Tributary-Specific Contribution to a Lacustrine Mixed-Stock Fishery of Brown Trout Salmo trutta (Salmonidae) in a Diverse Sub-Arctic Watershed

Mixed-stock fisheries which simultaneously cause mortality amongst several populations of a species of fish may occur where separate stocks partially or completely overlap in the geographic area of harvest. This study aims to analyze the population-of-origin of adult and subadult adfluvial lacustrine brown trout Salmo trutta, exploited in a mixed-stock fishery in Upper Tuloma Reservoir in Eastern Fennoscandia, using otolith microchemistry. To evaluate the origin of migratory brown trout captured in these mixed-stock harvest fisheries, we undertook otolith sampling of brown trout juveniles in fluvial spawning and rearing habitats in the reservoir watershed, including 13 natal tributaries across the catchment of the reservoir in both Russia and Finland. Harvested adult and subadult brown trout otoliths were sampled from the impoundment in the central area of the reservoir and we analyzed for stock-related character differences to compare with known populations, using trace element signatures, with fish from the mixed stock fishery. The assignments-of-origin in the mixed-stock fishery-harvest samples did not follow the known distribution of populations sampled from natal streams in the watershed. For example, brown trout from the largest tributary catchments of Lotta River and Nota River were less represented in the mixed-harvest sample, which was contradictory to their contribution to the overall spawning and rearing areas of the catchment. These results point towards the importance of maintaining the diversity of different spawning and rearing tributaries required for brown trout recruitment into mixed stock fisheries, and the potential of the existence of population structure of brown trout in the reservoir catchment. Our information suggests that it is important to develop conservation and management strategies for natal habitats in multiple streams utilized by adfluvial lacustrine brown trout populations inhabiting various catchments and that are harvested in mixed-stock fisheries.