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Changes in Irish Sea Benthos: Possible Effects of 40 years of Dredging
Estuarine, Coastal and Shelf Science (1999) 48, 739–750 Article No. ecss.1999.0476, available online at http://www.idealibrary.com on
Changes in Irish Sea Benthos: Possible Effects of 40 years of Dredging A. S. Hill, L. O. Veale, D. Pennington, S. G. Whyte, A. R. Brand and R. G. Hartnoll Port Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man IM9 6JA Received 28 August 1998 and accepted in revised form 22 January 1999 From 1946 to 1951 Dr N. S. Jones sampled the benthos around the south of the Isle of Man from over 200 sites. Multivariate methods have been used here to compare subsets of this historical data with recent data from the same locations: of these locations some have been subject to heavy scallop dredging over the intervening 40 plus years and some to little dredging. Clear changes were apparent regardless of scallop dredging intensity. Some of the changes in the heavily dredged areas were those expected to result from extreme physical disturbance—an increased polychaete mollusc ratio, loss of some fragile species, and an increase in the predominance of scavenger/predator species. However, changes in the lightly dredged areas also included the loss of a number of species including some potentially fragile tube-dwellers. Reasons for these changes were not apparent. 1999 Academic Press Keywords: benthos; dredging impacts; Irish Sea
Introduction Intensive bottom fishing activities, especially by gear such as scallop dredges that dig into the sea-bed, are expected to have major impacts upon benthic communities (for example Bullimore, 1985; Thrush et al., 1995, 1998; Hill et al., 1997; Collie et al., 1997). However, until recently the evidence for this has been largely anecdotal. Several approaches have been used to try and obtain more objective evidence on the effects of dredging. The simplest method is to compare dredged and undredged areas, or areas subject to very different levels of dredging (Hall et al., 1993). However, it becomes very difficult to isolate the effects of dredging from the complex array of other variable factors that impinge upon benthic fauna. Another approach has been to undertake controlled experimental dredging such that the effect can be monitored (e.g. Bullimore, 1985; Eleftheriou & Robertson, 1992; Thrush et al., 1995; Currie & Parry, 1996; Hill et al., 1997). A major problem has been to find suitable experimental sites that have not already been affected by dredging, and within which the intensity and location of dredging can be adequately controlled. It has sometimes been possible to use areas with low levels of commercial fishing (e.g. Eleftheriou & Robertson, 1992; Hall et al., 1993; Currie & Parry, 1996; Kaiser & Spencer, 1996), and less commonly areas closed to 0272–7714/99/060739+12 $30.00/0
commercial fishing have been utilized (Hill et al., 1997; Tuck et al., 1998). A third approach has been to close areas to commercial dredging and observe the subsequent changes in the benthic community (Hill et al., 1997). Again there are problems, first in designating comparable controls which are still subject to dredging, and second in terms of the time scale needed to detect ‘ recovery ’ from long-term dredging disturbance. Given the inherent problems in the above methods, there is a possible alternative—namely to use a control dating from before the initiation of intensive fishing. This is possible where adequate historical data on the sea-bed communities are available, and where intensive dredging is a relatively recent event. This approach still presents problems, in that temporal changes can still be attributed to other variables besides dredging, but it merits consideration as it may supplement the other methods. The scallop fishing grounds around the south of the Isle of Man conform to the requirements for such a temporal comparison. Over the period 1946 to 1951, Dr N. S. Jones of this laboratory carried out an extensive survey of the subtidal benthic epifauna and infauna around the south of the Isle of Man. That survey comprised sampling by trawl, dredge and grab at over 200 sites (Figure 1), with identification to species level (Jones, 1950, 1951, 1956). The results were published in a condensed form, but the original 1999 Academic Press
740 A. S. Hill et al.
been taken from within the area of Jones’ survey. Samples are available from: (1) areas known to have been subject to heavy dredging pressure and (2) areas thought to have not been dredged, or only lightly dredged, since the 1960s. These results were compared with the results of Jones’ survey using multivariate analyses, to investigate whether changes have occurred over time, and whether these changes are restricted to, more pronounced, or different in type, in heavily dredged areas. Materials and methods
F 1. Sites surveyed by N. S. Jones in the 1940s and 1950s. Sampling gears: Dredge ( ); Trawl ( ); Grab ( ).
raw data are available and have been recently updated and re-analysed (Pennington et al., 1998). It is also known that scallop dredging within the area was very limited in scale before the 1960s (Brand et al., 1991), and fortunately the area of Jones’ survey overlaps a number of the major scallop fishing grounds (Figure 2). Through various recent Port Erin research programmes, benthic faunal samples have
Details of the methods used to update, store and re-analyse the data from Jones’ original surveys have already been described (Pennington et al., 1998). The same multivariate methods (using the PRIMER computer package—Clarke & Warwick, 1994) were used to compare the original survey data with those collected during recent surveys of the same areas. Sites sampled during Jones’ surveys which coincided with recent data sets were isolated by extracting samples using calculated latitude and longitude values corresponding to positions of recent sampling sites. The recent samples were obtained as part of a MAFFfunded project to study the environmental impact of scallop dredging (referred to here as the ‘ dredge disturbance project ’) (Hill et al., 1997) and a research
F 2. Fishing grounds for the scallop (Pecten maximus) around the Isle of Man. Major fishing grounds are bounded by solid lines. The dotted outline indicates where scallops occur and are occasionally fished (modified from Hill et al., 1997).
Changes in Irish Sea benthos 741 A B CD E F G H I J K L MN O P Q R S T U V WX 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
F 3. Mean annual dredging effort (in metre hours) within each 5 nautical mile square in the northern Irish Sea from 1982 to 1995 (modified from Hill et al., 1997). >4000 m h yr 1 ( ), 1000–4000 m h yr 1 ( ), <1000 m h y 1 ( ). The grid positions of the grounds compared are: Bradda (K11), Chickens (J12) and Muddy sand (J10).
project to study muddy sand communities (the ‘ muddy sand project ’) (Whyte, 1994). Dredge disturbance project Data comparable with Jones’ records are available for the Chickens and Bradda Inshore commercial fishing grounds (see Figure 2). Bradda Inshore was the first area to be dredged for scallops in 1937, while dredging on the Chickens ground was first recorded in 1950 (Brand et al., 1991). The available fishing effort data (Figure 3) indicate that both Bradda Inshore and Chickens have been fished heavily each year, at least since 1982 when effort records began, but probably for much longer. The annual mean efforts on each ground were 7919 metre hours and 3899 metre hours per 5 nautical mile square respectively, equivalent to annual swept coverages of approximately 40 and 20% (though this would be more intense locally). Jones’ sampling employed a wide range of gear, and sites have been selected for comparison where similar gear types were used in the more recent studies. Chickens was sampled by Jones using a 0·1 m2 (chain rigged) Van Veen grab in 1952 (52 samples over two sites) and by a 0·1 m2 Day grab during the recent programme (12 samples over two sites); Bradda Inshore was sampled by Jones in 1946 (three samples) using an unspecified dredge type which contained a
canvas liner of small but unspecified mesh size. This was compared with recent data gathered using scallop dredges (28 samples), adapted with short queen teeth and a 1 cm mesh liner to catch a greater proportion of benthic animals, as well as 0·1–m2 Day grab data from the same area (14 samples). Recent data from both grabs and adapted scallop dredges were included in the comparison since the dredging gear used by Jones would have retained far more of the smaller species than our modern dredges, and his data included records for species that are commonly caught in Day grabs. Direct comparison of infaunal species abundance from the Chickens ground was possible between the historical and recent data sets because of the similarity of the grabbing apparatus, despite some differences in the efficiency of the gears used (Riddle, 1989). However, analysis of the epifaunal data was based on presence–absence descriptions of community composition. This is because comparisons were based on semi-quantitative dredging surveys, for which there are no available records of tow length for the earlier studies; but also because data from dredges were compared with data from grabs that sample much smaller areas. Univariate indices of community diversity and equitability were calculated for the Chickens data but were not possible for the Bradda data for reasons described above. Community composition was compared using the multivariate techniques of cluster analysis and multidimensional scaling ordination based on Bray-Curtis similarities after both fourthroot and presence–absence transformation of the quantitative Chickens data. Multivariate analysis of the Bradda Inshore data was based on the presence– absence transformation alone. Data were standardized to overcome effects of different sediment volumes, and the rarest 3% of species were removed. Determining whether changes in infaunal communities are a response to dredging is difficult. Analysis of the Chickens data therefore included an examination of the ratios of polychaetes to molluscs at each site. Expectations were that the increase in dredging activity since the 1950s would lead to more heavily disturbed communities and higher polychaete to mollusc ratios (see ICES, 1996 for discussion). Muddy sand project Another study was initiated in 1992 to specifically re-sample an area corresponding to a limited number of Jones’s sites, on the muddy sand ground north-west of Bradda Head. This area was considered to be
