Feeding Habits of the Blue Shark (Prionace glauca) in the Northwestern Pacific Based on Stomach Contents and Stable Isotope Ratios

The blue shark, Prionace glauca (L.), is one of the most abundant pelagic sharks, and determining their feeding habits is important for understanding the ocean ecosystem. Generally, stomach contents analysis has been used to evaluate its feeding habits; however simply analyzing stomach contents reveals short-term feeding habits. In this study, we examined feeding habits of blue sharks in the Northwestern Pacific based on results of both stomach contents and stable isotope analyses. The most dominant species and taxonomic group in the stomach were identified as anchovy and Oegopsida species, respectively. Mean δ13C and δ15N values of blue sharks were 18.5‰ and 12.1‰, respectively, which were higher than those of the prey items. Estimated proportional distribution of prey species varied according to discrimination factors used for stable carbon and nitrogen isotope ratios. Proportion of neritic or mesopelagic nekton, such as anchovy and lanternfish, was higher than that of other prey items including squids. We conclude that blue sharks capture prey ranging from neritic to mesopelagic animals through diel vertical migration.