Fisheries research vessels engaged in the International Bottom Trawl Survey in February 1993, May, August and November 1994, were joined by ornithologists from six insti tutions around the North Sea. Onboard oservations were designed to assess seasonal patterns in the spatial distribution of scaven ging seabirds in the North Sea and to study the attraction of fishing vessels for these birds. The selection and consumption of discards by seabirds was quantified during sessions of experimental discarding. Domi nance hierarchies were studied to obtain information on the vulnerability to robbery for each species. The spatial distribution of fishing vessels in the North Sea and discard practices were investigated and the results were used to analyse the attractiveness of different fisheries for seabirds, and to calculate the total number of active boats in different regions in the North Sea. Seabird distribution was studied by means of strip-transect counts. The attraction of fis hing vessels for seabirds was investigated by means of counts of birds associated with fishing research vessels and commercial fishing vessels. Trawler numbers were assessed by systematic counts using radar equipment. Discard experiments, in which the fate of measured discarded fish was studied were designed to assess consumption rates. Discarding practices were described from direct observations, interviews with fishermen and others, and literature. Eight species of seabirds utilised fishery waste on a large scale, at least during a part of the year. Consumption rates by seabirds, which were higher in winter than in summer, ranged from 95% for offal (entrails of gutted fish), to 80% for roundfish, 20% for flatfish and 6% for benthic invertebrates. All length classes of discards which occur normally in commercial fisheries can be consumed by seabirds. The median length of experimentally discarded roundfish consumed by seabirds ranged from 15 cm in a small species such as the Kittiwake to 25 cm in Gannets and Great Skuas. Fulmars and Kittiwakes were specialised feeders on offal. Discards size selection by different species of seabirds overlapped, leading to severe inter-specific competition. Many discards were stolen from smaller birds by larger species, with the result that success rates of the smaller birds were usually lower. Around 1500 active commercial fishing vessels were estimated to occur in the North Sea. The distribution of different types of fishing vessels are described. For none of the scavenging seabirds could the spatial distribution be fully explained by the presence or absence of commercial fishing vessels. The data obtained can be used to provide detailed advice when the potential effects of measures to reduce fishing effort in general or discards production in particular are evaluated. The effects of different measures can be predicted, using the information on fee ding success rates, vulnerability to robbery and prey selection. Anticipated scientific papers resulting from these studies include the distribution of seabirds in the North Sea, the distribution of fisheries in the North Sea, inter- and intra-specific relationships of scavenging seabirds, the analysis of inter-observer variance, and quantified information on discards utilisation by scavenging seabirds. Key words: Marine birds, census, diets, discards, ecological balance, feeding behaviour, feeding experiments, food preferences, interspecific relationships, nature conservation, marine fisheries, fishing effort, fishing gear, fishery management, ANE, North Sea, Skagerrak, Kattegat |
