Interspecific competition for resources is, alongside abiotic factors like climate and geography, a very important factor in shaping communities. Since competition is difficult to prove directly, resource partitioning is frequently used as an indication of the presence of interspecific competition. Over the last 20 years perch abundance in the Pasvik watercourse has increased immensely, presumably because of a rise in water temperature. In this study resource partitioning and feeding strategies of sympatric large sparsely rakered (LSR) whitefish (Coregonus lavaretus) and perch (Perca fluviatilis) in the littoral zone of two lakes in the Pasvik watercourse were studied. Stomach content and stable isotope (δ13C and δ15N) analyses were done to determine short- and long-term diets, respectively, of all four populations. In both lakes, Perch had a generalist population diet with some specialization on the individual level, and showed clear ontogenetic niche shifts. Small perch were specialized on relatively small invertebrates (crustaceans), intermediate sized perch had a more generalist diet consistent of larger invertebrates and fish, while the largest perch were specialized piscivores. LSR whitefish displayed a population specialization in molluscs. Therefore, the diet overlap between LSR whitefish and perch in Lake Tjærebukta was only 12%. Isotopic niche overlap was low as well (20.6%). In contrast, in Lake Skrukkebukta the diet overlap between the two species was relative high (53%). Overlap in isotopic niches was similar (48.5%). Here LSR whitefish had a more generalist feeding strategy at both population and individual level. In addition they had an ontogenetic shift in diet as LSR whitefish <250 mm had a mixed diet of small Eurycercus lamellatus, molluscs and insect larvae, while larger LSR whitefish had a diet dominated by different species of larger insect larvae. The low overlaps in diet and isotopic niches were a good indication of clear resource partitioning on both short- and long-term, between LSR whitefish and perch in the two study lakes. A difference in feeding strategies and ontogenetic dietary niche shifts of perch likely strengthened the resource partitioning in both lakes. The resource partitioning was more intense in Lake Tjærebukta, where perch has been present in high abundance for over a longer period than in Lake Skrukkebukta. Longstanding interspecific competition between perch and LSR whitefish in Lake Tjærebukta could have caused the distinct resource partitioning, while in Lake Skrukkebukta this process is likely still on-going, and has not fully established yet. If perch spreads to more areas/increases in density because of climate change, this might have a distinct effect on other whitefish populations as well