742 A. S. Hill et al.
relatively unaffected by the scallop fishery—mean annual dredging effort <1000 metre hours (Figure 3). Jones (1956) lists species counts for seven sites, each comprising 10–12 pooled Van Veen grab samples, but the unpooled data for five of these were found in one of his handwritten notebooks. These samples were taken at various times of the year throughout 1952. Whyte (1994) re-sampled the area with 10 Day grab samples taken in December 1992. Analysis was conducted as before, using both univariate and multivariate techniques, the latter on standardized, fourth-root transformed data, with 3% of species removed. Additionally, the polychaete: mollusc ratios were compared.
Total number of species 80 60 40 20 0
Between age groups Between site groups Age × site Na
Nb
Da
Between age groups Between site groups Age × site Na
Nb
Da
Shannon Wiener diversity 6
Between age groups Between site groups Age × site
2 0
Dredge disturbance project Species number, richness and Shannon-Weiner diversity were all significantly greater in the recent samples from the Chickens ground than the historical samples (Figure 4, two-way ANOVA) although there were also significant differences between individual sites within both the historical and recent samples. There were no differences in Pielou’s evenness but Simpson’s dominance was significantly greater in the historical samples. Historical species composition was clearly different from the recent composition on the Chickens ground, with cluster analysis separating the two groups at 12% similarity in both presence-absence (ANOSIM: Global R=0·825, P<0·01) and fourth-root (ANOSIM: global R=0·877, P<0·01) transformed data (Figure 5). Although there was overlap between the historical and recent samples in community composition and some differences between the sites sampled on each occasion (Figure 6), the suite of species that dominated the historical samples could be distinguished from those dominating the more recent samples. Differences in species composition included the presence of Brissopsis lyrifera and Echinocardium flavescens in the historical samples but not the recent samples. The appearance of these larger and fragile species in grab samples, albeit infrequently, suggests they occurred in quite high densities pre-1950. The bivalve Corbula gibba was also found in relatively high numbers in the historical samples but not in the recent samples. Conversely, a number of tube dwelling polychaetes were notably more abundant in the recent samples, these included Chone fauveli, Euchone rubrocincta and Ampharete lindstroemi. Lepidochiton asellus and Timoclea ovata were also more abundant in the recent samples. A higher polychaete to mollusc
Na
Nb
Da
Between age groups Between site groups Age × site
0.9 0.8 Na
Nb
Da
Between age groups Between site groups Age × site
0.2 0.1 Na
Nb
Da
57.4 <0.01 3.1
0.09
F P 97.9 <0.01 16.6 <0.01 2.5
0.12
F 4.0
P 0.05
3.1
0.08
0.6
0.44
Db
Simpson's dominance
0
F P 157.2 <0.01
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0
9.9 <0.01
Db
4
Results
71.4 <0.01
Db
Species richness 8 6 4 2 0
F P 149.9 <0.01
F P 11.8 <0.01 0.7
0.42
1.2
0.27
Db
F 4. Diversity and equitability indices are calculated for historical grab data (sites Na, Nb) and recent grab data (sites Da, Db) from the Chickens commercial ground. Results of two-way ANOVA are shown.
ratio was found within the recent samples (3·40·41 SE) compared with the historical samples (1·700·16 SE) and these differences were highly significant (t-test on natural log transformed data, t= 21·31, P<0·01). There is no reason why the gears used on the two surveys should sample bivalves or polychaetes differently. The canvas bag-lined dredge used by Jones on the Bradda Inshore ground was not identical to any of the modern sampling methods used in the recent study at that site, although samples taken with a combination of grabs and adapted scallop dredges would have sampled a similar range of species. Only three samples were taken by Jones at this site. This, and the mismatch between sampling gears, means that only very tentative analysis of presence–absence data is possible and any conclusions must be treated with caution.
Changes in Irish Sea benthos 743
F 5. Historical (Na & Nb) and recent (Da & Db) samples from the Chickens ground grouped by cluster analysis (a, c) and multidimensional scaling ordination (b, d) after fourth root (a, b) and presence–absence (c, d) transformation.
Historical and recent samples taken from the Bradda Inshore ground clearly differed in terms of species composition (Figure 7), though it must also be noted that the composition of recent samples differed with gear type, and for the fine mesh dredge samples between 1994 and 1996. Given the profound gear differences, no formal test of significance (ANOSIM) was attempted here. The only species found in the historical samples that were absent from the recent samples were three amphipods, Synchelidium haplocheles, Urothoe¨ marina and Bathyporeia gracilis, an isopod, Microjassa cumbrensis and a tube dwelling polychaete, Anothrobus gracilis (Figure 8). Notably, most of the epifaunal scavenger species present in the recent samples, and frequently found during surveys of the Bradda Inshore ground and on other fished grounds, were absent from the historical samples—e.g. Asterias rubens, Neptunea antiqua, Buccinum undatum, Cancer pagurus.
Resampling on muddy sand ground There was a significant reduction in the number of individuals per grab recorded in Whyte’s samples compared to Jones’ samples. Additionally, Pielou’s evenness increased and Simpson’s dominance decreased, both significantly, between 1952 and 1992 (Table 1). The MDS plot of the data shows Whyte’s samples clustering distinctly from Jones’, which themselves cluster fairly neatly into date groups (Figure 9). Subsequent formal analysis revealed a highly significant difference between the six datasets (ANOSIM: Global R=0·643, P<0·01), and multiple comparisons between all possible pairs of date groups—using an adjusted á value derived from the Dunn-S { ida´k method—all proved to be significant. The relative abundance of each species in each sample group is illustrated in Figure 10, incorporating also Jones’s two additional sets of 10 pooled grab
744 A. S. Hill et al.
Historical samples Na
Nb
Recent samples Da Db
Harmothoe fragilis Euchone rubrocincta Ampharete lindstroemi Chone fauveli Eumida bahusiensis Aonides paucibranchiata Euclymene spp Pholoe inornata Eulalia mustela Timoclea ovata Lepidochiton asellus Emarginula fissura Limaria hians Atylus vedlomensis Psammechinus miliaris Eulalia bilineata Harmothoë lunulata Ophiura albida Pectinaria koreni Scalibregma inflatum Hiatella arctica Ampelisca tenuicornis Polycirrus spp Amphipholis squamata Echinocyamus pusillus Notomastus latericeus Terebellides stroemi Owenia fusiformis Lumbrineris spp Harpinia spp Leptosynapta inhaerens Ampelisca spinipes Spiophanes bombyx Nephtys incisa Pectinaria auricoma Amphiura filiformis Laonice cirrata Trichobranchus glacialis Amphicteis gunneri Myriochele heeri Diplocirrus glaucus Minuspio cirrifera Streblosoma bairdi Gattyana cirrosa Arabella iricolor Phaxas pellucidus Corbula gibba Abra prismatica Nucula sulcata Dosinia lupinus Turritella communis Cardium scabrum Antalis entalis Aporrhais pespelecani Azorinus chamasolen Limatula subauriculata Cirolana borealis Mysia undata Nephrops norvegicus Anapagurus laevis Iphinoë serrata Leptognathia gracilis Ophiura affinis Alpheus glaber Echinocardium flavescens Amphiura chiajei Brissopsis lyrifera
F 6. Abundance of species (97%) across historical and recent samples from the Chickens ground. Shade intensity increases with abundance.
