Combined stomach contents and stable isotope analysis reveal feeding habits of Japanese jack mackerel ( Trachurus japonicus ) in the East China Sea

Japanese jack mackerel ( Trachurus japonicus ) is an economically important pelagic fish in the East China Sea. Knowledge is still limited regarding the feeding habits of this species in this area. Based on bottom trawl surveys conducted in the southern coastal area of Zhejiang during August in 2019 and 2020, we analyzed the feeding ecology of T. japonicus using stomach contents analysis and the stable isotope approach. The results indicated that the delta 13 C value for T. japonicus ranged from - 17.72 % to ;-14.30 %, and the delta 15 N value ranged from 10.89 % to 13.03 %. Two-way analysis of variance indicated significant annual and ontogenetic variations in the delta 13 C value, thereby suggesting changes in the final nutrient sources for T. japonicus in different growth stages and years. The trophic level of T. japonicus was estimated as from 3.35 to 4.46 . In general, the trophic level of T. japonicus increased with growth, although the ontogenetic variation was not significant. The main food groups identified for T. japonicus based on stomach contents analysis were fish, shrimp, crabs, cephalopods, and small crustaceans. The results obtained by stable isotope analysis showed that the nutrient contribution rate of small crustaceans to the diet of T. japonicus was highest in both years, followed by shrimp. The feeding habits of T. japonicus had obvious ontogenetic variations, and the average contribution of fish species increased as the size of the predator increased. The results obtained in this study provide detailed feeding information about T. japonicus in the East China Sea.