Long-Term Patterns in the Diets of Japanese Temperate Bass Lateolabrax japonicus Larvae and Juveniles in Chikugo Estuarine Nursery Ground in Ariake Bay, Japan

Larval and juvenile Japanese temperate bass (Lateolabrax japonicus) samples were collected from a wide range of spatial gradients (covering a distance of approximately 30 km) in Chikugo estuary, Ariake Bay, Japan over a period of 8 yr (1997-2004) in order to observe changes in diet. Gut contents were studied by separating, identifying, counting, and estimating the dry weight of prey organisms. Copepod samples were collected during each cruise to observe the numerical composition, abundance, and biomass in the estuary. Considerable spatial and temporal variations were observed in copepod distributions in ambient water and the diets of the fish. Two distinctly different copepod assemblages were identified in the estuary: one in the upper estuarine turbidity maximum (ETM), dominated by a single species Sinocalanus sinensis and the other in the lower estuary consisting of a multispecies assemblage, dominated by Oithona davisae, Acartia omorii, Paracalanus parvus, and Calanus sinicus. The gut content composition of the fish in the upper estuary was dominated by S. sinensis, while in the lower estuary, it consisted of P. parvus, O. davisae, and A. omorii. Within the size group analyzed (13.0-27.0 mm SL), the smaller individuals were found to feed on a mixed diet composed of smaller prey. The diets gradually shifted to bigger prey composed predominantly of S. sinensis for larger size groups. Greater proportions of empty guts were recorded in the smaller individuals and dropped with increasing fish size. Higher dry biomass of copepods in the environment, as well as higher dry weights of gut contents, were recorded in the upper estuary, indicating that the upper estuarine ETM areas are important nursery grounds for the early life stages of the Japanese temperate bass. The early life stages of the Japanese temperate bass are adapted to use the upstream nursery grounds and ascending to the nursery areas to use S. sinensis is one of the key survival strategies of the Japanese temperate bass in the Chikugo estuary.