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, perpen-
dicular 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.
L .O. Veale et al. J. Exp. Mar. Biol. Ecol. 255 2000 111 –129
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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
2
200 m 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 differ- ences. 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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.O. Veale et al. J. Exp. Mar. Biol. Ecol. 255 2000 111 –129
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
2
correlation coefficient Batschelet, 1981. The test statistic nr is compared with
2
tabulated values for x
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
