Vertical distribution,abundance and community structure of oncaeid copepods in the Oyashio region,western subarctic Pacific

The vertical distribution, abundance and community structure of oncaeid copepods were investigated in the Oyashio region, western subarctic Pacific. Samples were collected with a 0.10 mm mesh closing type net from five discrete layers down to a maximum depth of 2,000 m in September and December 1996 and in April and October 1997. The copepods were widely distributed from epipelagic to bathypelagic zones, and showed prominent peaks of abundance above the thermocline and/or between 250 and 1,000 m depth. Standing stocks of total oncaeid copepods ranged from 1.5 to 2.5×10⁵ inds m^–2 at 0–2,000 m in the water column, which are the same order of magnitudes as those reported in tropical, subtropical and polar regions by previous workers. A total of 38 species and two forms belonging to the genera Oncaea, Triconia, Spinoncaea, Conaea and Epicalymma, and two provisionally classified species of the family Oncaeidae were identified in this study. Of these, 14 species have already been recorded from the eastern subarctic Pacific. Several warm-water species were also found in December 1996 and/or October 1997, when the effect of warm-core rings originating from the Kuroshio Current was evident in the epipelagic zone. Dominant species throughout the whole water column were T. borealis, T. canadensis, O. grossa, O. parila, O. rimula, O. lacinia, Epicalymma spp. and Oncaea sp. A, and these eight species together always made up about half of the total oncaeid numbers. Community analysis revealed that species composition in epipelagic and upper mesopelagic zones varied temporally due to the changes of hydrographic conditions, whereas those in lower mesopelagic and bathypelagic zones were relatively stable, reflecting the constancy of deep environments.Communicated by O. Kinne, Oldendorf/Luhe