Since the environmental impacts of anthropogenic pressures have become more recognized, the employment of spatial management strategies has increased. However, in order to do this, it is essential to understand the environmental drivers and spatial distributions of marine assemblages. In the present study, the epibenthic communities within the highly-industrialised Firth of Clyde were investigated using a beam trawl at four separate locations south of the Isle of Cumbrae. Sediment type was found to be the main driver influencing species distribution, however high levels of substratum heterogeneity reduced the dissimilarity between trawl catches preventing further inferences to be drawn. Biological interactions, such as prey availability, and human disturbance were also found to influence the distribution of species within the firth. Particular species such as Alcyonium digitatum and Munida rugosa were found at exceedingly high numbers, whereas those previously commercially fished for, such as Pleuronectes platessa, were almost absent from the catch. Commercial fishing has dramatically altered the biodiversity within the Firth of Clyde, and may continue to do so in the future.
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Keywords: epifaunal assemblages, Firth of Clyde, spatial distribution, heterogeneity, sediment
First documented in the 14th century, beam trawls have long been employed in the fishing industry, and are currently used extensively in the North and Irish seas as a means of catching flatfish such as plaice (Pleuronectes platessa), and sole (Solea solea) (Kaiser and Spencer, 1995). In addition to benthic fish, large numbers of invertebrates - including some infauna - are also regularly sampled using the trawl. This is due to the gear's tickler chains penetrating the uppermost few centimetres of the sediment to improve fishing efficiency (Kaiser and Spencer, 1996). As a result, beam trawls are also commonly used in environmental epibenthic surveys to provide a semi-quantitative estimate of the total abundance of organisms within the designated area, and to assess assemblage changes as a result of environmental conditions or human activity (Rogers and Lockwood, 1989).
Since the environmental impacts of anthropogenic pressures such as commercial fisheries, transport routes and energy production have been recognized, the employment of spatial management strategies and other holistic approaches has increased (Reiss et al, 2010). However, in order to undertake such management and conservation, it is essential to understand the environmental drivers of marine assemblages, and the structure and spatial distributions exhibited by such communities. The benthic macrofauna are of particular importance due to their role in detrital decomposition (Andersen and Kristensen, 1992), nutrient cycling (Covich et al., 1999) and as food for higher trophic levels (Ojeda and Dearborn, 1991).
Over 60 years, long-term changes in benthic communities within the North Sea have been recorded as a result of exploitation by fisheries, and provide valuable information for ecosystem management and conservation (Frid et al., 2000). Here, the classic studies by Peterson (1914, 1918) first recognised the importance of sediment type in regards to the spatial distribution of benthic organisms. This is particularly important for sessile organisms and also for filter feeders for which fine sediments would pose difficulties (Bricelj and Malouf, 1984; Ellis et al., 2002). Other environmental factors such as temperature, depth and currents have also been suggested as reasons for the distribution of benthic assemblages (Frauenheim, 1989; Zühlke et al., 1991; Jennings et al., 1999). In addition, biological interactions, such as predator-prey relationships and bioturbation, have been recognised as providing further spatial and temporal heterogeneity within individual habitat types (Rhoads, 1974; Warwick and Uncles, 1980). However, in a large number of studies, sediment type remains the governing factor regarding species distribution (Jones, 1950; Young and Rhoads, 1971; Bloom et al., 1972; Lough et al., 1989; Amezcua and Nash, 2001).
Due to its high levels of shelter, the Firth of Clyde contains fine sediment with relatively large proportions of silt and clay (Tuck et al., 1997). As a deepwater port, and home to the UK's primary facility for coal imports, the firth receives considerable inputs of anthropogenic activity and pollution (Edgar et al., 1999). Webster et al. (2005) described the firth as arguably one of the most heavily contaminated water bodies in Scotland, with its largest source of contaminants derived from a historical sewage disposal site south of the Isle of Bute. Despite its termination in 1998, elevated concentrations of trace metals and altered benthic communities are still being reported as a result (Mojtahid et al., 2008; Webster et al., 2008).
