Spatial, Temporal and Dietary Overlap Between Harbour Seals Phoca vitulina and Fisheries in Erimo, Japan: Conflict at sea?

Abstract

Harbour seals are protected in Japan due to their restricted range, low population numbers and previous decimation by hunting, habitat damage and being caught incidentally in fishing nets. Since protection began in the mid-1980s, the total number of harbour seals in Japan has rebounded. With the rise in seal numbers, increased conflict with fisheries has occurred through depredation and the belief that seals compete with fisheries for prey. However, competition is unlikely if seals and fisheries take mostly different prey species and obtain fish from different areas or at different times. We studied the diet and foraging behaviour of harbour seals in Erimo, site of the largest population of harbor seals in Japan, from 2011-2012. We used satellite tags to track their movements through space and time, hard parts and DNA techniques to identify prey items in seal scats, then compared these results to local fisheries data. Of the 46 scats analysed, none contained salmon remains. Gadoids, sculpins and snailfish occurred in >70% of the scats. In contrast, the same species accounted for <15% in the mass of fisheries catches from 2009-2011. The foraging range of harbour seals overlapped with fisheries in all seasons, especially in autumn.