44
200
78
40
70
were significant at the 10% level (ANOVA, F1,4 5 6.16, P 5 0.068). The second baiting
occasion showed significant differences only for A. rubens (ANOVA, F2,5 5 16.71,
P , 0.01), and flatfish (ANOVA, F2,5 5 5.91, P 5 0.05). Subsequent Tukeys multiple
range comparison identified differences between the group comprising days 1–3 after
baiting and both the other two groups (days 1–3 before and days 4–5 after baiting) for
A. rubens, demonstrating that the aggregation has dispersed after 3 days.
Multivariate analysis showed a clear separation between the assemblages present
before and after baiting (e.g., the second baiting with mixed damaged benthos — Fig. 5).
Notably, days 4 and 5 after baiting grouped together with the days before baiting,
indicating a gradual shift back towards the pre-baiting structure, with the first day after
baiting (a1) the most distant from the pre-baiting samples (b1–3) (Fig. 5). A significant
difference in assemblage structure before and after baiting was demonstrated for this
baiting (ANOSIM: Global R 5 0.57, P 5 0.036) where groups comprised days 1–3
before, days 1–3 after, and days 4–5 after baiting. No significant changes in scavenger
assemblages were found for the other seven trials.
To assess the effects of damage to discards on the aggregation of scavengers,
undamaged and chipped (minor sub-lethal damage) queen scallops (n 5 15), Aequipecten
opercularis, were used as bait in separate 3-day periods. Asterias rubens again
dominated the scavenging assemblage (|50% of all animals observed). The immediate
increases in A. rubens abundance (day 1) are more or less equal for both damaged and
undamaged baits, but they appeared to remain at the damaged bait longer over the whole
3-day period (Fig. 6). There was no significant difference between A. rubens abundance
on damaged and undamaged queens, but there was a significant increase over unbaited
abundance after baiting with damaged queens (ANOVA: F1,4 5 9.79, P , 0.05). Both
Pagurus spp and Cancer pagurus abundances increased significantly after baiting with
both damaged and undamaged queens (ANOVA: F2,6 5 157.4, P , 0.01 and F2,6 5
810.3, P , 0.01, respectively), but there was no difference between aggregations on the
two different damage grades. Only Callionymus lyra demonstrated a significant
difference between abundance on damaged and undamaged queens, in addition to
increases over unbaited periods (ANOVA: F2,6 5 18.1, P , 0.05), being more abundant
after baiting with damaged queens. This species also showed a marked periodicity in
activity, being most active in the middle of the day.
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Fig. 5. Dendrogram and non-metric MDS plot showing the relationships between assemblages observed before
(b) and after (a) the second baiting with mixed damaged benthos. Data have been standardised, square-root
transformed, and no species have been excluded. The dotted lines indicate samples which are similar at the
75% level.
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Fig. 6. Hourly mean numbers of Asterias rubens and Callionymus lyra observed after baiting with undamaged queens (Aequipecten opercularis) (left) and those with
minor damage (shell chipping) (right). The horizontal lines indicate the unbaited average from the first baiting study. The vertical lines show the time of baiting. Night
and day are indicated by the black and white bar.
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Table 4
Summary of results of experiments using baits comprising different by-catch species to attract scavengers to a
baited video camera
Bait
Scavenging species showing significant increases
Mixed benthos
Asterias rubens, Astropecten irregularis,
Pagurus spp, Cancer pagurus, flatfish
Aequipecten opercularis
(damaged and undamaged)
Asterias rubens, Pagurus spp, Cancer
pagurus, Callionymus lyra
Pecten maximus
(undamaged)
Asterias rubens, Astropecten irregularis,
Pagurus spp
Asterias rubens
Flatfish
Buccinum undatum
Callionymus lyra, Liocarcinus spp, flatfish
Aggregation on undamaged animals was further demonstrated when the great scallop,
Pecten maximus, was used as bait. After baiting, significant increases in abundance were
found for Asterias rubens (ANOVA: F1,4 5 19.6, P , 0.05), Astropecten irregularis
(F1,4 5 25.2, P , 0.01) and Pagurus spp (ANOVA: F1,4 5 9.3, P , 0.05).
The scavenger responses to mono-specific baits of damaged Asterias rubens and
Buccinum undatum were considerably smaller than to mixed or bivalve baits. After
baiting with damaged A. rubens, only flatfish increased significantly (ANOVA: F1,4 5
10.3, P , 0.05), and after baiting with damaged B. undatum, significant increases were
observed in Callionymus lyra (ANOVA: F1,4 5 36.4, P , 0.01), flatfish (ANOVA: F1,4 5
106.6, P , 0.01), and Liocarcinus spp (ANOVA: F1,4 5 17.2, P , 0.05).
