Molecular-based diet analysis of the early post-larvae of Japanese sardine Sardinops melanostictus and Pacific round herring Etrumeus teres

Successful feeding on preferred prey could be important for the survival of fish larvae. However, high taxonomic resolution of prey types from damaged gut contents is difficult to achieve through morphological classification, especially for fragile organisms or immature stages. In this study, <10 mm early post-larvae of Japanese sardine Sardinops melanostictus and Pacific round herring Etrumeus teres were collected from Tosa Bay (Japan) during their main spawning periods. Diet and plankton communities present in the environment were investigated using molecular operational taxonomic units (MOTUs) clustered using eukaryotic metagenetic data from the 18S V9 region. There were no clear differences in the diets of the co-existing fish species. Fragile protists and gelatinous metazoans occasionally represented large proportions of gut content; however, copepods comprised the majority of the prey items. Among the copepods, the Calanidae MOTU derived from the large copepod Calanus sinicus was most consumed, followed by the Paracalanidae MOTU identified as small Paracalanus parvus s.l. The C. sinicus MOTU largely dominated inter mediate (0.5-1.0 mm) and large (1.0-2.0 mm) environmental mesoplankton communities, whereas the P. parvus s.l. MOTU was the most dominant in small-sized (0.1-0.5 mm) communities. Early post-larvae fish were considered to prefer eggs or nauplii of C. sinicus, which was the second-dominant MOTU in small-sized communities. These results suggested that both food availability and developmental stages of copepods determined prey preference.