Amphipods (Amphipoda: Gammaridea) from the Piltun Gray Whale Pasturing Region, Northeastern Sakhalin Island (Sea of Okhotsk)

Abstract

Gray whales (Eschrichtius robustus) depend on amphipods as a food source. At present there are only two pasturing regions of gray whales along the shore of Sakhalin Island, Sea of Okhotsk. The Piltun pasturing region characterized by a large supply of forage benthos, is the most traditionally used region. Macrobenthic samples were taken by collaborators at the Institute of Marine Biology for three years between 2003-2005 from the period from July to September during research expeditions to the region. About 46 species of amphipods belonging to 30 genera and 12 families were found at depths between 8-36m in the Piltun region. Only 5 species had a frequency of occurrence of more than 50%. They were: Monoporeia affinis (56%), Eohaustorius eous eous (79%), Eogammarus schmidti (81%) Grandifoxus longirostris (53%), Anonyx nugax (54%). The most abundant species in this region were: M. affinis, E. schmidti, E. eous eous, Anisogammarus pugettensis, Protomedeia fasciata and G. longirostris. The species M. affinis had high abundance (77% of total abundance) as well as great values of biomass (54% of total biomass). The species E. schmidti had low abundance (5%), but high biomass (30%). In contrast, the species E. eous eous had relatively high abundance (7%), but low biomass (3%). The size- structure of three dominant species (M. affinis, E. schmidti, E. eous eous) was studied from more than 9000 individuals. The results showed that the size of M. affinis varied from 2.6 to 16.8mm, the size of E. schmidti from 8.5-27.0mm, and the size of E. eous eous from 1.6-8.2mm. Thus, the large size of the species M. affinis, E. schmidti may help there contribution to total biomass and abundance but the small species E. eous eous contributes to total abundance despite its size due to high densities.