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Species composition and vertical distribution of oncaeid copepods, which are potentially important prey for juvenile fish, were investigated in the Kuroshio Extension region, the NW Pacific, in April, August, November 1998 and February 2001. Samples were collected from 8 discrete layers in the epipelagic zone (0-200 m depth) using MOCNESS (0.064 mm mesh) during both day and night. Thirty-five oncaeid species were identified. Oncaea (s.l.) zernovi and Spinoncaea ivlevi were numerically the dominant species comprising 20.0-48.2% and 15.2-26.8%, respectively, of adult oncaeid copepods in the epipelagic zone. Cluster analysis on all samples revealed that these were separated into three groups with discrete vertical ranges; the first one appearing in the 0-50 m depth surface layer in April and August and consisting mainly of Oncaea (s. str.), the second one located in the deepest layer and composed mostly of O. zernovi and S. ivlevi with some mesopelagic species, and the third one located above the second one and having intermediate species composition. Species-specific vertical distributions indicate that most oncaeid populations shifted downward from August to November, when the thermocline remarkably descended. However, most Oncaea spp. did not show a downward shift with the thermocline, and were positively correlated to appendicularian abundances, suggesting that appendicularian houses, known to be oncaeid habitats and to provide food, were a possible factor affecting their vertical distribution. Niche partitioning, allowing coexistence of congeners, might be explained by differences in body size and distribution layers in Oncaea and by differences in distribution layer in Triconia.

Seasonal changes in reproduction rates of three dominant oncaeids, Oncaea media, O. venusta f. venella and O. venusta f. -1, and effects of environmental factors on them were examined in the surface layer (0-30 m) in the Kuroshio Extension and adjacent waters off the Pacific coast of central Japan. The biomass of oncaeid copepods peaked in spring, and remained at about 14% of total copepod biomass throughout the year. Monthly mean specific egg production rates of O. media, O. venusta f. -1 and O. venusta f. venella ranged from 0 to 0.038 day )1 , from 0.026 to 0.051 day )1 and from 0.022 to 0.049 day )1 , respectively, and were relatively higher from winter to spring. Specific egg production rates of both O. media and O. venusta f. venella were positively related to primary production in the euphotic layer. Analysis of temperature effects on reproduction parameters indicated that low temperature was one cause for high specific egg production rates of O. venusta f. -1 in winter, mainly because of an increase in clutch size. Environmental factors thus affect reproduction of the dominant oncaeids in the surface layer of the Kuroshio Extension, and an increase in temperature and decline in primary production would reduce their reproduction.