Changes in Irish Sea benthos 745
10
Brissopsis lyrifera, were among the most significantly different species. There was a highly significant difference between the polychaete:mollusc ratios calculated from Jones’ (1·00·21 SE) and Whyte’s (1·580·37 SE) individual samples (t-test: t= 2·79, P<0·01), using natural log transformed data, with a greater number of polychaetes found in the recent (1992) sample group.
(a)
Bray-Curtis similarity
20 30 40 50 60 70 80
Discussion
90 100
(b)
Dredge disturbance project d
g (1994)
d
f (1996)
f (1994)
Stress = 0.034
F 7. Bradda Inshore ground. Historical samples taken using a canvas-lined dredge (d), and recent samples taken using a 0·1 m2 Day grab (g) and queen dredge lined with a fine (1 cm) mesh (f). Cluster analysis (a) and MDS ordination (b) are based on presence–absence data.
samples. Replicate grabs have been pooled for ease of presentation. Species consistently contributing less than 1% of total numbers were excluded, and species were sorted in order of importance in Whyte’s data set. Some species appear more abundant in the recent samples e.g. the brittlestars, Amphiura chiajei and Amphipholis squamata, the bivalve Nucula sulcata and the worm, Lumbrineris sp. Conversely, a large number of species were found in the historical samples, but were absent from the recent ones, possibly reflecting the difference in samples size. However, the potentially vulnerable tube-dwelling amphipod, Ampelisca spp., and the urchins, Echinocardium flavescens and
Of the large number of samples taken by Jones during the 1946 to 1952 period only a small number could be realistically compared with samples taken during recent sampling programmes at Port Erin. However, it was possible to compare sites that were located on the Bradda Inshore and Chickens commercial scallop fishing grounds. Both of these grounds have been fished for scallops on a commercial basis from at least 1950 and the indications were that each ground had been fished fairly consistently each year, so they represented good sites for comparison with the recent data sets. Comparisons were further hampered by mismatches in the types of gear used to take samples at each location. The gear used to sample the Bradda Inshore ground in Jones’ study was a dredge of unspecified type with a canvas bag liner, presumably of a very small mesh size. This would have filled quickly and consequently it would have been towed for only short distances over the sea bed, retaining a large proportion of the smaller species. It is likely that the samples taken with this gear would have resembled those of a modern anchor dredge which has not been used during the recent programme. The Day grab samples and dredge samples (with a 1 cm mesh liner) used during the recent programme at these sites represented the closest possible comparison, although the differences in gear efficiency limited the analysis of the Bradda data to presence–absence comparisons only. In 1950 the Chickens ground was sampled with a 0·1 m2 Van Veen grab which was comparable to the 0·1 m2 Day grab used at the corresponding sites in the recent study. However, the former is a lighter piece of equipment, and may be less efficient (Riddle, 1989). A further confounding factor is the nature of the sediments at the time of sampling, which may have been different in 1946–1950. Only very limited data are available in the historical data sets that describe substratum type and this would not assist the detection of significant differences between samples at an infaunal level.
1 cm mesh lined dredge
1 cm mesh lined dredge
Historical samples Canvas bag lined dredge
Aug 1994
Jan 1996
Oct 1947 Aug 1947 Sep 1947
746 A. S. Hill et al. Recent samples Grab samples
Moerella donacina Nematoneris unicornis Polycarpa spp Bodotria arenosa Lumbrineris gracilis Amphipholis squamata Owenia fusiformis Aonides paucibranchiata Chone fauveli Pomatoceros triqueter Euclymene spp Notomastus latericeus Lepidochiton asellus Pista cristata Echinocymus pusillus Timoclea ovata Atylus vedlomensis Lysiannassa plumosa Ampelisca diadema Maera othonis Melphidipella macra Phtisica marina Ophelia spp Echinocardium flavescens Photis longicaudata Orchomene nana Microjassa cumbrensis Bathyporeia gracilis Synchelidium haplocheles Anobothrus gracilis Urothoë marina Aequipecten opercularis Porania pulvillus Ophiocomina nigra Pecten maximus Cellaria fistulosa Alcyonium digitatum Aporrhais pespelecani Nemertesia antennina Echinus esculentus Eledone cirrhosa Ophiothrix fragilis Scyliorhinus canicula Cancer pagrus Pisidia longicornis Agonus cataphractus Buccinum undatum Anseropoda placenta Neptunea antiqua Astropecten irregularis Crossaster papposus Luidia sarsi Asterias rubens Marthasterias glacialis
Aug 1994 xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x x x xx x xx x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x x x x xxxx x xx x x x x x x x x x x x xx x x xx x x x x x x x xxx x x xx x x x
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x x xx x xxx xx x x x xx x x x x x x x x x x x x x x xx x x x x x x x x x x x x x x xx x x x x x x x x x x x x x xx x x x x x x x x x x x xx x x x x x xx x x x xxx x x x x x x x x x x xxx x x x x x xx x x x xxx x x
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F 8. Bradda Inshore ground. The presence or absence of species in historical canvas-lined dredge samples compared with recent grab and 1 cm mesh-lined dredge samples.
Despite these limitations it is argued that meaningful comparisons have been made by adopting a cautious approach in the interpretation of the results. Comparison of the community structure on the Chickens ground was limited to infaunal species since only grabbing gear was used in the 1950 surveys. Notable differences included the occurrence of the fragile echinoids, Brissopsis lyrifera and Echinocardium flavescens, and high densities of Corbula gibba, a
burrowing bivalve, in the historical samples that were absent from the recent samples. Other differences included higher abundances of a number of tubedwelling worms, a sedentary chiton and a bivalve in the recent samples relative to the historical samples. These sedentary species might be expected to do less well after prolonged periods of dredging, although it is difficult to predict the response of individual species to the effects of dredging, and dredging may result in
Changes in Irish Sea benthos 747 T 1. Comparison of some univariate indices of community structure calculated from samples collected by Norman Jones in 1952 (N=52) and Se´amus Whyte in 1992 (N=10) Jones (1952)
Total number of species, S Total number of individuals, N Species richness, d Shannon Weiner diversity, H Pielou’s evenness, J Simpson’s dominance, SI a
Whyte (1992)
Mean
se
Mean
se
t
P
18·26 55·60 4·34 2·22 0·81 0·19
1·13 4·75 0·20 0·06 0·01 0·01
12·90 23·70 3·76 2·33 0·92 0·12
1·03 1·83 0·24 0·07 0·01 0·01
1·81 2·83 1·26 0·73 3·28 2·31
0·07 <0·01b 0·21 0·47 <0·01b 0·02a
Significant at the 5% level. bSignificant at the 1% level.
bringing greater numbers of stones to the surface to serve as attachment surfaces. One interpretation of the infaunal data is to apply the hypothesis that disturbed areas support a greater proportion of polychaetes to molluscs (ICES, 1996). This hypothesis was supported by the Chickens data and is good evidence that the differences in the assemblages were due to dredge disturbance. There is evidence that one of the effects of dredging is to create coarser, and possibly less mobile sediments (ICES, 1973; Hill et al., 1997; see Kaiser & Spencer, 1996 for discussion of mechanism) which will favour many sedentary species. Long-term
F 9. Muddy sand ground. Multidimensional scaling plot of grab sample data collected by Norman Jones (J) in 1952 (a—February, b—April, c—June, d—September, e—December) and Se´amus Whyte (W) in December 1992.
