Een verkennend onderzoek De exploitatie van op zee overboord geworpen vis en snijafval door zeevogels The exploitation of discards and offal by seabirds: a pilot study

This paper reports on field experiments on board the fishery research vessel Tridens in November 1992, meant to study the consumption of discards and offal by associated seabirds. This pilot study was conducted in the first place to check whether or not methods developed by Anne Hudson and Bob Furness (Glasgow University) for whitefish trawlers around Shetland and Nephrops-fisheries in West Scotland could be applied to Dutch beamtrawl fisheries. Between 9 and 11 November, the ship was beamlrawling at some 90 km to the northwest of Umuiden, the Netherlands (figure 1). Associated birds were counted during different activities of the ship (table 1). Obviously, considerably larger numbers of birds were joining the vessel when it provided discards/offal as compared to normal steaming or towing without discarding or gutting (table 2). Fish and offal were sampled on board and discarded experimentally, recording the fate of fish after measuring to the nearest cm. (table 3-4). The various species of associated seabirds were found to stay in different positions behind the ship: small gulls (e.g. Kittiwake and Common Gull) flew close to the stern, while larger gulls followed the ship at a larger distance (figure 2). Fulmars and Gannets kept on flying large circles, while Great Skuas visitied the associated flock only briefly. Two-thirds (67.2%) of all discarded biota were consumed by seabirds (table 4). Kittiwakes proved to be very successful in picking up the offal and smaller fish near the stern alighting immediately after the offal was thrown into the sea. When fish were dropped, or when larger quantities were thrown at once, the biota were floating further away from the ship (which kept on steaming during all activities), and Kittiwakes were driven away by the other seabirds. All discarded Whitings, the most numerous fish in the discard fraction, could be consumed by one or the other seabird species (table 5). Kittiwakes picked up most of the smaller specimens (<23 cm; table 6, figure 3), but tried to lift several of the larger fish (up to 26 cm). Behind the ship, a dominance hierarchy was found, in which Gannet and Great Black-backed Gulls were most powerful (figure 4). However, Kittiwakes, being small, fast and manoeuvrable birds, obtained an impressive part of the discarded biota. It is concluded that the methods developed in Scotland were reasonably good to study discard consumption in beamtrawlers. However, the constantly moving ship made it impossible to discard and record more than a few particles at a time; a situation which is only partly realistic. The data are now seriously biased towards the smaller scavengers near the vessel.