Influence of Physical Parameters on Zooplankton Variability during Early Ocean Life of Juvenile Chum Salmon in the Coastal Waters of Eastern Hokkaido, Okhotsk Sea

摘要

Zooplankton variability during the early life of juvenile chum salmon and its relationship to physical environmental variability were studied from 2002 to 2004 in the Okhotsk Sea along the coast of eastern Hokkaido. During the study, the zooplankton taxa, Hydrozoa, Cladocera, Copepoda, Euphausiacea, Echinodermata and Appendiculata were commonly found. Cladocera, Copepoda, and Appendiculata were numerically dominant. At the boundary of the Soya Warm Current, the predominant zooplankton taxa shifted from Copepoda to Cladocera-Appendiculata. The predominant species of Copepoda, Cladocera and Appendiculata taxa were divided into two groups, based on their abundance and preferred water temperature and salinity. Group I was made up of cold-water species that prefer temperatures < 12°C and salinities < 33.6 psu. Group II was made up of warm-water species that prefer temperatures >12°C and salinities > 33.6 psu. Several species in Group I are important components in the diets of juvenile chum salmon. In Group I the cold-water copepod, Pseudocalanus newmani, was the most numerous species in Abashiri Bay from 2002 to 2004. Pelagic cold-water copepods (Neocalanus spp. other than Neocalanus cristatus) and neritic cold-water copepods (Eurytemora herdmani and Tortanus discauda-tus) were more abundant in 2004 than in 2002 or 2003. We theorize that the abundance of pelagic copepods may be related to the timing of the retreat of sea ice, and that the abundance of neritic copepods may be related to the displacement of less saline water under the influence of terrestrial inputs. The Soya Warm Current, sea ice, and less saline water are thought to be key factors affecting zooplankton variability and, by extention, influencing the life modes of juvenile chum salmon in the Okhotsk Sea.