Seasonal changes in the density and vertical distribution of nauplii, copepodites and adults of the genera Oithona and Oncaea (Copepoda) in the surface water of Toyama Bay, southern Sea of Japan

Abstract

In this study, we examined seasonal changes in the vertical population density of each developmental stage (nauplius and copepodite) of Oithona and Oncaea species in the surface water of Toyama Bay from 1997 to 1999. Fur- thermore, we investigated the detailed vertical distribution (0 m, 25 m and 50 m depth) of the naupliar stages, which are regarded as an important food source for fish larvae. Major changes in the community structure of nauplii, which oc- curred at different sampling depths and in different seasons, are considered to have been influenced by the occurrence of adults and changes in the hydrographic conditions, including temperature and water currents. In winter to early sum- mer (January to June) nauplii of Oithona similis and Oithona atlantica, copepodites and adults, were abundant, while those of Oithona plumifera, and other species of Oithona and Oncaea showed a higher density between August and De- cember. During the spring season, when many fish larvae appeared, O. similis nauplii occurred at higher densities of more than 10 5 indiv. m � 3 in the upper layer (0 m and 25 m depth) than those of O. atlantica. The weighted mean depth of O. at- lantica nauplii was significantly deeper than that of O. similis nauplii. The respective vertical distributions of the nauplii of O. atlantica and O. similis were similar to the distributions of adults. Oithona similis nauplii are suggested to be a potentially important food item for fish larvae in spring because of their high density and similar vertical distribution to many fish larvae.