Geographical Variation in Carbon and Nitrogen Stable Isotope Ratios and Feeding Habits of Japanese Jack Mackerel (Trachurus japonicus)

Abstract

Japanese jack mackerel (Trachurus japonicus) is subject to substantial and escalating commercial harvesting activities, yet its habitat structure is poorly understood. We analyzed the carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) of this species to evaluate geographical fluctuations in these ratios, allowing a delineation of the jack mackerel feeding habits. Fish specimens were collected across the East China Sea (ECS) and Sea of Japan (JS), and isotopic data were retrieved from two previous studies. Six subareas were considered: the southern, central, and northern ECS; Tsushima Strait; and eastern and western JS. The residuals of isotope ratios predicted from fork length showed significant geographical variation in the ratios (a lower δ¹³C in the JS than in the ECS and Tsushima Strait; a lower δ¹⁵N in the eastern JS and southern ECS than in other areas). Comparisons of stable isotope ratios between jack mackerel and prey candidates also revealed geographical differences in potential prey selection. The potential prey was identified as anchovy larvae and larger Copepoda in the western and eastern JS, respectively, whereas various prey species contributed to the diet in the other areas. Both δ¹³C and δ¹⁵N increased with fork length, suggesting ontogenetic changes in foraging habitat, associated with habit shifts into demersal layers of jack mackerel during their growth. Collectively, our results suggest that stable isotope ratio values—and consequently, the feeding habits—of jack mackerel differed between the ECS and JS but that they possess a limited migration range after an ontogenetic shift to demersal layers.