2017 Volume 64 Issue 1 Pages 22-26
We investigated the summer–autumn distributions of the subarctic copepod Neocalanus plumchrus (Marukawa) as food for young chum salmon, Oncorhynchus keta (Walbaum) in the surface waters of southwestern Okhotsk Sea, using zooplankton samples collected by vertical hauls (0–50 m depth) with Norpac net (mouth diameter: 45 cm, mesh opening: 0.315 mm) in July 2007, August 2012, September 2007, October 2004 and 2005. From investigation of the zooplankton samples, almost all 5th stage copepodids (CV) N. plumchrus were not observed during late September to early October due to the seasonal descent into deep layers. This disappearance from surface layer was 1 to 4 (mean: 2.5) months later than those in the adjacent waters (Oyashio region and Japan Sea). They developed lipid accumulation from L (low lipid store) or M (medium lipid store) type in summer into H (high lipid store) type in early autumn, maintaining high abundance as observed in late July. The development of lipid accumulation was considered to be closely related with the food availability for CV, sustained by the stably high primary production at the subsurface chlorophyll a maximum from summer to autumn, under the lower temperature due to the presence of the Intermediate Cold Water. Thus, the lipid-rich CV may serve as a large-sized and high-caloric food source for seasonal migratory fishes (e.g. young chum salmon) in the summer–autumn pelagic ecosystem in the Okhotsk Sea.
