2005年5月の中部日本海能登半島周辺海域におけるカイアシ類ノープリウスの水平分布

抄録

The central Sea of Japan around the Noto Peninsula is an important nursery ground for Japanese anchovy. The larvae of Japanese anchovy mainly feed on copepod nauplii. However, little information on the spatial distribution of copepod nauplii has been available in this region. In this study, we studied the species-specific distribution of copepod nauplii in May 2005, which corresponds to the main spawning season of Japanese anchovy. The most dominant species (>8.0 ind. L^-1) were Oithona atlantica N2–N6, Oithona similis N2–N6 and Paracalanus sp. N2–N6. All these species were in high abundance off Niigata. From a comparison with environmental parameters, nauplii of Oithona nana showed positive correlations with temperature and salinity. Conversely, nauplii of O. atlantica, O. similis, Paracalanus sp. and Pseudocalanus spp. had negative correlations with temperature and salinity. Cluster analysis based on the abundance of the eight dominant species classified the 60 stations into four groups (A–D). Occurrence of each group was separated geographically. Group D, which had the largest numbers of stations but the lowest abundance of naupliar, was distributed from Wakasa Bay to Toyama Bay. These areas were warm, saline and low in chlorophyll a, indicating that group D was characterized as an oligotrophic naupliar community. The other three groups (A–C) were mainly concentrated within smaller areas off Niigata, where community diversity was relatively higher. River runoff (ice-melt water from the mountains) from various large rivers in this area may provide a nutrient input, phytoplankton bloom and induce massive reproduction of copepods. This mechanism may maintain a high biological community diversity of copepod nauplii in this region.