Population structure of Euphausia pacifica in the Kuroshio-Oyashio transitional waters off northeastern Japan

Abstract

Population structure, abundance and biomass of Euphausia pacifica were surveyed during three seasons (January, February, May and October) mainly in the Kuroshio–Oyashio transitional waters off northeastern Japan between 1999 and 2002. Larval stages, juveniles and adults showed aggregated distributions. The abundance of juveniles and adults collected at night was significantly higher than those collected during the day. Active spawning and recruitment to larval stages occurred in May when the concentration of chlorophyll a was high. High abundance of juveniles occurred in October and based on back-calculating likely ages of the animals, another significant spawning period may have occurred between August and September. Both the abundance of adults and the biomass were higher in October. Substantial contribution to the total biomass by small adults (8–15 mm) occurred in January–February and October but by larvae and large adults in May. The proportion of males tended to decrease with increasing size for each season. The somatic growth pattern was estimated to be identical between sexes, thus it is considered that a higher mortality rate occurs in males as they grow. However, in this study using the cohort technique was found not to be suitable to determine the growth rate, and it is considered that an instantaneous growth rate incubation technique is needed to allow accurate measurement of the growth rate in the future.