Estimated bycatch of harbour porpoise (Phocoena phocoena) in two coastal gillnet fisheries in Norway, 2006-2008. Mitigation and implications for conservation

Using data collected during 2006-2008 from a monitored segment (18 vessels) of the Norwegian coastal fleet (vessels <15 m) of gillnetters targeting monkfish and cod, we used general additive models (GAMs) to derive bycatch rates of harbour porpoise. These bycatch rates were then applied to fishery catch data on the target species to estimate the total number of porpoise taken by the coastal gillnet fisheries. The two best models estimated bycatches of 20,719 and 20,989 porpoises during 2006-2008, with CVs 36% and 27%, respectively. Thus, about 6900 harbour porpoises are taken annually in the coastal monkfish and cod gillnet fisheries. Although no abundance estimate is available for the coastal harbour porpoise population, this annual bycatch is likely not sustainable according to the management objectives defined by ASCOBANS. In the cod gillnet fishery, harbour porpoise bycatch rates decreased rapidly with increasing depth to 50 m and then levelled off. In the monkfish gillnet fishery, bycatch rates decreased linearly with increasing depth throughout the depth range fished. To reduce harbour porpoise bycatches, we recommend that large mesh nets associated with the monkfish fishery to be prohibited at depths less than 50 m. We also recommend to conduct experiments using Acoustic Deterrent Devices (ADDS or 'pingers') on nets set deeper than 50 m. If these devices prove successful in reducing porpoise bycatch, we propose that ADDs should be implemented in the Norwegian coastal gillnet fisheries for cod and monkfish. (C) 2013 Elsevier Ltd. All rights reserved.