changes in infaunal species composition are likely to be in response to a number of factors in combination, including direct effects upon the animals by the gear, but also through substratum alteration. Analysis of the Bradda Inshore data revealed a small number of infaunal species that were sampled in 1946 but not recorded in the recent samples. Given the small sample sizes taken in 1946, and the problems of interpreting infaunal changes in terms of fishing activity, the significance of these differences remains doubtful. More interesting, however, was the absence of a number of epifaunal scavenger and predator species from the samples taken in 1946. These species are dominant in most epifaunal samples taken from Bradda Inshore in recent times including those used in this comparison. This is consistent with predictions of the effects of fishing where high densities of scavengers would be expected on fished grounds (ICES, 1996) as they exploit the damaged and discarded material, and in this case of asteroid echinoderms, survive better the effects of being caught in dredges. However, the recent data were only compared with three historical samples which would have effectively covered a fairly small area. This study has demonstrated that community structure has changed considerably since 1950 despite the problems of comparing samples taken with different gear types in surveys that were not designed specifically for this purpose. In the case of the Chickens infaunal data changes in the ratio of polychaetes to molluscs link these changes to dredging. In addition, the predominance of scavengers in the benthic communities on Bradda Inshore also indicates a strong effect of dredging. However, these comparisons would be improved significantly by re-surveying more of the sites originally sampled in 1950. It would also be preferable to re-sample the sites using gear types that are identical to those used in the earlier surveys in
Jones E
Jones D
Jones C
Jones B
Jones A
Jones F2
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748 A. S. Hill et al.
Amphiura chiajei Nucula sulcata Abra prismatica Lumbrineris spp Owenia fusiformis Corbula gibba Amphipholis squamata Notomastus latericeus Amphiura filiformis Glycera rouxi Myriochele heeri Phoronis muelleri Glycera gigantea Chaetozone setosa Phaxas pellucidus Pectinaria spp Iphinoe serrata Azorinus chamasolen Goniada maculata Turritella communis Laonica cirrata Molgula occulta Echinocyamus pusillus Spio filicornis Antalis entalis Dosinia lupinus Harpinia pectinata Amphicteis gunneri Trichobranchus glacialis Diplocirrus glaucus Brissopsis lyrifera Prionospio spp. Parvicardium scabrum Astrorhiza limicola Echinocardium flavescens Ampelisca spp
F 10. Muddy sand ground. Standardized infaunal species abundance in samples collected by Jones in 1952 and Whyte in 1992. Shading intensity is proportional to abundance. 1% of species have been removed.
order to overcome the confounding effects of differences in gear efficiency. Some sediment description is also available in the historical data although comparable observations had not been made for most of the
sites sampled during the recent surveys. Additional surveys employing acoustic methods of seabed discrimination (e.g. sidescan sonar, RoxAnn) would, therefore, help to examine any changes in the surface
Changes in Irish Sea benthos 749
structure of sediments which would be valuable in the interpretation of changes of infaunal communities in particular. Resampling on muddy sand ground The distinct differences between the data produced by Jones in 1952–1953 and Whyte in 1992 suggest that the community has altered dramatically in the interim period. Given that this site was chosen for re-sampling because apparently it had not been exposed to high levels of commercial fishing, it must be assumed that other factors have caused the change. These may include long-term natural biological cycles or a change in the substratum type brought about by natural or anthropogenic causes. It has been suggested that dredging activities may resuspend large amounts of sediment, and the lighter fractions (silt) may be translocated and deposited elsewhere (Hill et al., 1997; ICES, 1996). This effect could potentially lead to a change in substratum type in areas which are not fished directly, and smothering of sensitive filter feeding species, though other filter feeders may benefit from an increased food supply (Hill et al., 1977; Morton, 1977). It has been considered that the low mean annual fishing effort recorded for the 55 nautical mile square enclosing this site could have been concentrated around the study site, resulting in a localized area of higher fishing disturbance. This has been ruled out, however, as the substratum at the site is soft mud, and does not support any scallops, and as such would not be fished by commercial fishermen. It is apparent that some potentially vulnerable, tube-dwelling species have decreased in abundance since the 1950s, but further interpretation of the results obtained would require much greater knowledge of the ecology of the species involved. However, the significant increase in the polychaete:mollusc ratio from 1952 to 1992 suggests that the observed changes in community structure could be related to fishing disturbance effects (ICES, 1996), albeit indirectly. Conclusions It is clear that the observed changes in the benthic communities around the Isle of Man since the 1940s and 1950s may be partially due to the increases in commercial dredging effort, but this seems not to be the only cause. In order to interpret the significance of these changes, further study into the actual species involved is required: in particular some assessment of the direct effects of dredging on these species is necessary.
Acknowledgements This work would not have been possible without the original survey data of the late Dr Norman Jones: he generously made those available to us. The work was partly funded from two contracts: A study of temporal changes in Irish Sea benthos—contract number EPG 1/9/87 from D.O.E.; An assessment of the effects of scallop dredging on benthic communities—contract number CSA 2332 from MAFF. References Brand, A. R., Allison, E. H. & Murphy, E. J. 1991 North Sea scallop fisheries: a review of changes. In An international compendium of scallop biology and culture (Shumway, S. E. & Sandifer, P. A., eds). World Aquaculture Society, Baton Rouge, pp. 204–218. Bullimore, B. 1985 An investigation into the effects of scallop dredging within the Skomer Marine Reserve. Skomer Marine Reserve Subtidal Monitoring Project. Report to the Nature Conservancy Council. SMRSMP Rep. 3. Clarke, K. R. & Warwick, R. M. 1994 Change in Marine Communities: An Approach to Statistical Analysis and Interpretation. NERC, UK, 144 pp. Collie, J. S., Escanero, G. A. & Valentine, P. C. 1997 Effects of bottom fishing on the benthic megafauna of Georges Bank. Marine Ecology Progress Series 155, 159–172. Currie, D. R. & Parry, G. D. 1996 Effects of scallop dredging on a soft-sediment community: a large-scale experimental study. Marine Ecology Progress Series 134, 131–150. Eleftheriou, A. & Robertson, M. R. 1992 The effects of experimental scallop dredging on the fauna and physical environment of a shallow sandy community. Netherlands Journal of Sea Research 30, 289–299. Hall, S. J., Robertson, M. R., Basford, D. J. & Heaney, S. D. 1993 The possible effects of fishing disturbance in the northern North Sea: An analysis of spatial patterns in community structure around a wreck. Netherlands Journal of Sea Research 31, 201–208. Hill, A. S., Brand, A. R., Veale, L. O. & Hawkins, S. J. 1997 Assessment of the effects of scallop dredging on benthic communities. University of Liverpool Final Report to MAFF. Contract CSA 2332, 112 pp. ICES 1973 Effects of trawls and dredges on the sea-bed. International Council for the Exploration of the Sea, Report of the Chairman of ICES Gear and Behaviour Committee CM 1973/B:2. Copenhagen, Denmark. ICES 1996 Report of the working group on ecosystem effects of fishing activities. International Council for the Exploration of the Sea Working Group Report, CM 1996/Assess/Env:1. Copenhagen, Denmark. Jones, N. S. 1950 Marine bottom communities. Biological Reviews 25, 283–313. Jones, N. S. 1951 The bottom fauna off the south of the Isle of Man. Journal of Animal Ecology 20, 132–144. Jones, N. S. 1956 The fauna and biomass of a muddy sand deposit off Port Erin, Isle of Man. Journal of Animal Ecology 25, 217–252. Kaiser, M. J. & Spencer, B. E. 1996 The effects of beam trawl disturbance on infaunal communities in different habitats. Journal of Animal Ecology 65, 348–359. Morton, J. W. 1977 Ecological effects of dredging and dredge spoil disposal: a literature review. U.S. Fish and Wildlife Services. Technical Papers 94. Pennington, D., Veale, L. O. & Hartnoll, R. G. 1998 Re-analysis of an historical benthic data set from the Irish Sea. Estuarine, Coastal and Shelf Science 46, 769–776. Riddle, M. J. 1989 Bite profiles of some benthic grab samplers. Estuarine, Coastal and Shelf Science 29, 285–292.
