Spatio-temporal associations between harbour porpoise Phocoena phocoena and specific fisheries in the German Bight

Abstract

Interactions between cetaceans and fisheries are a major concern of cetacean conservation and fisheries management, as by-catch and resource competition occur as a result. To assess potential interaction between porpoises and fisheries in the German Bight, high-resolution spatial and temporal data were analysed in order to reveal association patterns between cetaceans and fisheries in the German Exclusive Economic Zone (EEZ) of the North Sea. To facilitate interpretation of resulting patterns, additionally, relationships of fish assemblages were analysed. Vessel monitoring system (VMS) data from 2006 and fisheries surveillance data from 2002 to 2006 were acquired to calculate fishing effort, aerial survey data from 2002 to 2006 to evaluate porpoise distribution, and trawl survey data from 2000 to 2007 to assess fish assemblage. Relationships were analysed in terms of seasonal association and overlap. Results showed that porpoise distribution was positively associated with sandeel fisheries in all seasons and with gillnet fisheries in summer. Overlaps for sandeel and gillnet fisheries were high in summer, while negligible in spring and autumn. In spring, high overlap was found for large beam trawlers targeting sole and plaice. In parallel, porpoise distribution was positively linked to fish assemblage components representing sandeel and poor cod in summer and flatfish in spring. Our results suggest that, especially in summer, resource competition between porpoises and industrial sandeel fisheries is likely and the risk of by-catch in gillnets is increased. In spring, interference with sole and plaice fisheries is to be expected.