Incidental catches of marine-mammals in pelagic trawl fisheries of the northeast Atlantic

Abstract

Marine mammal by-catch in 11 pelagic trawl fisheries operated by four different countries in the northeast Atlantic was studied. Observers accompanied commercial fishing vessels and monitored 374 tows totalling 1771 h of towing during 377 days fishing. Three species of marine mammal were identified in by-catches (white-sided dolphin, Lagenorhynchus acutus, common dolphin, Delphinus delphis and grey seal Halichoerus grypus) and a fourth, bottlenose dolphin Tursiops truncatus, was probably present. Dolphins were caught in four of the 11 fisheries and seal in one. In those fisheries with cetacean by-catch, rates varied from 0.0606 to 0.1000 per tow and 0.0107 to 0.0137 per hour of towing and were highest in the French sea bass fishery and lowest in the French tuna fishery. Grey seals were caught in the Irish Celtic Sea herring fishery at a rate of 0.0513 per tow or 0.0396 per hour of towing. The mean ±SD dolphin catch rate for all fisheries combined was 0.048±0.013 per tow (one dolphin per 20.7 tows), or 0.0185±0.0019 per hour of towing (one dolphin per 98 h of towing) and, for all marine mammals, 0.059±0.019 (1 per 17.0 tows) or 0.0124±0.0121 (1 per 80.6 h of towing). 95% confidence intervals, calculated on untransformed data, for all fisheries combined were 0.4–1.6 dolphins per 100 h of towing. No operational factors were correlated with by-catch rates but the haul-back procedure was identified as a potentially important factor. All dolphin by-catches occurred during the night which may be a due to an association between cetaceans and trawlers at night. White-sided dolphins and grey seals were observed feeding around the net during towing and this behaviour may make them more vulnerable to capture. Operational difficulties in observing by-catch and potentially significant annual fluctuation in catch rates warrant further observer studies of these and other trawl fisheries.