Distribution and Structure of Faunal Assemblages and Their Associated Physical Conditions on the Atlantic Continental Shelf of the British Isles
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International Council for the Not to be cited without prior Exploration of the Seas reference to the author Theme session P: Physical-biological Interactions: Experiments, Models and Observations ICESCM 2004/P:03 Distribution and structure of faunal assemblages and their associated physical conditions on the Atlantic continental shelf of the British Isles Jim Ellis and Stuart Rogers There has been an increased use of groundfish surveys as platforms for the sampling of macro-epibenthic fauna over wide geographical areas in recent years. Indeed, internationally coordinated studies have provided spatially comprehensive data for the North Sea. In contrast, the invertebrate fauna off the western coasts of the UK are more poorly known. Macro-epifaunal samples collected by 4m-beam trawl were used to determine the structure and diversity of demersal assemblages occurring along the western seaboard of the British Isles. Samples were collected during 2003 from the Celtic Sea/Bristol Channel (ICES division VIIf-g), Irish Sea (VII a) and the western English Channel (VIIe). Cluster analysis was used to determine the similarity of assemblages in the area, and the distribution of these assemblages was examined in relation to depth, substrate, latitude, water temperature, salinity and tidal stress. Stations in the Irish Sea were characterised by species typical of inshore grounds, including Pagurus bernhardus, Asterias rubens, Ophiura ophiura and Liocarcinus holsatus, with stations on the coarser grounds of the central Irish Sea and English Channel were typified by Pagurus prideaux. The relationships between epibenthic assemblages and the physical environment are discussed. Keywords: Epibenthic monitoring, Jim Ellis and Stuart Rogers, CEFAS Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk, NR33 0HT, U.K. [tel: +44 (0)1502 524300, fax: +44 (0)1502 513865, e-mail [email protected]]. INTRODUCTION It has become apparent in recent years that there is a lack of knowledge of the distribution and structure of offshore benthic species and communities, although such information is required on wide geographical scales (e.g. by ICES area and OSPAR region). Whereas the majority of published accounts of benthic communities have focused on detailed site-specific studies, such localised studies are rarely compatible, due to the differences in timing, gears and sampling protocol. Over recent years, the use of groundfish and monitoring surveys as platforms for the sampling of macro-epifauna and megafauna over wide geographical areas has been undertaken for many sea areas. Regional studies include 4m-beam trawl sampling in the eastern English Channel (Kaiser et al., 1999), southern North Sea (Ellis and Rogers, 1999) and Irish Sea/Bristol Channel (Rees et al., 1999; Ellis et al., 2000), and 2m-beam trawls sampling in the North Sea (Jennings et al., 1999; Rees et al., 1999; Callaway et al., 2002) and Celtic Sea (Ellis et al., 2002). Other studies have incorporated data from such surveys for specific taxa (e.g. Ellis and Rogers, 2000; Ellis et al., 2002; Porter et al., 2002). The aim of the current study was to describe the structure, composition and distribution of macro-epibenthic invertebrate assemblages in the western English Channel, Bristol Channel, St George’s Channel and Irish Sea using samples collected during a 4m-beam trawl groundfish survey, to determine which physical variables best explained the observed patterns in these assemblages and to determine whether these spatial patterns in invertebrate assemblages had corresponding spatial differences with regards to cephalopods and demersal fishes. MATERIALS AND METHODS Sampling stations and sampling protocol Demersal fish and invertebrates were collected from 182 stations in the western English Channel (ICES Division VIIe), Bristol Channel (VIIf) and Irish Sea and St George’s Channel (VII a) during a groundfish survey undertaken by the R.V. Corystes in September 2003 (Figure 1). Fishing was conducted with a 4-metre beam trawl with chain mat and 40 mm stretched mesh cod-end (see Kaiser & Spencer, 1994). Trawling speed was 4 knots and tow duration was 30 minutes at most stations, sampling approximately 15,000 m2 per tow. Certain stations, which were known to result in large quantities of shell debris, macroalgae or excessive numbers of juvenile flatfish, were sampled for 15 minutes. All fish, cephalopods and commercial shellfish were identified, counted, weighed and measured (see Rogers et al., 1998). The remaining invertebrate catch was weighed and a representative sub-sample of known weight sorted. Invertebrates were identified to the lowest taxonomic level possible, weighed and all