Migration of Seatrout (Salmo trutta); effects of increasing temperatures and water flow

Abstract

This study investigates the impact of rising temperatures and water flow on the downstream migration of sea trout (Salmo trutta) smolts in Haga å, a small stream in southwestern Sweden. The research aimed to assess how environmental changes affect smolt migration patterns and smoltification status. A fish trap installed since 2022 was used to capture and monitor smolts. Data on water flow rates and temperature were collected from the Swedish Meterorological and Hydrological Institute (SMHI) and compared across three years (2022-2024). The results indicate a dramatic decline in migrating smolts, with a 96% reduction observed from the initial year to 2024. This decline was coupled with increasing water temperatures and variable water flow, which may have influenced the timing and success of smolt migration. Notably, higher water temperatures potentially caused earlier migration, affecting smolt survival in the ocean due to mismatch with optimal marine conditions. This study highlight the urgent need to monitor and adapt management strategies for small stream like Haga å to mitigate the impacts of climate change on keystone species such as the sea trout.