750 A. S. Hill et al. Thrush, S. F., Hewitt, J. E., Cummings, V. J. & Dayton, P. K. 1995 The impact of habitat disturbance by scallop dredging on marine benthic communities: what can be predicted from the results of experiments? Marine Ecology Progress Series 129, 141–150. Thrush, S. F., Hewitt, J. E., Cummings, V. J., Dayton, P. K., Cryer, M., Turner, S. J., Funnell, G. A., Budd, R. G., Milburn, C. J. & Wilkinson, M. R. 1998 Disturbance of the marine benthic habitat by commercial fishing: Impacts at the scale of the fishery. Ecological Applications 8, 866–879.
Tuck, I. D., Hall, S. J., Robertson, M. R., Armstrong, E. & Basford, D. J. 1998 Effects of physical trawling disturbance in a previously unfished sheltered Scottish sea loch. Marine Ecology Progress Series 162, 227–242. Whyte, S. G. 1994 Changes in the macrobenthic invertebrate fauna of a muddy-sand deposit off the west coast of the Isle of Man, over a period of 40 years. University of Liverpool, M.Phil. thesis, 122 pp.
Changes in Irish Sea Benthos: Possible Effects of 40 years of Dredging A. S. Hill, L. O. Veale, D. Pennington, S. G. Whyte, A. R. Brand and R. G. Hartnoll Port Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man IM9 6JA Received 28 August 1998 and accepted in revised form 22 January 1999 From 1946 to 1951 Dr N. S. Jones sampled the benthos around the south of the Isle of Man from over 200 sites. Multivariate methods have been used here to compare subsets of this historical data with recent data from the same locations: of these locations some have been subject to heavy scallop dredging over the intervening 40 plus years and some to little dredging. Clear changes were apparent regardless of scallop dredging intensity. Some of the changes in the heavily dredged areas were those expected to result from extreme physical disturbance—an increased polychaete mollusc ratio, loss of some fragile species, and an increase in the predominance of scavenger/predator species. However, changes in the lightly dredged areas also included the loss of a number of species including some potentially fragile tube-dwellers. Reasons for these changes were not apparent. 1999 Academic Press Keywords: benthos; dredging impacts; Irish Sea
Introduction Intensive bottom fishing activities, especially by gear such as scallop dredges that dig into the sea-bed, are expected to have major impacts upon benthic communities (for example Bullimore, 1985; Thrush et al., 1995, 1998; Hill et al., 1997; Collie et al., 1997). However, until recently the evidence for this has been largely anecdotal. Several approaches have been used to try and obtain more objective evidence on the effects of dredging. The simplest method is to compare dredged and undredged areas, or areas subject to very different levels of dredging (Hall et al., 1993). However, it becomes very difficult to isolate the effects of dredging from the complex array of other variable factors that impinge upon benthic fauna. Another approach has been to undertake controlled experimental dredging such that the effect can be monitored (e.g. Bullimore, 1985; Eleftheriou & Robertson, 1992; Thrush et al., 1995; Currie & Parry, 1996; Hill et al., 1997). A major problem has been to find suitable experimental sites that have not already been affected by dredging, and within which the intensity and location of dredging can be adequately controlled. It has sometimes been possible to use areas with low levels of commercial fishing (e.g. Eleftheriou & Robertson, 1992; Hall et al., 1993; Currie & Parry, 1996; Kaiser & Spencer, 1996), and less commonly areas closed to 0272–7714/99/060739+12 $30.00/0
commercial fishing have been utilized (Hill et al., 1997; Tuck et al., 1998). A third approach has been to close areas to commercial dredging and observe the subsequent changes in the benthic community (Hill et al., 1997). Again there are problems, first in designating comparable controls which are still subject to dredging, and second in terms of the time scale needed to detect ‘ recovery ’ from long-term dredging disturbance. Given the inherent problems in the above methods, there is a possible alternative—namely to use a control dating from before the initiation of intensive fishing. This is possible where adequate historical data on the sea-bed communities are available, and where intensive dredging is a relatively recent event. This approach still presents problems, in that temporal changes can still be attributed to other variables besides dredging, but it merits consideration as it may supplement the other methods. The scallop fishing grounds around the south of the Isle of Man conform to the requirements for such a temporal comparison. Over the period 1946 to 1951, Dr N. S. Jones of this laboratory carried out an extensive survey of the subtidal benthic epifauna and infauna around the south of the Isle of Man. That survey comprised sampling by trawl, dredge and grab at over 200 sites (Figure 1), with identification to species level (Jones, 1950, 1951, 1956). The results were published in a condensed form, but the original 1999 Academic Press
740 A. S. Hill et al.
been taken from within the area of Jones’ survey. Samples are available from: (1) areas known to have been subject to heavy dredging pressure and (2) areas thought to have not been dredged, or only lightly dredged, since the 1960s. These results were compared with the results of Jones’ survey using multivariate analyses, to investigate whether changes have occurred over time, and whether these changes are restricted to, more pronounced, or different in type, in heavily dredged areas. Materials and methods
F 1. Sites surveyed by N. S. Jones in the 1940s and 1950s. Sampling gears: Dredge ( ); Trawl ( ); Grab ( ).
raw data are available and have been recently updated and re-analysed (Pennington et al., 1998). It is also known that scallop dredging within the area was very limited in scale before the 1960s (Brand et al., 1991), and fortunately the area of Jones’ survey overlaps a number of the major scallop fishing grounds (Figure 2). Through various recent Port Erin research programmes, benthic faunal samples have
Details of the methods used to update, store and re-analyse the data from Jones’ original surveys have already been described (Pennington et al., 1998). The same multivariate methods (using the PRIMER computer package—Clarke & Warwick, 1994) were used to compare the original survey data with those collected during recent surveys of the same areas. Sites sampled during Jones’ surveys which coincided with recent data sets were isolated by extracting samples using calculated latitude and longitude values corresponding to positions of recent sampling sites. The recent samples were obtained as part of a MAFFfunded project to study the environmental impact of scallop dredging (referred to here as the ‘ dredge disturbance project ’) (Hill et al., 1997) and a research
F 2. Fishing grounds for the scallop (Pecten maximus) around the Isle of Man. Major fishing grounds are bounded by solid lines. The dotted outline indicates where scallops occur and are occasionally fished (modified from Hill et al., 1997).
Changes in Irish Sea benthos 741 A B CD E F G H I J K L MN O P Q R S T U V WX 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
F 3. Mean annual dredging effort (in metre hours) within each 5 nautical mile square in the northern Irish Sea from 1982 to 1995 (modified from Hill et al., 1997). >4000 m h yr 1 ( ), 1000–4000 m h yr 1 ( ), <1000 m h y 1 ( ). The grid positions of the grounds compared are: Bradda (K11), Chickens (J12) and Muddy sand (J10).
