Effects on benthic scavengers of discards and damaged benthos produced by the beam-trawl fishery in the southern North Sea

Abstract

The beam-trawl fishery for flatfish produces large amounts of dying discards as well as damaged and disturbed benthos. The importance of these food sources to scavenging benthic species was investigated. To identify epibenthic species showing scavenging behaviour and to detect their food preferences, traps (370 overall) baited with different kinds of carrion (fish, crustaceans, molluscs, echinoderms, polychaetes) were deployed at 14 locations in the southern North Sea. Liocarcinus holsatus, Pagurus bernhardus, Asterias rubens, ophiurids, and small gadoids were the main active scavengers feeding on different kinds of food, while lyssianid amphipods (Orchomene nanus, Scopelocheirus hopei) fed mainly on crustacean carrion. Estimated attraction areas were largest for gadoids, hermit crabs, and swimming crabs. Immigration of fish and swimming crabs into experimentally trawled areas was observed by sampling repeatedly in a trawled strip. Differences in the response to trawling (feeding activity, food niche breadth, and resource partitioning) between selected fish species are described. Limanda limanda, Pleuronectes platessa, Merlangius merlangus, Callionymus lyra, and Eutrigla gurnardus all showed strong responses, using different feeding strategies. In situ clearance rates of dead discarded fish exposed on the sea floor were estimated. Discards as well as moribund benthos were consumed within a few days. The balance between the amount of food materials produced by beam-trawl fishery and the amount of food consumed by potential scavengers was translated in a model of the food flows as mobilized by beam trawling. In general, the direct importance of the additional food resource for populations of scavengers is considered to be relatively small. However, the importance may be relatively larger for scavenging fish than for invertebrates. It is estimated that after a single beam trawling about 1.27 g m 2 ash-free dry biomass, or 6% to 13% of the annual secondary production of macrobenthos per unit area, would suddenly become available to scavengers and to the detritus food chain. It is suggested that beam trawling leads to shortcuts in trophic relationships and therefore may enhance secondary production. 2000 International Council for the Exploration of the Sea