The influence of sediment type on the distribution of the lesser sandeel, Ammodytes marinus
Introduction
Sandeels are an important component of food webs in the North Atlantic (Sherman et al., 1981, Harwood and Croxall, 1988, Furness, 1990, Sparholt, 1990, Wanless et al., 1998). They also support the largest fishery in the North Sea, with recent annual landings of around a million tonnes (ICES, 1997). The magnitude of the fishery and the importance of sandeels to marine predators has led to concern over the potential impact of sandeel harvesting on the North Sea ecosystem (Monaghan, 1992). Recent expansions in the distribution of exploitation have heightened this concern and identified a need for a more detailed knowledge of sandeel distribution, particularly outside fished areas (ICES, 1997). Studies of sandeel distribution in parts of Shetland and the Firth of Forth have been useful in explaining the foraging locations of sandeel predators, such as piscivorous seabirds (Monaghan et al., 1996, Wright and Begg, 1997). However, information on sandeel distribution at a much larger scale is required in order to define where possible areas of competition between marine predators and fisheries could arise.
Of the five species of sandeels inhabiting the North Sea, the lesser sandeel, Ammodytes marinus (Raitt, 1934) is the most abundant and comprises over 90% of sandeel fishery catches (ICES, 1997). As with other sandeel species, A. marinus has a close association with sandy substrates into which they burrow, following a planktonic larval phase (Macer, 1966, Reay, 1970). Observations on the availability of A. marinus to fisheries (Macer, 1966) and their occurrence in sediment (Cameron, 1958) suggest that this species rarely emerges from the seabed between September and March, with the exception of spawning in December and January (Macer, 1966, Gauld and Hutcheon, 1990). Even during the season when A. marinus is active, fish tend to emerge only during daylight hours in order to feed (Winslade, 1974) and they tend to forage over the sediments they inhabit (Macer, 1966, Reay, 1970).
The sandeel's habit of burrowing can be seen both as an anti-predator behaviour and an energy conservation strategy. Evasion by burrowing in sand has frequently been observed in response to predators foraging near the seabed (Girsa and Danilov, 1976, Pearson et al., 1984, Pinto et al., 1984), although some predators are capable of capturing buried sandeels (Hobson, 1986). The lack of both a swim-bladder and fins capable of compensatory movements means that it is energetically costly for sandeels to remain in open water when they are not feeding (Reay, 1970). Burrowing may also allow sandeels to maintain their position without being displaced by currents close to the seabed, as has been suggested for flatfish (Arnold and Weihs, 1978).
Several studies have provided descriptions of the sediment, depth and water circulation that sandeels are associated with (Macer, 1966, Reay, 1970, Scott, 1973, Pinto et al., 1984, Scott and Scott, 1988). In common with several sandeel species, A. marinus inhabits shallow (<150 m), turbulent sandy areas, such as the edges of sand banks (Macer, 1966, Reay, 1970). From choice experiments it is clear that sandeels prefer sandy sediments to those that are predominantly gravel or silt (Pinto et al., 1984). Interest in identifying profitable feeding areas for predators has led some to infer sandeel distribution based on this available information and maps of sediment distribution (e.g. Wanless et al., 1997). However, these studies acknowledge that the lack of detailed knowledge about the relative importance of sediment composition may lead to an over-estimate of the extent of sandeel habitat.
The present study uses information collected on sediment fraction, particle size and depth from sites where sandeels were caught in benthic samples to consider the relative significance of these physical factors in influencing distribution. A general additive model framework was used to assess which physical factors significantly contribute to explaining sandeel occurrence and abundance. Apparent sediment preferences indicated from field observations were then investigated using sediment choice experiments.
Section snippets
Benthic surveys
The bottom topography of grounds where sandeels were present was examined using a variety of underwater video systems during surveys of the waters around Shetland (1985, 1990, 1993) and the Firth of Forth (1991). Low light video cameras were deployed by means of drifting frames (1990 and 1995) and towed bodies (1985) and using a remotely operated video system (SPRINT ROV, 1991). All video cameras were deployed to within a metre of the seabed. Bottom seabed features, such as the presence of sand
Description of sandeel grounds
Underwater video observations at grounds around Shetland, Orkney and the Firth of Forth showed the presence of sand ripples up to approximately 1.5 m apart and with height-to-length ratios ranging from <0.1 to 0.25. In general, the ripples were tangential to the tidal stream and tended to be symmetrical, indicating bed formations produced by oscillatory flow (Fig. 3). The characteristics of these sand ripples were consistent with descriptions of rolling-grain, 2D and 3D vortex ripples (Sleath,
Discussion
In common with previous investigations, the present study indicates that post-settled sandeels actively select specific substrates for burying. Around Shetland, A. marinus appears to have a strong preference for medium to very coarse sands (median particle size 0.25–2 mm). This range is slightly broader than the 0.35–1.35 mm particle range reported by Reay (1970) for sandeel grounds in the English Channel. The present study also provides direct evidence for such a preference based on the
Acknowledgements
This work was funded by The Scottish Executive, The Danish Institute for Fisheries Research, and contract XIV/C1 94/071 from the Commission of the European Communities. We would like to acknowledge the help of S. Greenstreet, A. McIntosh, H. Mosegaard, A. Nielsen, P. Lewy, J. Hislop, R. Hutcheon, A.D. Hawkins and and the masters and crews of FRV ‘Clupea’, RRS ‘Challenger’ and RV ‘Dana’. Wim van Raaphorst also provided useful comments on a draft of the manuscript.
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