Amphipoda in the diet of nekton in the Okhotsk Sea

Abstract

Amphipods (mainly hyperiids) are among the top-five taxonomic groups in the diet of many nekton species. Feeding of 106 nekton species dwelled in the Okhotsk Sea is considered (though 93 % of the stomach samples were collected from only 20 mass species). Amphipods are found in the food of 83 species and are absent for 23 species. Taxonomic composition of Amphipoda coincides in plankton and stomach samples; 3 species of hyperiids dominate here and there: Themisto pacifica, T. libellula, Primno macropa. The portion of amphipods in food is the highest for fish with length of 30–40 cm, decreases for smaller and larger fish, and practically disappears for fish with the length > 60 cm. In the food of mass commercial fish species, as pollock, cod, herring, silverfish, char, and pink, chum, sockeye and cherry salmon, the portion of hyperiids in the food could be almost 100 %, with CI of 200–700 ‱. During the daytime, the portion of large-sized hyperiids decreases up to 3 times, as they descend deeper than 200 m. T. pacifica have the highest biomass in the southern Okhotsk Sea, with a long-term tendency to increasing, and the lowest biomass in the northern Okhotsk Sea, with a tendency to decreasing both in the northern and central parts of the sea. P. macropa have similar spatial patters of distribution, but dynamics of their biomass has negative trend in the northern part, positive trend in the central part, and no trend in the southern part of the sea. On the contrary, the biomass of T. libellula is the highest in the northern Okhotsk Sea.