Feeding habits of the cutlassfish Trichiurus japonicus in Osaka Bay, Japan

The feeding ecology of the cutlassfish Trichiurus japonicus in Osaka Bay was investigated using stomach content analysis and stable isotope ratios from monthly samples collected between November 2018 and February 2022. The index of relative importance indicated that fish dominated the diet (95.97%), with Engraulis japonicus as the primary prey. As body size increased, fish prey became more dominant, with larger individuals consuming relatively large prey such as Konosirus punctatus. When E. japonicus was not detected in stomach contents, substantial amounts of crustaceans such as Leptochela gracilis and cephalopods were found, indicating opportunistic use of alternative prey. Comparison with 1970s data showed that the importance of E. japonicus as prey has remained unchanged. Stable isotope analysis (δ¹³C: −17.4 to −14.2‰; δ¹⁵N: 15.8–19.9‰) showed consistently high trophic position with little seasonal variation, contrasting with clear seasonal and ontogenetic patterns in stomach contents. T. japonicus is a high-level predator highly dependent on E. japonicus, with opportunistic use of alternative prey. This study demonstrates the complementary value of integrating multiple analytical approaches for understanding trophic ecology in complex coastal ecosystems.