Factors Affecting Harbour Porpoise Bycatch Occurrence In the Swedish Skagerrak and Kattegat Seas

Sammanfattning: Incidental catch (bycatch) in fisheries is one of the major threats to marine mammal populations worldwide. The harbour porpoise (Phocoena phocoena, Linneaus 1758) is one of the most common cetaceans in northern Europe. Previous studies have reported high bycatch numbers of harbour porpoises in gillnet fishing operations in the North Sea and Skagerrak Sea. With the aim of disentangling the driving factors behind bycatch of harbour porpoises, I looked at the relationship between occurrence of harbour porpoise bycatch and properties of fishery and porpoise density. Parameters of fishing effort, gear characteristics and operations of 951 sets of strings of cod and pollack bottom set gillnets were collected by an on-board observer programme in the Swedish Skagerrak and Kattegat Seas during 1995-1997. Porpoise density data was generated from satellite tracking locations of 116 individuals from 1997 to 2016. Generalised linear models with binomial distribution were applied to identify the relationships between the occurrence of porpoise bycatch and seven explanatory variables extracted as soak time, string length, net height, mesh size, net bagginess, water depth and porpoise density. During the observer programme, a total of 21 porpoises were bycaught during 10,174 km'h of observed fishing effort. The total bycatch was estimated to be 2.6% of the population abundance at that time, i.e. above the maximum sustainable total anthropogenic removal (1.7%). Soak time, string length and water depth were identified to be positively related to bycatch occurrence. The findings provide guidance for bycatch mitigation measures in terms of adjusting fishing operations, improving gear and establishing separate management units. Modelling can be used to predict possible spatio-temporal high-risk hot spots for harbour porpoise as well as other marine-living species to optimise future monitoring efforts.