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Dredging of the channel to maintain the deep waterways also remains a considerable threat to benthic communities, as does dredging for scallops (Pecten maximus) and trawling for other commercially important species such as the Norway lobster, Nephrops norvegicus (Kaiser and Spencer, 1996; Hauton et al., 2003). Seagrass cover and maerl habitats have been found to significantly decrease as a result, leading to subsequent losses in associated biodiversity (Spencer and Moore, 2000; Thurstan and Roberts, 2010). Additionally, overfishing of demersal fish has also led to significant changes in benthic assemblages: landings of whiting, flounder, cod and hake have dramatically decreased by up to 99% since 1984 (Thurstan and Roberts, 2010), leading to the closure of such demersal fisheries at the start of the 21st century (McIntyre et al., 2012). Just recently, a study conducted within the firth suggested that the ecosystem may be recovering, and has led to uncertainty regarding the reliability of the previous data compiled by Thurstan and Roberts (2010) (Heath and Speirs, 2011).
The aim of this study was to provide semi-quantitative data regarding the presence and relative abundance of epifaunal species upon the soft substrata of a sheltered area within the Firth of Clyde. The feeding and trophic habits of organisms within the assemblages were also investigated, alongside spatial distributions in regards to substrate type. All of the aforementioned anthropogenic inputs could also have direct impacts upon the results of this study.
2. MATERIALS & METHODS
2.1 Study area and survey design
The Firth of Clyde is situated on the western coast of Scotland, consisting of a large area of coastal water and several small islands. The water within the firth is sheltered from the Atlantic Ocean by the Kintyre Peninsula, and as a result, the majority of substrata generally consist of fine, muddy sediments (Tuck et al., 1997). Possessing a single town, Millport, on the south of the island, the Isle of Cumbrae is situated approximately 1 mile from the mainland and is further sheltered by surrounding islands within the firth. Located near to the town, the University Marine Biological Station Millport (UMBSM) regularly undertakes sampling of the surrounding benthos biodiversity. Sampling during this study took place in the waters south to south east of the Isle of Cumbrae, where high levels of shelter were present, and where the impacts of pollution may also be apparent.
2.2. Sampling methodology and processing
During late June, 2012, four separate locations surrounding the Isles of Cumbrae in the Fairlie Channel were sampled aboard the UMBSM RV Aora (55°44'N; 04°55'W). A 3m beam trawl fitted with a 70mm stretched mesh (35mm from knot to knot) was employed with durations ranged from 10 to 21 minutes at a speed of 2.3 to 2.8 knots (Table 1). Due to differences in towing direction and local currents, towing speed varied. The start and finish co-ordinates were logged onboard the trawler; the paths of each of the trawls are displayed in Figure 1. Trawls 1 and 2 were positioned in relatively close proximity to one another, whilst Trawls 3 and 4 were taken a similar distance apart approximately 1 mile south. Whilst the depths of each of the trawls varied, the water surrounding the Isles of Cumbrae is generally rather deep, maintained due to dredging to allow large boats to reach the shipping terminal. The duration, depth and speed of each of the four trawls are shown in Table 1, alongside the dominant sediment-type(s) thought to be present in the area.
Particularly large catches were sub-sampled by 50%, and then relative abundance counts for those trawls were later doubled to allow semi-quantitative comparison between all trawl locations. In this study, Trawls 1, 2, and 4 were sub-sampled onboard directly after the completion of the trawl. Subsequently, intertidal algae were removed from the sample and the remaining organisms were retained and later identified as closely to species level as possible at the laboratory. The most abundant species and general taxa were counted, to provide relative abundance information for each site.