project to study muddy sand communities (the ‘ muddy sand project ’) (Whyte, 1994). Dredge disturbance project Data comparable with Jones’ records are available for the Chickens and Bradda Inshore commercial fishing grounds (see Figure 2). Bradda Inshore was the first area to be dredged for scallops in 1937, while dredging on the Chickens ground was first recorded in 1950 (Brand et al., 1991). The available fishing effort data (Figure 3) indicate that both Bradda Inshore and Chickens have been fished heavily each year, at least since 1982 when effort records began, but probably for much longer. The annual mean efforts on each ground were 7919 metre hours and 3899 metre hours per 5 nautical mile square respectively, equivalent to annual swept coverages of approximately 40 and 20% (though this would be more intense locally). Jones’ sampling employed a wide range of gear, and sites have been selected for comparison where similar gear types were used in the more recent studies. Chickens was sampled by Jones using a 0·1 m2 (chain rigged) Van Veen grab in 1952 (52 samples over two sites) and by a 0·1 m2 Day grab during the recent programme (12 samples over two sites); Bradda Inshore was sampled by Jones in 1946 (three samples) using an unspecified dredge type which contained a
canvas liner of small but unspecified mesh size. This was compared with recent data gathered using scallop dredges (28 samples), adapted with short queen teeth and a 1 cm mesh liner to catch a greater proportion of benthic animals, as well as 0·1–m2 Day grab data from the same area (14 samples). Recent data from both grabs and adapted scallop dredges were included in the comparison since the dredging gear used by Jones would have retained far more of the smaller species than our modern dredges, and his data included records for species that are commonly caught in Day grabs. Direct comparison of infaunal species abundance from the Chickens ground was possible between the historical and recent data sets because of the similarity of the grabbing apparatus, despite some differences in the efficiency of the gears used (Riddle, 1989). However, analysis of the epifaunal data was based on presence–absence descriptions of community composition. This is because comparisons were based on semi-quantitative dredging surveys, for which there are no available records of tow length for the earlier studies; but also because data from dredges were compared with data from grabs that sample much smaller areas. Univariate indices of community diversity and equitability were calculated for the Chickens data but were not possible for the Bradda data for reasons described above. Community composition was compared using the multivariate techniques of cluster analysis and multidimensional scaling ordination based on Bray-Curtis similarities after both fourthroot and presence–absence transformation of the quantitative Chickens data. Multivariate analysis of the Bradda Inshore data was based on the presence– absence transformation alone. Data were standardized to overcome effects of different sediment volumes, and the rarest 3% of species were removed. Determining whether changes in infaunal communities are a response to dredging is difficult. Analysis of the Chickens data therefore included an examination of the ratios of polychaetes to molluscs at each site. Expectations were that the increase in dredging activity since the 1950s would lead to more heavily disturbed communities and higher polychaete to mollusc ratios (see ICES, 1996 for discussion). Muddy sand project Another study was initiated in 1992 to specifically re-sample an area corresponding to a limited number of Jones’s sites, on the muddy sand ground north-west of Bradda Head. This area was considered to be
742 A. S. Hill et al.
relatively unaffected by the scallop fishery—mean annual dredging effort <1000 metre hours (Figure 3). Jones (1956) lists species counts for seven sites, each comprising 10–12 pooled Van Veen grab samples, but the unpooled data for five of these were found in one of his handwritten notebooks. These samples were taken at various times of the year throughout 1952. Whyte (1994) re-sampled the area with 10 Day grab samples taken in December 1992. Analysis was conducted as before, using both univariate and multivariate techniques, the latter on standardized, fourth-root transformed data, with 3% of species removed. Additionally, the polychaete: mollusc ratios were compared.
Total number of species 80 60 40 20 0
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Between age groups Between site groups Age × site Na
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Shannon Wiener diversity 6
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Dredge disturbance project Species number, richness and Shannon-Weiner diversity were all significantly greater in the recent samples from the Chickens ground than the historical samples (Figure 4, two-way ANOVA) although there were also significant differences between individual sites within both the historical and recent samples. There were no differences in Pielou’s evenness but Simpson’s dominance was significantly greater in the historical samples. Historical species composition was clearly different from the recent composition on the Chickens ground, with cluster analysis separating the two groups at 12% similarity in both presence-absence (ANOSIM: Global R=0·825, P<0·01) and fourth-root (ANOSIM: global R=0·877, P<0·01) transformed data (Figure 5). Although there was overlap between the historical and recent samples in community composition and some differences between the sites sampled on each occasion (Figure 6), the suite of species that dominated the historical samples could be distinguished from those dominating the more recent samples. Differences in species composition included the presence of Brissopsis lyrifera and Echinocardium flavescens in the historical samples but not the recent samples. The appearance of these larger and fragile species in grab samples, albeit infrequently, suggests they occurred in quite high densities pre-1950. The bivalve Corbula gibba was also found in relatively high numbers in the historical samples but not in the recent samples. Conversely, a number of tube dwelling polychaetes were notably more abundant in the recent samples, these included Chone fauveli, Euchone rubrocincta and Ampharete lindstroemi. Lepidochiton asellus and Timoclea ovata were also more abundant in the recent samples. A higher polychaete to mollusc
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F 4. Diversity and equitability indices are calculated for historical grab data (sites Na, Nb) and recent grab data (sites Da, Db) from the Chickens commercial ground. Results of two-way ANOVA are shown.
ratio was found within the recent samples (3·40·41 SE) compared with the historical samples (1·700·16 SE) and these differences were highly significant (t-test on natural log transformed data, t= 21·31, P<0·01). There is no reason why the gears used on the two surveys should sample bivalves or polychaetes differently. The canvas bag-lined dredge used by Jones on the Bradda Inshore ground was not identical to any of the modern sampling methods used in the recent study at that site, although samples taken with a combination of grabs and adapted scallop dredges would have sampled a similar range of species. Only three samples were taken by Jones at this site. This, and the mismatch between sampling gears, means that only very tentative analysis of presence–absence data is possible and any conclusions must be treated with caution.
Changes in Irish Sea benthos 743
F 5. Historical (Na & Nb) and recent (Da & Db) samples from the Chickens ground grouped by cluster analysis (a, c) and multidimensional scaling ordination (b, d) after fourth root (a, b) and presence–absence (c, d) transformation.
Historical and recent samples taken from the Bradda Inshore ground clearly differed in terms of species composition (Figure 7), though it must also be noted that the composition of recent samples differed with gear type, and for the fine mesh dredge samples between 1994 and 1996. Given the profound gear differences, no formal test of significance (ANOSIM) was attempted here. The only species found in the historical samples that were absent from the recent samples were three amphipods, Synchelidium haplocheles, Urothoe¨ marina and Bathyporeia gracilis, an isopod, Microjassa cumbrensis and a tube dwelling polychaete, Anothrobus gracilis (Figure 8). Notably, most of the epifaunal scavenger species present in the recent samples, and frequently found during surveys of the Bradda Inshore ground and on other fished grounds, were absent from the historical samples—e.g. Asterias rubens, Neptunea antiqua, Buccinum undatum, Cancer pagurus.
Resampling on muddy sand ground There was a significant reduction in the number of individuals per grab recorded in Whyte’s samples compared to Jones’ samples. Additionally, Pielou’s evenness increased and Simpson’s dominance decreased, both significantly, between 1952 and 1992 (Table 1). The MDS plot of the data shows Whyte’s samples clustering distinctly from Jones’, which themselves cluster fairly neatly into date groups (Figure 9). Subsequent formal analysis revealed a highly significant difference between the six datasets (ANOSIM: Global R=0·643, P<0·01), and multiple comparisons between all possible pairs of date groups—using an adjusted á value derived from the Dunn-S { ida´k method—all proved to be significant. The relative abundance of each species in each sample group is illustrated in Figure 10, incorporating also Jones’s two additional sets of 10 pooled grab
744 A. S. Hill et al.
Historical samples Na
Nb
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Harmothoe fragilis Euchone rubrocincta Ampharete lindstroemi Chone fauveli Eumida bahusiensis Aonides paucibranchiata Euclymene spp Pholoe inornata Eulalia mustela Timoclea ovata Lepidochiton asellus Emarginula fissura Limaria hians Atylus vedlomensis Psammechinus miliaris Eulalia bilineata Harmothoë lunulata Ophiura albida Pectinaria koreni Scalibregma inflatum Hiatella arctica Ampelisca tenuicornis Polycirrus spp Amphipholis squamata Echinocyamus pusillus Notomastus latericeus Terebellides stroemi Owenia fusiformis Lumbrineris spp Harpinia spp Leptosynapta inhaerens Ampelisca spinipes Spiophanes bombyx Nephtys incisa Pectinaria auricoma Amphiura filiformis Laonice cirrata Trichobranchus glacialis Amphicteis gunneri Myriochele heeri Diplocirrus glaucus Minuspio cirrifera Streblosoma bairdi Gattyana cirrosa Arabella iricolor Phaxas pellucidus Corbula gibba Abra prismatica Nucula sulcata Dosinia lupinus Turritella communis Cardium scabrum Antalis entalis Aporrhais pespelecani Azorinus chamasolen Limatula subauriculata Cirolana borealis Mysia undata Nephrops norvegicus Anapagurus laevis Iphinoë serrata Leptognathia gracilis Ophiura affinis Alpheus glaber Echinocardium flavescens Amphiura chiajei Brissopsis lyrifera
F 6. Abundance of species (97%) across historical and recent samples from the Chickens ground. Shade intensity increases with abundance.
