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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
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
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 examined100 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
L .O. Veale et al. J. Exp. Mar. Biol. Ecol. 255 2000 111 –129
117 Table 1
Taxa observed throughout the duration of the study, and their percentage and rank occurrence in all video frames examined
Phylum Species taxa
Rank occurrence
occurrence Crustacea
Atelecyclus rotundatus 0.2
15.5 Cancer pagurus
4.1 7
Inachus spp 0.3
12 Liocarcinus spp
15.5 3
Pagurus spp 17.1
2 Mollusca
Eledone cirrhosa 0.1
18 Nudibranch 1
0.1 18
Nudibranch 2 0.3
12 Nudibranch 3
0.3 12
Echinodermata Asterias rubens
45.2 1
Astropecten irregularis 12.3
4 Crossaster papposus
0.02 21
Porania pulvillus 1.9
8 Ophiocomina nigra
0.2 15.5
Ophiura spp 0.3
12 Ophiothrix fragilis
0.3 12
Pisces Agonus cataphractus
0.5 9
Callionymus lyra 10.9
5 Flatfish
4.5 6
Synagnathus sp 0.1
18 Scyliorhinus caniculata
0.01 22
Triglidae 0.05
20
2
bait within the 2 m 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
Time of day Interaction
white or red day or night
Light3Time of day Asterias rubens
F 5 9.5, P , 0.05 F 5 0.6, P 5 0.47
F 5 0.1, P 5 0.71 Astropecten irregularis
F 5 89.0, P , 0.01 F 5 49.7, P , 0.01
F 5 40.0, P , 0.01
Cancer pagurus F 5 1.1, P 5 0.33
F 5 1.1, P 5 0.33 F 5 17.9, P , 0.01
Liocarcinus spp F 5 6.3, P , 0.05
F 5 4.8, P 5 0.07 F 5 0.6, P 5 0.46
Callionymus lyra F 5 0.1, P 5 0.78
F 5 1.3, P 5 0.29 F 5 0.3, P 5 0.60
Pagurus spp F 5 22.0, P , 0.01
F 5 0.0, P 5 0.95 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, F
5 18.61, P , 0.05, Astropecten irregularis ANOVA, F 5 8.24, P ,
1,4 1,4
0.05, Cancer pagurus ANOVA, F 5 22.47, P , 0.01 and the data for Pagurus spp
1,4
L .O. Veale et al. J. Exp. Mar. Biol. Ecol. 255 2000 111 –129
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.
L .O. Veale et al. J. Exp. Mar. Biol. Ecol. 255 2000 111 –129
121 Table 3
2
Maximum densities of some species observed at the bait within the 2 m field of view, compared to
background density estimates obtained from 16 diver surveys in the area at the time Species
Maximum density Mean back-
Size of observed at bait
ground density aggregation
2 2
Nos. per 2 m Nos. per 2 m
3 increase field of view
Asterias rubens 8
0.18 44
Astropecten irregularis 8
0.04 200
Pagurus spp 7
0.09 78
Liocarcinus spp 4
0.10 40
Callionymus lyra 5
0.07 70
were significant at the 10 level ANOVA, F 5 6.16, P 5 0.068. The second baiting
1,4
occasion showed significant differences only for A . rubens ANOVA, F
5 16.71,
2,5
P , 0.01, and flatfish ANOVA, F 5 5.91, P 5 0.05. Subsequent Tukeys multiple
2,5
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: F 5 9.79, P , 0.05. Both
1,4
Pagurus spp and Cancer pagurus abundances increased significantly after baiting with both damaged and undamaged queens ANOVA: F
5 157.4, P , 0.01 and F 5
2,6 2,6
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: F
5 18.1, P , 0.05, being more abundant
2,6
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.
L .O
. V
eale et
al .
J .
Exp .
Mar .
Biol .
Ecol .
255 2000
111 –
129
123 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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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
Asterias rubens, Pagurus spp, Cancer damaged and undamaged
pagurus, Callionymus lyra Pecten maximus
Asterias rubens, Astropecten irregularis, undamaged
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: F 5 19.6, P , 0.05, Astropecten irregularis
1,4
F 5 25.2, P , 0.01 and Pagurus spp ANOVA: F
5 9.3, P , 0.05.
1,4 1,4
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: F
5
1,4
10.3, P , 0.05, and after baiting with damaged B . undatum, significant increases were
observed in Callionymus lyra ANOVA: F 5 36.4, P , 0.01, flatfish ANOVA: F
5
1,4 1,4
106.6, P , 0.01, and Liocarcinus spp ANOVA: F 5 17.2, P , 0.05.
1,4
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–
2
Circular Correlation Coefficient: nr 5 10.02, P , 0.05, badly damaged A . rubens
2 2
CCCC: nr 5 11.19, P , 0.05, damaged Aequipecten opercularis CCCC: nr 5 23.20,
2
P , 0.001, and undamaged A . opercularis CCCC: nr 5 31.08, P , 0.001, but was
2
not correlated with water direction during two control periods studied CCCC: nr 5
2
1.52, P . 0.05 and nr 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
2 2
two baitings with mixed damaged benthos CCCC: nr 5 5.11, P . 0.05 and nr 5 1.56, P . 0.05.
4. Discussion