The results described in the preceding paragraphs are summarised in Table 4.
Direction of arrival of Asterias rubens was significantly correlated with water current
direction when a number of baits were used: badly damaged Cancer pagurus (Circular–
Circular Correlation Coefficient: nr 2 5 10.02, P , 0.05), badly damaged A. rubens
(CCCC: nr 2 5 11.19, P , 0.05), damaged Aequipecten opercularis (CCCC: nr 2 5 23.20,
P , 0.001), and undamaged A. opercularis (CCCC: nr 2 5 31.08, P , 0.001), but was
not correlated with water direction during two control periods studied (CCCC: nr 2 5
1.52, P . 0.05 and nr 2 5 9.29, P . 0.05). In all significant cases, the mean direction of
arrival of A. rubens was opposite to that of the prevailing water current. Surprisingly,
however, significant correlations with water current direction were not detected after the
two baitings with mixed damaged benthos (CCCC: nr 2 5 5.11, P . 0.05 and nr 2 5 1.56,
P . 0.05).
4. Discussion
The scavenging assemblages identified in this study were dominated by invertebrates,
mainly the starfish, Asterias rubens, but also Astropecten irregularis, and the crabs
Pagurus spp and Liocarcinus spp. In contrast to other studies where gadoid fish have
been abundant (Kaiser and Spencer, 1996), fish species occurred only in low abundances. The sizes of the aggregations, although significantly larger than background
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125
levels, are relatively small. Aggregations of hermit crabs, Pagurus bernhardus, of up to
330 m 22 at baited bags in the Irish Sea have been recorded. These differences may be
related to background population densities of the scavenging species, or to the
availability of natural food sources (Ramsay et al., 1997b).
The use of red light to film animal activity at night enabled a more realistic
assessment of the scavenger assemblage including nocturnal species, whilst minimising
the artificial attraction of the light itself, although not completely removing it. The diel
activities of both Astropecten irregularis and Cancer pagurus were significantly related
to the type of light used, both being more abundant under white light. C. pagurus is
known to be more active at night (Bergman et al., 1990; Skajaa et al., 1998), so light
avoidance behaviour may have been expected, but was not observed. Another Astropecten species (A. articulatus) is known to be crepuscular (Beddingfield et al., 1991), so
conceivably their observed nocturnal activity may have been related to the low light
levels from the video work.
In a previous experiment in the Irish Sea, scavenger numbers were recorded as
peaking between 8 and 14 h after baiting (Kaiser and Spencer, 1996). However, the
fluctuations apparent in Figs. 3 and 4 (present study) suggest that more peaks may have
been observed had the period of observation been extended. In the present study, the
aggregations of some species remained for up to 72 h after baiting, before returning to
background levels. This may have been related to the size of the aggregation: fewer
scavengers would take longer to consume a bait of a given size. In the case of Asterias
rubens, the prolongation of the aggregation may be related to their post-prandial
behaviour: having fed to satiation, A. rubens generally remain motionless for some time
(Jangoux, 1982), presumably digesting their meal, and would continue to be recorded on
the video.
The change in composition of the assemblage observed after baiting (Fig. 5) implies
differential reactions to the bait in the species recorded; some aggregated, some did not.
One of the most pronounced differences was the increased relative abundance of
Pagurus spp; it comprised 19% of the total assemblage abundance averaged over the 3
days before baiting, and 46% after. This observation concurs with other work in the Irish
Sea (Ramsay et al., 1996, 1997a) and in the Clyde (Nickell and Moore, 1991, 1992a,b)
where P. bernhardus was found to be the most abundant scavenger. The observed
change in assemblage composition is obviously very localised and specific to the small
area surrounding the baited camera: it is simply a temporary disruption of the spatial
distribution of these species over the seabed.
Some authors (e.g., Kaiser and Ramsay, 1997; Ramsay et al., 1997b; Thrush et al.,
1998) have suggested that community level changes may already have occurred on
heavily fished areas. In effect, the dietary advantages conferred upon scavenging species
(e.g. Pagurus spp and Asterias rubens) shown here, may lead to long-term benefits at the
population level. Although the present study is of too short a time span to verify this
population change, the mechanism by which such a shift could occur is demonstrated.
Given the ubiquity of A. rubens around the Isle of Man, this species must play a major
role in discard recycling, and previous studies have suggested an increase in A. rubens
abundance in the Irish Sea since the onset of the commercial scallop fishery in the 1930s
(Hill et al., 1999).
