Inter-oceanic comparison of planktonic copepod ecology (vertical distribution, abundance, community structure, population structure and body size) between the Okhotsk Sea and Oyashio region in autumn

Several aspects of the ecology of planktonic copepods (vertical distribution, abundance, community structure, population structure and body size) were evaluated and compared between communities in the autumn, in the Okhotsk Sea and the adjacent Oyashio region in the western North Pacific. Vertically, copepods were concentrated primarily at depths of 250 to 500m in the Okhotsk Sea but near the surface in the Oyashio region. The abundances of most of the copepods were greater in the Oyashio region with the exception of Metridia okhotensis, which showed significantly greater abundance in the Okhotsk Sea (30 times greater) and dominated the copepod community, accounting for approximately 70% of total copepod abundance. The population structure of the dominant copepods in the Okhotsk Sea was dominated by late copepod stages, suggesting that these copepods were in the resting phase. The prosome lengths of most of the copepods were larger in the Okhotsk Sea than in the Oyashio region and the larger body size is probably due to the lower habitat temperatures. The special ecological characteristics of planktonic copepods in the Okhotsk Sea are possibly related to the development of a strong pycnocline in the Okhotsk Sea. The consequences of differences in copepod communities between regions were discussed from the viewpoints of life cycle timing and the scale of active vertical flux.