Trophic segregation in multispecies community of salmonids in the Penzhina River lower course (Kamchatsky Krai, Russia)

Trophic relationships are presented for the juveniles of Coregonus sardinella, C. subautumnalis, C. pidschian, Prosopium cylindraceum, and Thymallus arcticus mertensii jointly inhabiting the Penzhina River lower reaches in summer. This community has no analogues in the North-Okhotsk region in terms of freshwater salmonid diversity. We analysed the stomachs content, indicative parasites infestation and stable isotopes ratio in the muscles. We demonstrated the similarity of food niches for the following pairs of species P. cylindraceum – T. а. mertensii and C. sardinella – C. subautumnalis. The summer diet of the species pair P. cylindraceum – T. а. mertensii mainly consisted of chironomid larvae. At the same time, mysids dominated in the diet of the species pair C. sardinella – C. subautumnalis. Coregonus pidschian mainly fed on gammarids. The following parasites have been found in juvenile fishes: Chloromyxum tuberculatum, Myxobolus spр., Proteocephalus longicollis, Triaenophorus nodulosus, Diplostomum sp., Metecercaria spp., Pronoprymna petrowi, Cystidicola farionis, Salmonema ephemeridarum, Raphidascaris acus, Pseudocapillaria salvelini, Echinorhynchus cotti, and Salmincola extensus; herewith, the intensity of invasion was low for all parasites. Only T. а. mertensii has been infested by C. tuberculatum, T. nodulosus, and C. farionis. Only C. pidschian was infested by P. longicollis. Only C. sardinella and C. subautumnalis have been infested by P. petrowi, while Salmincola extensus has infested only P. cylindraceum in the studied ecosystem. Thymallus arcticus mertensii and C. pidschian are significantly different from other fish species in term of infestation by the helminths E. сotti and P. salvelini. In the δ 13С–15N isotopes space, the fish juveniles formed three groups: 1) with high 15N level (C. sardinella – C. subautumnalis), 2) with low 13С level (C. pidschian), and 3) with low 15N – high 13С level (P. cylindraceum – T. а. mertensii). The complexity of trophic relationships of juvenile salmonids in the River Penzhina and an increased feeding competition in the species pairs threaten the functioning of the unique River Penzhina salmonid community under the conditions of extreme fishing pressure in the Northern Far East