In Ariake Bay, catches of the manila clam Ruditapes philippinarum have decreased considerably due to the low rate of larval return to adult habitats, shrinkage of larval networks among each local population, and shortening of the occurrence period of larvae. In this study, we examined the seasonal and annual occurrence trends of planktonic clam larvae and the environmental factors related to the quantity of larvae based on surveys conducted in the Ariake Sea between 2015 and 2018. In the Ariake Sea, emergence peaks with population densities of more than 1,000 m⁻³ individuals occurred during the spring and fall spawning seasons. In spring, the peak onset often occurred between late April and May, but trends also varied from year to year. In autumn, the peak period was observed at approximately the same time every year, from mid-October to November. The peak emergence of plankton larvae was thought to be related to water temperature.

It has been thought that food availability for fish larvae was poor in the Kuroshio because of the low standing stocks of plankton in the oligotrophic conditions under thermal stratification throughout the year. Despite a potential risk or disadvantage for larval survival and growth, Kuroshio and its neighbouring waters are nursery grounds for the early life stages of various fishes. Here, we compared the growth activity of fish larvae among 15 taxonomic groups, including 11 families in the Kuroshio and its neighbouring waters, based on protein synthetase activity and protein contents. Protein-specific (_spAARS), individual-based aminoacyl tRNA synthetase activities (_iAARS), and protein contents (PRO) of fish larvae ranged from 2 to 232 nmol PPi mg protein⁻¹ h⁻¹, from 1 to 21 μmol PPi ind⁻¹ d⁻¹, and from <1 to 26 mg ind⁻¹, respectively. _spAARS, _iAARS and PRO were variable among the taxonomic groups and were not classified between mesopelagic groups and the others. Compiling these measurements among all taxonomic groups, a significant negative correlation was found for _spAARS to PRO. The correlation showed no significant difference between the Kuroshio and its neighbouring coastal waters, indicating that growth activities to fish larvae body mass were comparable in the Kuroshio and its neighbouring waters. Based on nonmetric multidimensional scaling on _spAARS, _iAARS and PRO among 15 taxonomic groups, two different groups were classified for fish larvae, represented by the high _iAARS under the high PRO but low _spAARS (Group 1) and the low _iAARS under the low PRO but high _spAARS (Group 2), representing the different life strategies for larval growth among the taxonomic groups. The present findings suggest that these biochemical indices are useful for evaluating the growth activity of fish larvae among various taxonomic groups and that food availability is not poor enough to support larval growth in the Kuroshio and its neighbouring waters.