Trophic Relationships of Coregonid and Percid Fishes Evaluated by the Stomach Content and Stable Isotopes in Subarctic Lake Imandra

Fish communities undergo climate-induced shifts; it is crucial to study the trophic interactions of various fish species in order to understand the extent to which fish trophic niches overlap and the degree of competition between them. We investigated the food web structure, feeding habits, and trophic positions of common fish in the subarctic Lake Imandra. Two methods were used: SCA (stomach content analysis) and SIA (stable isotope analysis). Perch (Perca fluviatilis) and whitefish (Coregonus lavaretus, large sparsely rakered morph) had similar trophic positions (TP = 3.69 ± 0.55 and 3.67 ± 0.55, respectively; p > 0.05); both species were generalists. The diet similarity (the index of relative importance of food items in stomach contents) of perch and whitefish was 48%, with aquatic insects (Trichoptera) being common items in both fish. According to carbon isotope values (δ13C), vendace (Coregonus albula), smelt (Osmerus eperlanus), and burbot (Lota lota) were more closely related with pelagic food sources (δ13C ranged from −27 to −25‰), whereas perch, whitefish, and ruffe (Gymnocephalus cernua) were more fuelled by benthic food web compartments (δ13C ranged from −24 to −21‰). The highest average nitrogen values (δ15N) were found in smelt and ruffe, 15.0 ± 0.7‰ and 14.2 ± 1.9‰, respectively. Perch and whitefish overlap significantly in their isotopic composition (δ13C and δ15N), demonstrating 36% overlap in the combined 40% ellipses (according to Stable Isotope Bayesian Ellipses in R) of the isotopic space. This study confirms the existence of distinct food competition between these two species in a subarctic lake.