Changes in Irish Sea benthos 745
10
Brissopsis lyrifera, were among the most significantly different species. There was a highly significant difference between the polychaete:mollusc ratios calculated from Jones’ (1·00·21 SE) and Whyte’s (1·580·37 SE) individual samples (t-test: t= 2·79, P<0·01), using natural log transformed data, with a greater number of polychaetes found in the recent (1992) sample group.
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F 7. Bradda Inshore ground. Historical samples taken using a canvas-lined dredge (d), and recent samples taken using a 0·1 m2 Day grab (g) and queen dredge lined with a fine (1 cm) mesh (f). Cluster analysis (a) and MDS ordination (b) are based on presence–absence data.
samples. Replicate grabs have been pooled for ease of presentation. Species consistently contributing less than 1% of total numbers were excluded, and species were sorted in order of importance in Whyte’s data set. Some species appear more abundant in the recent samples e.g. the brittlestars, Amphiura chiajei and Amphipholis squamata, the bivalve Nucula sulcata and the worm, Lumbrineris sp. Conversely, a large number of species were found in the historical samples, but were absent from the recent ones, possibly reflecting the difference in samples size. However, the potentially vulnerable tube-dwelling amphipod, Ampelisca spp., and the urchins, Echinocardium flavescens and
Of the large number of samples taken by Jones during the 1946 to 1952 period only a small number could be realistically compared with samples taken during recent sampling programmes at Port Erin. However, it was possible to compare sites that were located on the Bradda Inshore and Chickens commercial scallop fishing grounds. Both of these grounds have been fished for scallops on a commercial basis from at least 1950 and the indications were that each ground had been fished fairly consistently each year, so they represented good sites for comparison with the recent data sets. Comparisons were further hampered by mismatches in the types of gear used to take samples at each location. The gear used to sample the Bradda Inshore ground in Jones’ study was a dredge of unspecified type with a canvas bag liner, presumably of a very small mesh size. This would have filled quickly and consequently it would have been towed for only short distances over the sea bed, retaining a large proportion of the smaller species. It is likely that the samples taken with this gear would have resembled those of a modern anchor dredge which has not been used during the recent programme. The Day grab samples and dredge samples (with a 1 cm mesh liner) used during the recent programme at these sites represented the closest possible comparison, although the differences in gear efficiency limited the analysis of the Bradda data to presence–absence comparisons only. In 1950 the Chickens ground was sampled with a 0·1 m2 Van Veen grab which was comparable to the 0·1 m2 Day grab used at the corresponding sites in the recent study. However, the former is a lighter piece of equipment, and may be less efficient (Riddle, 1989). A further confounding factor is the nature of the sediments at the time of sampling, which may have been different in 1946–1950. Only very limited data are available in the historical data sets that describe substratum type and this would not assist the detection of significant differences between samples at an infaunal level.
1 cm mesh lined dredge
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746 A. S. Hill et al. Recent samples Grab samples
Moerella donacina Nematoneris unicornis Polycarpa spp Bodotria arenosa Lumbrineris gracilis Amphipholis squamata Owenia fusiformis Aonides paucibranchiata Chone fauveli Pomatoceros triqueter Euclymene spp Notomastus latericeus Lepidochiton asellus Pista cristata Echinocymus pusillus Timoclea ovata Atylus vedlomensis Lysiannassa plumosa Ampelisca diadema Maera othonis Melphidipella macra Phtisica marina Ophelia spp Echinocardium flavescens Photis longicaudata Orchomene nana Microjassa cumbrensis Bathyporeia gracilis Synchelidium haplocheles Anobothrus gracilis Urothoë marina Aequipecten opercularis Porania pulvillus Ophiocomina nigra Pecten maximus Cellaria fistulosa Alcyonium digitatum Aporrhais pespelecani Nemertesia antennina Echinus esculentus Eledone cirrhosa Ophiothrix fragilis Scyliorhinus canicula Cancer pagrus Pisidia longicornis Agonus cataphractus Buccinum undatum Anseropoda placenta Neptunea antiqua Astropecten irregularis Crossaster papposus Luidia sarsi Asterias rubens Marthasterias glacialis
Aug 1994 xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x x x xx x xx x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x xx x x x x x x x x x x x x x x x xxxx x xx x x x x x x x x x x x xx x x xx x x x x x x x xxx x x xx x x x
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x x xx x xxx xx x x x xx x x x x x x x x x x x x x x xx x x x x x x x x x x x x x x xx x x x x x x x x x x x x x xx x x x x x x x x x x x xx x x x x x xx x x x xxx x x x x x x x x x x xxx x x x x x xx x x x xxx x x
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F 8. Bradda Inshore ground. The presence or absence of species in historical canvas-lined dredge samples compared with recent grab and 1 cm mesh-lined dredge samples.
Despite these limitations it is argued that meaningful comparisons have been made by adopting a cautious approach in the interpretation of the results. Comparison of the community structure on the Chickens ground was limited to infaunal species since only grabbing gear was used in the 1950 surveys. Notable differences included the occurrence of the fragile echinoids, Brissopsis lyrifera and Echinocardium flavescens, and high densities of Corbula gibba, a
burrowing bivalve, in the historical samples that were absent from the recent samples. Other differences included higher abundances of a number of tubedwelling worms, a sedentary chiton and a bivalve in the recent samples relative to the historical samples. These sedentary species might be expected to do less well after prolonged periods of dredging, although it is difficult to predict the response of individual species to the effects of dredging, and dredging may result in
Changes in Irish Sea benthos 747 T 1. Comparison of some univariate indices of community structure calculated from samples collected by Norman Jones in 1952 (N=52) and Se´amus Whyte in 1992 (N=10) Jones (1952)
Total number of species, S Total number of individuals, N Species richness, d Shannon Weiner diversity, H Pielou’s evenness, J Simpson’s dominance, SI a
Whyte (1992)
Mean
se
Mean
se
t
P
18·26 55·60 4·34 2·22 0·81 0·19
1·13 4·75 0·20 0·06 0·01 0·01
12·90 23·70 3·76 2·33 0·92 0·12
1·03 1·83 0·24 0·07 0·01 0·01
1·81 2·83 1·26 0·73 3·28 2·31
0·07 <0·01b 0·21 0·47 <0·01b 0·02a
Significant at the 5% level. bSignificant at the 1% level.
bringing greater numbers of stones to the surface to serve as attachment surfaces. One interpretation of the infaunal data is to apply the hypothesis that disturbed areas support a greater proportion of polychaetes to molluscs (ICES, 1996). This hypothesis was supported by the Chickens data and is good evidence that the differences in the assemblages were due to dredge disturbance. There is evidence that one of the effects of dredging is to create coarser, and possibly less mobile sediments (ICES, 1973; Hill et al., 1997; see Kaiser & Spencer, 1996 for discussion of mechanism) which will favour many sedentary species. Long-term
F 9. Muddy sand ground. Multidimensional scaling plot of grab sample data collected by Norman Jones (J) in 1952 (a—February, b—April, c—June, d—September, e—December) and Se´amus Whyte (W) in December 1992.
