Interactions Between Dutch Midwater Trawl and Atlantic White-sided Dolphins(Lagenorhynchus acutus) Southwest of Ireland

Abstract

Incidental catches of cetaceans in the Dutch pelagic trawl fishery are largely restricted to late-winter early-spring in an area along the continental slope southwest of Ireland. Available evidence indicates that annual variations in such incidents are large. Using combined mid-water trawl by-catch and dolphin stomach content data, the hypothesis is put forward that Atlantic white-sided dolphin (Lagenorhynchus acutus), normally a more oceanic species, may actively search for mackerel (Scomber scombrus) closer to shore in early-spring, at least in some years. Supporting data were from an observer program during 1992–94, which covered seven trips representing 5% of the annual effort in the fishery. Also over the period 1989–94, a total of 71 records of by-catch incidents were collected (involving a minimum of 312 individuals), of which 41 occured in 1994 (172 individuals). Approximately 90% of the incidents occurred late-winter early-spring, when both the mackerel and horse mackerel (Trachurus trachurus) pelagic fisheries were operating in the same area, southwest of Ireland. Incidence frequency peaked by late February and March, when mackerel is known to move into the area during its southward migration. The Atlantic white-sided dolphin was the main cetacean species in the by-catch (83% of all identified individuals). Other species recorded included long-finned pilot whale (Globicephala melas), short-beaked common dolphin (Delphinus delphis), bottlenose dolphin (Tursiops truncatus) and the white-beaked dolphin (Lagenorhynchus albirostris). The stomachs of 47 Atlantic white-sided dolphins, 11 common dolphins, two bottlenose dolphins and one white-beaked dolphin were examined. Fresh mackerel remains were found in nearly all white-sided dolphin stomachs, whereas fresh horse mackerel remains occured only in stomachs of bottlenose and common dolphins. Deep-water fish otolith incidents suggested that white-sided, common and bottlenose dolphin had completely different diets before moving to the southwest of Ireland.