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L.O. Veale et al. / J. Exp. Mar. Biol. Ecol. 255 (2000) 111 – 129
There appeared to be an elevated presence of Asterias rubens after baiting with
damaged Aequipecten opercularis compared to undamaged ones, but it was not
demonstrable statistically. The difference appears to lie not in the initial aggregation
levels, but in the persistence of the aggregation over 3 days, implying that A. rubens are
initially attracted just as strongly to undamaged queen scallops as to damaged ones, but
disperse more rapidly. It is interesting to note that the undamaged A. opercularis (only
15 individuals) still attracted up to seven times the unbaited levels of A. rubens. It is
generally believed that scavengers are attracted by the odour plume released from
damaged tissues (Sainte-Marie and Hargrave, 1987), although other stimuli may be
involved (Lapointe and Sainte-Marie, 1992). In this case, the excretion of normal
metabolic by-products from such an unnaturally high density of undamaged A.
opercularis may have stimulated aggregation. It seems likely that even undamaged
animals among the discards will be exposed to increased predator encounter rates and
concomitant elevated stress levels (Crowder and Murawski, 1998), for example Ramsay
and Kaiser (1998) showed that apparently undamaged whelks, which had been rolled to
simulate movement in fishing gear, were significantly less likely to exhibit an escape
response than unrolled ones.
The Irish Sea scallop fishery is subject to various legislations, including a minimum
legal landing size of 110 mm. The dredge teeth and belly ring diameter exert some size
selection on the target species (Dare et al., 1993), but undersized scallops will inevitably
be retained, and subsequently discarded, especially on grounds with large amounts of
dead shell, which quickly block up the dredge belly. These scallops, which may be
damaged or exhausted, in addition to those damaged by the dredge on the seabed but not
retained, may suffer considerable predation mortality. Related scallop restocking studies
have demonstrated high predation levels in small scallops seeded in unnaturally high
densities (Barbeau et al., 1996, 1998). This has obvious implications for fisheries
management.
Mono-specific baits of Asterias rubens, Buccinum undatum and Pecten maximus
appear to be generally less attractive to scavengers than the mixed baits, and both the
damaged and undamaged Aequipecten opercularis baits. It seems likely that A.
opercularis, a component of the mixed baits, is a particularly attractive prey to
scavenging species. This may be due to a combination of its energy value as a food
source and its relative accessibility (the queen scallop shell is much more fragile than
that of a scallop or a whelk). It is also possible that Echinus esculentus is an attractive
prey, but mono-specific baits of this species were not used. The lack of trials using all
by-catch taxa individually obviously limits the interpretation possible here, but it can be
concluded that scavengers can discern prey species from odour plumes. This has
previously been demonstrated for some scavenging species, e.g. hermit crabs (Thacker,
1996; Rittschof and Hazlett, 1997).
The movement of scavengers towards dredge tracks or discarded by-catch is probably
stimulated by chemosensory stimuli (Warner, 1979; Lapointe and Sainte-Marie, 1992;
Nickell and Moore, 1992a,b). The bottom current velocity will therefore play an
important role in both the distribution of carrion odours, and the speed of aggregation of
the different scavenging species. There is some supporting evidence from these
experiments, i.e. Asterias rubens move up current towards the bait. However, the lack of
L.O. Veale et al. / J. Exp. Mar. Biol. Ecol. 255 (2000) 111 – 129
127
correlation observed after baiting with mixed damaged benthos appears to contradict
this, especially as these baits attracted the greatest number of scavengers. Possibly the
larger total numbers of animals aggregating on these baits provided other cues (e.g.,
auditory) which attracted randomly distributed A. rubens from the surrounding area
(including upstream).
The findings of this study are specific to the area under investigation, and other studies
(e.g., Ramsay et al., 1997b) have identified large variation between scavenger responses
in different habitats. However, the present study was sited in a closed area adjacent to
one of the most productive scallop grounds in the Irish Sea (Brand et al., 1991), and so
responses may be considered typical for the seabed types dredged for scallops in this
area.
These findings demonstrate that a wide range of scavenger species benefit from the
input of by-catch material, over an extended time period after discarding. The prey
species vary in attractiveness to scavengers, and there is evidence to suggest that the
inclusion of queen scallops, Aequipecten opercularis, in the discards may increase the
size of scavenger aggregation. The damage sustained by the by-catch may not be as
instrumental in determining attraction of predators as initially assumed. It is apparent
that undamaged animals in unnaturally high densities may be sufficient to bring about a
scavenger aggregation.
Acknowledgements
The authors would like to thank the crew of the RV Sula and all the divers who
helped, especially M. Bates and M. Mosley. This study was funded by the Ministry of
Agriculture, Fisheries and Food as part of contract number CSA 2332. The work also
drew on the resources provided by the Isle of Man Department of Agriculture, Fisheries
and Forestry scallop research project for which we are most grateful. [RW]
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