Browsing by Author "Kauppi, Kati"

Brown trout is classified as vulnerable species in the Baltic Sea region and endangered in Finland, and wide conservation efforts have been conducted to restore the populations. Understanding the migration patterns of the species allows e.g. more accurate assessment of the population stock sizes and needed measures for conservation. Well-known life history strategy of brown trout (Salmo trutta) includes habitat shifts such as downstream marine feeding migration and upstream spawning migration. More recently, this simplified life history has been observed to be variable among populations and individuals. Environmental and biological factors, such as water discharge, temperature and fish size are known drivers for downstream migration. For this study, data derived from a field study conducted by Natural Resources Institute Finland was analyzed together with environmental data to understand the factors influencing the timing of the out-migration in an urban brook (Longinoja brook), in Helsinki, Finland. The PIT tagging method and automatic antennas were used to track the movements of the fish in the brook. Results indicated that the fish out-migration occurs in peaks. Individuals migrating in spring seemed to favor decreasing water flow and water temperatures between 6-9 °C, and their migration peak was specific to late April and early May. During the autumn and winter seasons, increase in water flow and a relatively cold (1-4 °C) water temperature created suitable conditions for the fish to start moving downstream. Fish migrating in the spring were smaller in size compared to those migrating during winter. The study results demonstrate how brown trout can adopt multiple and complex life history strategies even in the same population. Suitable environmental conditions for downstream migrations were diverse overall, but especially during winter. Motivation and reason for out-migration might differ between the seasons, and future research should focus on understanding the behavioral differences between the individuals within the populations better.