Estimation of eel larvae diet and feeding environment based on oceanographic observations and metagenomics

To clarify the natural diets of Pacific eel larvae, the eukaryotic composition of gut contents was compared with the particulate organic matter (POM) in environmental seawater using next-generation 18S rRNA gene sequences. And oceanographic observation was conducted in the oligotrophic tropical Northwestern Pacific areas where eel larvae feed. The marine environments of the area where leptocephali feed generally showed similarities across different geographical areas (horizontally) but differed vertically among the three layers of 10–50 m, 100 m-subsurface chlorophyll maximum (SCM), and 200 m. The SCM formed in the upper part of the thermocline was shown to have the optimal temperature for leptocephalus development. Dinoflagellates were the predominant eukaryotes in both gut contents and POM samples, and zooplankton was more abundant in gut contents than in POM. The composition of gut contents did not show the differences in geographical areas, fish species, and body size. The composition of POM also showed no geographical difference, but differed vertically among the three layers of 10–50 m, 100 m-SCM, and 200 m. When eel gut contents compared with POM at different depths in seawater, the composition was most similar to POM at the SCM. The SCM layer is estimated to be the location where eel larvae feed, given both the availability of their food and their growth in this layer. Thus, many migratory fish may prefer the tropical oligotrophic ocean for their spawning and nursery grounds.