Temporal variations in the copepod community structure during the sand lance fishery season near the Akashi Strait, eastern Seto Inland Sea

Around the Akashi Strait, about half of the catch of the sand lance in Harima-Nada and Osaka Bay is caught in winter to early spring. The annual catch has continued to decrease after the 1990s, and it is suggested that the decline in catch was caused by copepod shortage as food. In the present study, frequent samplings of copepods were conducted from December to early April around the Akashi Strait for three years to evaluate copepod abundance and biomass. Around the Akashi Strait, the dominant species by abundance were Paracalanus spp., Oithona spp. and Microsetella norvegica, but by biomass, the most dominant was Calanus sinicus. Cluster analysis showed that copepod adult communities, based on biomass, were classified into five groups, and the representative species were Oithona similis, Subeucalanus crassus, M. norvegica, Euchaeta plana and C. sinicus, respectively. Especially from February to early April, most of the copepod communities were classified into cluster group 5, characterized by a high biomass of C. sinicus. This period coincides with the active feeding on copepods by sand lance. These results suggest that C. sinicus is the most important prey item for sand lance around the Akashi Strait. In addition, the temporal recruitment of pelagic species such as S. crassus and E. plana contributed to an increase in copepod biomass. There is concern that environmental shifts will lead to declines in C. sinicus, resulting in a decline in the total copepod biomass in this area.