2.3 Data analysis
Raw data was inputted into Excel and later transferred to the statistical package, SPSS (Statistical Package for Social Sciences) version 19.0, for analysis. The Kolmogorov-Smirnov test was first applied to test for normality in the distribution of the data, and then the appropriate parametric tests were undertaken. The significance level of 0.05 was employed. Differences in species abundances between trawl sites were tested by the Independent Samples T-test. To further compare species diversity between trawl locations, the similarity index, Sørensen's Qualitative Index, was used:
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Sørensen's Qualitative Index: Cs = 2j__
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3.1. Abundance of epifauna
A total of 71 species were sampled across all four beam trawls, including crustaceans (17), cnidarians (15), echinoderms (11) and fish (9). Of the 71 species, 49 were free-living and 22 were sessile. All sessile organisms encountered were suspension feeders. A complete list of all species encountered, and their presence/absence at each site, is provided at the end of this report (Appendix). The most abundant species was the soft coral, Alcyonium digitatum; in Trawl 2, there was an estimated abundance of 598 individuals (Figure 2). Trawls 1 and 2 had high numbers of A. digitatum as opposed to Trawls 3 and 4 (Figure 2). Similarly, squat lobsters (Munida rugosa) were also highly numerous; 368 individuals were sampled in Trawl 2. As found with A. digitatum, Trawl 1 and Trawl 2 collected large numbers of squat lobsters, yet Trawls 3 and 4 did not (Figure 2). Although only small abundances were sampled, whelks such as Buccinum undatum were only present in Trawls 1 and 2.
A variety of species were found in relatively large numbers during Trawl 4 as opposed to the rest of the trawls (Figure 2). For example, hermit crabs, such as Pagurus bernhardus and Pagurus prideaux, were collected in significantly higher abundances in Trawl 4 as opposed to Trawls 1-3 (Independent Samples T-test: t = 14.850, P = 0.004, df = 2). Similarly, the common starfish, Asterias rubens, was also found in significantly higher numbers in Trawl 4 (Independent Samples T-test: t = 7. 401, P = 0.018, df = 2). Flatfish were also sampled and a similar pattern was recorded (Independent Samples T-test: t = 26.500, P = 0.001, df = 2). In Trawl 4, 63 individuals were sampled, whilst a mean average of 9 (SD = 1.73) individuals were sampled throughout Trawls 1-3.
In contrast, some species were found in extremely low numbers, with little variation between trawl locations. For example, no heart urchins were sampled at all during this study, and relatively few brittle stars were sampled either. Smooth-armed brittle stars were low to absent across the whole of the study site, whilst spiny-armed brittlestars were slightly more abundant but were still relatively scarce (max. 6 per trawl). Interestingly, very low numbers of N. norvegicus were collected throughout the study: the organism was absent from Trawls 1 to 3, and only 8 were present in Trawl 4. Scorpionfish (primarily Myoxocephalus scorpius) were also found in rather low numbers.
3.2. Comparison between sites
Table 2 illustrates the qualitative similarity found between all four trawl sites in regards to species composition. There was no great dissimilarity found between any of the four sites: values generated by the Sørensen's Qualitative Index ranged from 0.44 - 0.58, where 1.00 indicates complete similarity and 0.00 indicates complete dissimilarity. In terms of species composition, the most similar beam trawl sites were 1 and 4 (0.58), whilst the most dissimilar were 2 and 3 (0.438). However, due to the low range between the values produced, very little further information can be drawn from the results of this analysis.
4.1. Macrobenthic assemblages and drivers of distribution
The results of this study suggest that a minimum of two separate assemblages were sampled. The first was sampled by Trawls 1 and 2, whilst the second assemblage was sampled by Trawls 3 and 4. This was indicated by relatively large numbers of A. digitatum and squat lobsters in Trawls 1 and 2 as opposed to Trawls 3 and 4 (Figure 2). However, the exceedingly high numbers of hermit crabs and A. rubens in Trawl 4 suggest that there may be a third distinct assemblage sampled. Such disparate species compositions may be a result of encountering various sediment types (Peterson, 1914).
The octocoral, A. digitatum, had its highest abundances in Trawls 1 and 2, as the sediment in this area is thought to contain some hard substrate such as cobbles to which the organism can attach. The coral was found in lower abundances in Trawls 3 and 4 as here there is a larger amount of soft substrate and therefore fewer viable attachment points for the soft coral. Furthermore, being a passive suspension feeder (Fabricus et al., 1995) the muddy sediment may pose difficulties if re-suspended. Fine sediments are indicative of high shelter and low-velocity water movements (Little, 2000), yet A. digitatum is reliant on ambient currents to move plankton and other food particles towards its feeding structures (Migné and Davoult, 2002). Therefore, the lower abundance of A. digitatum sampled by Trawls 3 and 4 may be a result of a lower water velocity and sediment grain size.
