Intraspecific differences in the diet of Kuril harbor seals (Phoca vitulina stejnegeri) in Erimo, Hokkaido, using DNA barcoding diet analysis

Abstract

Competition between Kuril harbor seals (Phoca vitulina stejnegeri) and salmon set-net fishing industries has become a serious problem with the recent increase in the number of seals in Erimo, Hokkaido, Japan. We aimed to understand the detailed dietary structure of Kuril harbor seals focusing on intraspecific differences and verify whether “problem seals” who habitually use salmon set-nets could be characterized by intrinsic factors such as sex and maturity. We estimated the diet of Kuril harbor seals in two fishing seasons using DNA barcoding diet analysis on colon contents and verified intraspecific differences in their diet. In spring, their diets showed different tendencies between maturity stage; each adult seal fed on different prey items, suggesting that they avoid the intraspecies competition over food during the breeding season. Additionally, it was implied that some adult females habitually stole from salmon set-nets. Our dietary analysis showed dietary changes of Kuril harbor seals with different tendencies depending on maturity or sex, suggesting that problem individuals who habitually use set-nets can be characterized by intrinsic factors. This detailed dietary information can offer an accurate assessment of seal predation effects on fishing targets and the selective management of Kuril harbor seals, especially in mitigating seal–commercial fishery conflicts.