| Summary: | Over the last 20 years there has been a large change in the fish community structure in Lille Rostavatn, northern Norway. The relative contribution of the cold-water adapted fish species, Arctic charr ( Salvelinus alpinus ) and burbot ( Lota lota ), has distinctly decreased whereas the more temperate-adapted fish species, grayling ( Thymallus thymallus ) and brown trout ( Salmo trutta ), have increased, presumably because of a rise in water temperature which is shown to be a significant increase in the Målselv watercourse from 1991 to 2018. In this study the spatial and temporal variation in relative contribution, habitat use, and diet of these four fish species were studied from four years of gill net sampling in August and October between 1997 and 2018, exploring general patterns in resource partitioning and possible climaterelated changes in habitat and dietary utilization. Arctic charr dominated the total catches, followed by grayling. A distinct resource segregation was observed between the cold-water and the temperate-adapted fish species according to habitat use and the selection of specific prey types, especially in the littoral habitat. The habitat partitioning was particularly clear in August, when the cold-water adapted fish species decreased in densities in the littoral habitat over time. In October, at lower water temperatures, Arctic charr and burbot, showed a minor increase in the littoral catches. The dietary overlap between the species were in general low and the intraspecific dietary similarity showed high overlap between the four years, indicating small changes in resource utilization over time. Analysis of stomach content revealed a narrow trophic niche of Arctic charr as they fed mainly on zooplankton in all habitats of the lake. Previous field studies from Lille Rostavatn have demonstrated that Arctic charr are adapted to a planktivore diet due to strong interspecific resource competition for the benthic resources by burbot and in the more recent years this may have been amplified due to increasing grayling and brown trout population. The diet of burbot was dominated mainly by benthic invertebrates in all habitats whereas grayling and brown trout consumed surface insects and benthic invertebrates. These findings suggest that interspecific competition leading to changes in resource partitioning under a changing climate is not the key factor influencing the shift in fish community structure in Lille Rostavatn. The observed resource segregation might be a result of biotic interactions mediated by abiotic conditions.
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