Quantitative distribution and feeding of euphausiids in the Okhotsk Sea

Euphausia in the Okhotsk Sea is one of the most abundant groups of zooplankton, its portion in the total zooplankton biomass in the epipelagic layer is 25.4-37.4 %, depending on season, the total stock is 83.2 · 106 t in spring, 66.3 · 106 t in summer, 57.4 · 106 t in autumn, and 73.3 · 106 t in winter (mean assessments for 1986-2012). The highest biomass is observed in spring - 369 mg/m3, on average, and the minimum one in autumn - 256 mg/m3, on average. Two species, Thysanoessa raschii and Thysanoessa longipes , are the most abundant. Year-to-year fluctuations of Euphausia abundance are prominent: up to 5 times, but it never causes any catastrophic change in their populations. Sharp declines usually are followed by sharp rises in their number. Mean daily ration of euphausiid in the Okhotsk Sea is 7.5-8.5 % of the body weight. Annually they consume 2012.7 · 106 t of prey, including 1636.5 · 106 t of phytoplankton and 341.8 · 106 t of micro-heterotrophs. About 11.4 % of the annual production of phytoplankton and 26.0 % of the annual production of micro-heterotrophs are grazed by euphausiids