Vertical segregation and population structure of ontogenetically migrating copepods Neocalanus cristatus, N. flemingeri, N. plumchrus, and Eucalanus bungii during the ice-free season in the Sea of Okhotsk

The Sea of Okhotsk is a semi-enclosed marginal sea of the subarctic North Pacific. One of the prominent characteristics of this area is the fast ice coverage during winter and early spring. We investigated the life cycles and vertical distributions of four species of large suspension feeding copepods, Neocalanus plumchrus, N. flemingeri, N. cristatus , and Eucalanus bungii in the southern part of the Sea of Okhotsk during the ice-free season. Neocalanus plumchrus and N. cristatus showed annual life cycles while a major part of the population of N. flemingeri and E. bungii showed biennial life cycles. Their occurrences in the surface layer for grazing and development were 1–2 months later than in the adjacent area of the Pacific Ocean, and they remained longer in the surface layer for development. Consequently, the seasonal overlap among the copepods in the surface productive layer was intensified. In contrast, the vertical distribution of the four species of copepods during the growing period extended to deeper layers and became more finely segregated than those of Oyashio region and open ocean of the North Pacific. Thus, the seasonal segregations in the Oyashio region are replaced by vertical segregation in the Sea of Okhotsk. A major factor modifying the seasonal occurrences in the surface layer was considered to be the presence of fast ice during winter and early spring which greatly reduces the photosynthesis in the water column.