Abstract

Throughout their range, marine mammals are vulnerable to incidental capture in fishing gear. Bycatch is a particularly important issue for cetaceans, which generally have lower potential rates of population growth and are thus more susceptible to the effects of bycatch mortality than other marine mammals. Spatial and temporal gaps in our knowledge of cetacean distributions can make it difficult to mitigate and manage bycatch. Here, we modelled the spatial and temporal distribution of two species of cetacean for which bycatch is known to occur; the common dolphin (Delphinus delphis) and the harbour porpoise (Phocoena phocoena) and the spatial and temporal distribution of six different gear types. The models were integrated into a Productivity Susceptibility Analysis (PSA) to calculate risk in both spatial and temporal dimensions. Models showed temporal variation in spatial distribution for both species but particularly for harbour porpoise, whilst common dolphins were distributed in association with areas of high primary productivity where pelagic fish are likely to shoal. The risk assessment indicated two areas in the study region where risk from bycatch is likely to be particularly high, with gillnets being assessed as the highest risk gear type. Results should be used to guide fisheries and bycatch management through an integrated ecosystem approach, and could be used as a guide to where further observer effort should be focused. Results also provide valuable information for the designation of Marine Protected Areas for marine mammals.