With its diet mainly consisting of A. digitatum and thriving in such current-exposed locations, the nudibranch, Tritonia hombergii, was present in all four beam trawl samples (Thompson, 1962). Although no quantitative data was recorded for this organism, it can be assumed that more T. hombergii were present at the locations of Trawls 1 and 2 due to its close association with A. digitatum. As stated previously, all sessile organisms encountered had a suspension-feeding trophic mode, and very few deposit feeding organisms were recorded. This infers that the sediment type was sandy to firm mud (Rhoads and Young, 1970). Such trophic group segregation (as a result of differing loads of suspended food) has been frequently reported over large areas, and may explain why so few deposit feeders were sampled during this study (Sanders, 1960; Parker, 1963).
Spiny-armed brittle stars were also encountered in low numbers; they regularly form discrete, dense aggregations so it is possible that they were present within the area but were missed by the trawl. Similarly, smooth-armed brittle stars were also sampled in low numbers. However, this may be due to their preference for clean sand which was not particularly abundant at the study site. This is consistent with the results of this study in which the trawls that passed over substrate types such as soft mud, cobbles and shells did not encounter any such species.
4.2. Sediment type and substratum heterogeneity
Despite sampling four supposedly different sedimentary habitats, the differences between entire species compositions at each site were not substantial, inferring that the species diversity may not have been entirely dependent upon sediment type. Moreover, one would have expected the species compositions found at sites closest to one other to be the most similar; however, this was not the case. This outcome may be an error resulting from the statistical index used, in which the sample sites were only compared qualitatively, leading to less accurate data being produced. The differences in species abundance between sites were not taken into account therefore slight changes in species diversity were not discernible. Due to the small range of values produced, and the reliability of the index often questioned, very little can be further inferred.
Interestingly, Zühlke et al. (2001) also found that sediment type showed no significant relationship in regards to species richness. Furthermore, no statistically significant correlation was produced between sediment mud content and epifaunal abundance at 255 stations in the North Sea (Reiss et al., 2010). However, both this current study and that undertaken by Reiss et al. (2010) employed beam trawls as a means of collecting epifaunal data, and it can be argued that sediment type was not accurately sampled. It is generally impractical to collect sedimentary samples along the whole of the trawl transect, so sedimentary conditions may either be characterised by the results of a 0.1 m grab prior to the tow (as in Reiss et al., 2010), or through the use of visual inspection and previous knowledge regarding the sample site (as used in this study). As suggested by Rees et al. (1999), a combination of underwater photography and acoustic methods may solve the problem associated with mapping the sediment throughout the trawl. Due to the inevitable substratum heterogeneity present within all sampling locations, it is possible that in this study a number of sediment types were sampled throughout each trawl, and were not wholly represented by the sediment information provided. This would explain the higher-than-expected similarity between sampling sites. It is also important to note that only a subsample of each population was documented during this study: the epifaunal assemblages at each site were not fully recorded by the trawls, and a number of species currently present within the study location may not have been sampled.
For example, a relatively low number of fish were collected using the beam trawl. All species present in the catch were benthic, and no pelagic vertebrates were recorded. This is possibly a combined result of net avoidance behaviour and the low position of the trawl within the water column. Alternatively, there may simply be a low abundance of benthic fish present within the sample sites due to sediment preferences or food availability. This is consistent with low numbers of scorpion fish (e.g. M. scorpius), which favour rocky substrate (Stal et al., 2007) of which there was little throughout the study area. Highest numbers of such fish were present within Trawls 1 and 2 where cobbles were thought to be present (Figure 2; Table 1).
