Presence and behavior of harbor porpoises (Phocoena phocoena) around set nets revealed using passive acoustic monitoring

Abstract

Harbor porpoises (Phocoena phocoena) are often captured as bycatch in set net fisheries, which may have a large impact on their population and reduces the efficiency of fishing operations. Therefore, this study aimed to use passive acoustic monitoring using “A-tag” stereo acoustic data loggers to observe the presence and movement of harbor porpoises in two large set nets off Usujiri, Hakodate, Hokkaido, Japan, from April to May 2013–2015. Bycatch of harbor porpoises in the set nets was surveyed from onboard the set net fishing boat. Harbor porpoises were observed in the final trap of the set net on 10 separate days; however, bycatch was confirmed only once. These results indicate that the number of harbor porpoises captured as bycatch represents only a fraction of the number that are present in the set net, demonstrating their ability to escape from the net.