Stable Isotope Analysis Revealed Ontogenetic Changes in Trophic Ecology and Migration Patterns of Sepia esculenta in the Northern Coastal Waters of China

Golden cuttlefish (Sepia esculenta), the most economically important cephalopod in the northern coastal seas of China, had experienced greatly reduced population biomass due to continuous fishing pressure in recent decades. Understanding the trophic ecology and clarifying the spatial distribution of wintering ground can help develop management plans for this species. In this study, the ontogenetic changes in the trophic ecology of S. esculenta in the Qingdao coastal water were determined, and the migration patterns were studied using stable isotope analysis. Cluster analysis based on isotopic values divided S. esculenta with different lengths into groups: 11-20, 21-100, and 121-200 mm. A significant difference in the delta C-13 values between the groups 11-20 mm (-17.10 parts per thousand) and 21-100 mm (-15.89 parts per thousand) illustrates an ontogenetic change in the feeding habits. Due to the migratory habits of S. esculenta, the delta C-13 value of the group 121-200 mm (-16.39 parts per thousand) was lower than that of the group 21-100 mm. The delta N-15 values of S. esculenta were found to increase in length, and there was a clear linear relationship between different S. esculenta groups, suggesting that the wintering ground may locate in the same latitude as the spawning ground (i.e., the middle Yellow Sea). Furthermore, the trophic relationship between S. esculenta and coexisting species was assessed, revealing that the group 11-20 mm of S. esculenta has some overlap of carbon isotope space with other species, suggesting that these species may feed on it as prey. Thus, slightly increasing the length to more than 20 mm may reduce the pressure of being the prey of post-release juveniles of S. esculenta and improve the effect of release.