changes in infaunal species composition are likely to be in response to a number of factors in combination, including direct effects upon the animals by the gear, but also through substratum alteration. Analysis of the Bradda Inshore data revealed a small number of infaunal species that were sampled in 1946 but not recorded in the recent samples. Given the small sample sizes taken in 1946, and the problems of interpreting infaunal changes in terms of fishing activity, the significance of these differences remains doubtful. More interesting, however, was the absence of a number of epifaunal scavenger and predator species from the samples taken in 1946. These species are dominant in most epifaunal samples taken from Bradda Inshore in recent times including those used in this comparison. This is consistent with predictions of the effects of fishing where high densities of scavengers would be expected on fished grounds (ICES, 1996) as they exploit the damaged and discarded material, and in this case of asteroid echinoderms, survive better the effects of being caught in dredges. However, the recent data were only compared with three historical samples which would have effectively covered a fairly small area. This study has demonstrated that community structure has changed considerably since 1950 despite the problems of comparing samples taken with different gear types in surveys that were not designed specifically for this purpose. In the case of the Chickens infaunal data changes in the ratio of polychaetes to molluscs link these changes to dredging. In addition, the predominance of scavengers in the benthic communities on Bradda Inshore also indicates a strong effect of dredging. However, these comparisons would be improved significantly by re-surveying more of the sites originally sampled in 1950. It would also be preferable to re-sample the sites using gear types that are identical to those used in the earlier surveys in
Jones E
Jones D
Jones C
Jones B
Jones A
Jones F2
Jones F1
Whyte
748 A. S. Hill et al.
Amphiura chiajei Nucula sulcata Abra prismatica Lumbrineris spp Owenia fusiformis Corbula gibba Amphipholis squamata Notomastus latericeus Amphiura filiformis Glycera rouxi Myriochele heeri Phoronis muelleri Glycera gigantea Chaetozone setosa Phaxas pellucidus Pectinaria spp Iphinoe serrata Azorinus chamasolen Goniada maculata Turritella communis Laonica cirrata Molgula occulta Echinocyamus pusillus Spio filicornis Antalis entalis Dosinia lupinus Harpinia pectinata Amphicteis gunneri Trichobranchus glacialis Diplocirrus glaucus Brissopsis lyrifera Prionospio spp. Parvicardium scabrum Astrorhiza limicola Echinocardium flavescens Ampelisca spp
F 10. Muddy sand ground. Standardized infaunal species abundance in samples collected by Jones in 1952 and Whyte in 1992. Shading intensity is proportional to abundance. 1% of species have been removed.
order to overcome the confounding effects of differences in gear efficiency. Some sediment description is also available in the historical data although comparable observations had not been made for most of the
sites sampled during the recent surveys. Additional surveys employing acoustic methods of seabed discrimination (e.g. sidescan sonar, RoxAnn) would, therefore, help to examine any changes in the surface
Changes in Irish Sea benthos 749
structure of sediments which would be valuable in the interpretation of changes of infaunal communities in particular. Resampling on muddy sand ground The distinct differences between the data produced by Jones in 1952–1953 and Whyte in 1992 suggest that the community has altered dramatically in the interim period. Given that this site was chosen for re-sampling because apparently it had not been exposed to high levels of commercial fishing, it must be assumed that other factors have caused the change. These may include long-term natural biological cycles or a change in the substratum type brought about by natural or anthropogenic causes. It has been suggested that dredging activities may resuspend large amounts of sediment, and the lighter fractions (silt) may be translocated and deposited elsewhere (Hill et al., 1997; ICES, 1996). This effect could potentially lead to a change in substratum type in areas which are not fished directly, and smothering of sensitive filter feeding species, though other filter feeders may benefit from an increased food supply (Hill et al., 1977; Morton, 1977). It has been considered that the low mean annual fishing effort recorded for the 55 nautical mile square enclosing this site could have been concentrated around the study site, resulting in a localized area of higher fishing disturbance. This has been ruled out, however, as the substratum at the site is soft mud, and does not support any scallops, and as such would not be fished by commercial fishermen. It is apparent that some potentially vulnerable, tube-dwelling species have decreased in abundance since the 1950s, but further interpretation of the results obtained would require much greater knowledge of the ecology of the species involved. However, the significant increase in the polychaete:mollusc ratio from 1952 to 1992 suggests that the observed changes in community structure could be related to fishing disturbance effects (ICES, 1996), albeit indirectly. Conclusions It is clear that the observed changes in the benthic communities around the Isle of Man since the 1940s and 1950s may be partially due to the increases in commercial dredging effort, but this seems not to be the only cause. In order to interpret the significance of these changes, further study into the actual species involved is required: in particular some assessment of the direct effects of dredging on these species is necessary.
Acknowledgements This work would not have been possible without the original survey data of the late Dr Norman Jones: he generously made those available to us. The work was partly funded from two contracts: A study of temporal changes in Irish Sea benthos—contract number EPG 1/9/87 from D.O.E.; An assessment of the effects of scallop dredging on benthic communities—contract number CSA 2332 from MAFF. References Brand, A. R., Allison, E. H. & Murphy, E. J. 1991 North Sea scallop fisheries: a review of changes. In An international compendium of scallop biology and culture (Shumway, S. E. & Sandifer, P. A., eds). World Aquaculture Society, Baton Rouge, pp. 204–218. Bullimore, B. 1985 An investigation into the effects of scallop dredging within the Skomer Marine Reserve. Skomer Marine Reserve Subtidal Monitoring Project. Report to the Nature Conservancy Council. SMRSMP Rep. 3. Clarke, K. R. & Warwick, R. M. 1994 Change in Marine Communities: An Approach to Statistical Analysis and Interpretation. NERC, UK, 144 pp. Collie, J. S., Escanero, G. A. & Valentine, P. C. 1997 Effects of bottom fishing on the benthic megafauna of Georges Bank. Marine Ecology Progress Series 155, 159–172. Currie, D. R. & Parry, G. D. 1996 Effects of scallop dredging on a soft-sediment community: a large-scale experimental study. Marine Ecology Progress Series 134, 131–150. Eleftheriou, A. & Robertson, M. R. 1992 The effects of experimental scallop dredging on the fauna and physical environment of a shallow sandy community. Netherlands Journal of Sea Research 30, 289–299. Hall, S. J., Robertson, M. R., Basford, D. J. & Heaney, S. D. 1993 The possible effects of fishing disturbance in the northern North Sea: An analysis of spatial patterns in community structure around a wreck. Netherlands Journal of Sea Research 31, 201–208. Hill, A. S., Brand, A. R., Veale, L. O. & Hawkins, S. J. 1997 Assessment of the effects of scallop dredging on benthic communities. University of Liverpool Final Report to MAFF. Contract CSA 2332, 112 pp. ICES 1973 Effects of trawls and dredges on the sea-bed. International Council for the Exploration of the Sea, Report of the Chairman of ICES Gear and Behaviour Committee CM 1973/B:2. Copenhagen, Denmark. ICES 1996 Report of the working group on ecosystem effects of fishing activities. International Council for the Exploration of the Sea Working Group Report, CM 1996/Assess/Env:1. Copenhagen, Denmark. Jones, N. S. 1950 Marine bottom communities. Biological Reviews 25, 283–313. Jones, N. S. 1951 The bottom fauna off the south of the Isle of Man. Journal of Animal Ecology 20, 132–144. Jones, N. S. 1956 The fauna and biomass of a muddy sand deposit off Port Erin, Isle of Man. Journal of Animal Ecology 25, 217–252. Kaiser, M. J. & Spencer, B. E. 1996 The effects of beam trawl disturbance on infaunal communities in different habitats. Journal of Animal Ecology 65, 348–359. Morton, J. W. 1977 Ecological effects of dredging and dredge spoil disposal: a literature review. U.S. Fish and Wildlife Services. Technical Papers 94. Pennington, D., Veale, L. O. & Hartnoll, R. G. 1998 Re-analysis of an historical benthic data set from the Irish Sea. Estuarine, Coastal and Shelf Science 46, 769–776. Riddle, M. J. 1989 Bite profiles of some benthic grab samplers. Estuarine, Coastal and Shelf Science 29, 285–292.
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