Furthermore, Trawl 4 passed over substrate primarily characterised by soft mud (Table 1), and here, highest abundances of flatfish were sampled as opposed to Trawls 1 to 3. This reflects the enhanced ability for the benthic organisms to bury themselves in the finer sediment (Damalas et al., 2009). Such preferences have been reported by Gibson and Robb (2005) in which laboratory experiments demonstrated that juvenile P. platessa consistently settled on the finest of four sediments. However, opposing outcomes have been reported in the Wadden Sea where plaice were absent from muddy sites and distribution was mainly attributed to prey abundance (Jager et al., 1999). Habitat selectivity has also been found to decrease with organism age and size (Stoner and Ottmar, 2003). It, therefore, may be an accumulation of such factors which has led to flatfish being present within all four beam trawls. Flatfish, in particular larger organisms, may leave finer sediments in order to search for higher abundances of prey as the need to remain buried from pelagic predators (Ansell and Gibson, 1993) is reduced when larger sizes are reached (Stoner and Ottmar, 2003). Flatfish prey includes bottom-dwelling crustaceans and other small invertebrates such as polychaetes and bivalves (Gibson et al., 1998).
4.3. Sampling Nephrops norvegicus abundance
The crustacean, N. norvegicus, is of high commercial importance within the Firth of Clyde, with the Nephrops fishery landings worth an estimated £89.3 million in 2006 (Milligan et al. 2009). Despite their obvious abundance within the firth, very few individuals were sampled during this study (Figure 2). Large local variations in N. norvegicus densities, size composition and growth have been studied, and are particularly apparent within the continuous Nephrops bed in the Firth of Clyde (Bailey et al., 1986; Tuck et al., 1997). It is now accepted that differences in sediment type influence the ability of the crustaceans to construct suitable burrows, and therefore their distribution, as soft muddy sediments are preferred (Chapman and Bailey, 1987; Smith and Papadopoulou, 2003). Due to their burrowing lifestyle, and variability in emergence patterns, the current trawl catch rates are poorly representative of the N. norvegicus population (Chapman and Rice, 1971; Campbell et al., 2009). Moreover, the low catchability of the gear, especially in regards to mesh size, further prevented reliable samples to be undertaken.
This study employed a beam trawl with a mesh size of 70mm, whereas Tuck et al. (1997) used a 22mm mesh, and was able to sample substantial densities of the organism. In contrast, a recent study using a 70mm mesh found substantial populations of N. norvegicus in areas identified as heavily-trawled locations (Murray and Cowie, 2011) which were close to Trawls 3 and 4 in this investigation. However, the study undertook the trawl to coincide with feeding and emergence patterns of the crustacean. It has been found that the Norway lobster feeds at the hours of 10pm - 2am and 6am - 10am (Parslow-Williams et al., 2007). This is consistent with the results of the current study in which Trawl 4 was the only trawl undertaken before 10am, and also the only trawl to have N. norvegicus present within the catch. Therefore, it is clear that if such species are to be studied further, greater care must be undertaken to ensure trawls coincide with emergence patterns relating to tide, time of day and ambient light levels (Tuck et al., 1997).
4.4 Environmental implications of fishing within the Firth of Clyde
High levels of otter trawling and bottom disturbance, through fishery activity, have altered the biodiversity within the Firth of Clyde (Thurstan and Roberts, 2010). Landings of cod and whiting have reduced by up to 99% between 1985 and 2009, with similar trajectories documented for plaice, flounder and haddock (Thurstan and Roberts, 2010). Such findings may explain the low abundances of flatfish sampled during this study. Reductions in such fish have enabled the explosion of N. norvegicus and P.maximus abundances due to reduced predation (Thomas, 1965) and the less complex environments brought about by bottom trawling and dredging (Thurstan and Roberts, 2010). Discards from the Nephrops fishery are considerably large, with 9kg of bycatch for every 1kg of N. norvegicus caught (Bergmann et al., 2002). The low survival rate of such organisms once re-entered to the water (Bergmann and Moore, 2001) echoes the considerable impacts such fisheries are having upon the biodiversity within the firth.
Further indirect impacts upon benthic communities have been recorded, such as changes in sediment size and composition after long periods of dredging activity (Bradshaw et al., 2002). The immediate re-suspension of finer sediment fractions ultimately leads to the coarsening of marine sediment (Langton and Robinson, 1990). Being as sediment type is regarded as the main environmental driver of species distribution and assemblage composition, it can be concluded that the biodiversity within the Firth of Clyde may continue to alter in response to the current inputs of anthropogenic activity and fishing effort.