non-colonial species counted. Data were subsequently converted to catch per unit effort (CPUE) for both biomass and numbers per hour. Physical variables Surface water and bottom temperature and salinity were recorded from a micro-CTD attached to the headline of the beam trawl. Surface water temperature and salinity were also recorded from a continuous data logger, though these data were only used when no data from the micro-CTD were available (e.g. in the case of battery failure). Although no data on the sediments of the trawl stations were available, the general sediment type was estimated from British Geological Survey Charts. For data analysis, sediments were allocated values on the following scale: 1=mud; 2=sandy mud; 3=muddy sand; 4=sand; 5=gravely sand; 6=sandy gravel; and 7=gravel. To provide an additional indication of the ground type, the total weight of rocks in each catch was recorded, and the weight of shell debris in the sub-sample was raised to total catch. Tidal stress was estimated from a two-dimensional hydrodynamic model of the north-west European shelf, originally developed at the Proudman Oceanographic Laboratory, which had been used to predict the depth-mean M2 tidal current at a spatial resolution of 1/8° longitude by 1/12° latitude (approximately 8km). Bed stresses due to the M2 tide were calculated using a quadratic expression, with stress dependent on the predicted maximum ellipse current and an appropriate bed friction coefficient, in this case with an assumed value of 0.0025. Data from this model were used to provide an indication of tidal stress at trawl stations, with stations between values of tidal stress interpolated. Data analysis The PRIMER analytical package (Clarke and Warwick, 1994) was used for the cluster analysis of trawl data, using the Bray-Curtis similarity on fourth-root transformed CPUE data (kg.hr-1). This transformation was undertaken to downweight the importance of abundant, large-bodied taxa (e.g. common starfish Asterias rubens) and increase the relative importance of smaller- bodied taxa that may be more indicative of ground type. Stations with similar catch compositions were assumed to reflect sites with similar macro- epibenthic assemblages. Discriminating species for each assemblage were identified using the similarity of percentages procedure (SIMPER), which determines the contribution of each species to the average dissimilarity between clusters. SIMPER analysis was also undertaken to examine differences in fish catches (numerical catch data) between groups of stations with the similar macro-epinbenthic invertebrate catches. The association between physical variables with the similarity of macro- epibenthic invertebrates was determined using the BIOENV routine (Clarke & Warwick, 1994). The biological similarities of the catches at stations were compared with the following physical variables: latitude, depth, the weights of rocks and broken shell in the catch, surface water temperature and salinity (as recorded during the research cruise) and M2 tidal current. The weights of rocks and broken shell were also transformed (ln(1+weight)) for this analysis. BIOENV analysis was also undertaken to determine which physical variables were best correlated with the catch composition of the dominant taxa. The association between physical variables with the similarity of fish catches was also determined. RESULTS Cluster analysis indicated five broad category of macro-epibenthic assemblage, though these clusters could generally be divided into more spatially defined sub-clusters. The majority of stations were broadly divided into two main clusters, broadly equating with shallow-water sandy habitats (61 stations) and coarser grounds (81 stations) that were typically further from shore, though several parts of the western English Channel had coarser grounds close inshore. The distribution of these broad assemblage types are illustrated in Figure 2. Inshore assemblage The inshore sandy grounds (mean similarity = 44.2%) were dominated by a few common inshore species, such as Asterias rubens, sand star Astropecten irregularis, common hermit crab Pagurus bernhardus, common brittlestar Ophiura ophiura; harbour crab Liocarcinus depurator, swimming crab L. holsatus, common whelk Buccinum undatum and sea mouse Aphrodita aculeata (Table 1). Inshore sandy grounds were sub-divided latitudinally between those in the south (including parts of Cardigan Bay, Carmarthen Bay, Swansea Bay, Bideford Bay and Lyme Bay), and those sites further north in the Irish Sea (e.g. Liverpool Bay, Solway Firth, Dundrum Bay). The principle differences between these two regions were that spider crab Maja squinado and curly weed Alcyonidium diaphanum were more abundant in the southerly, and A. irregularis, L. depurator and L. holsatus more abundant in the northerly area. This assemblage was typical of