Downstream migration and survival of Atlantic salmon (Salmo salar L.) kelts and fallbacks when passing a hydropower plant

Abstract: Repeat spawners of Atlantic salmon may be of great importance for population stability, the return spawners can significantly contribute to recruitment of juveniles. To have a significant proportion of repeat spawners in the population, it is necessary that salmons can successfully migrate back to the sea after spawning upriver. Studying fish behaviour and movement connected to hydropower plants is important in order to improve fish passage solutions and aid future management. In this study, we use acoustic telemetry to look at migration and survival of kelts and fallbacks at the power station Stornorrfors in river Umeälven, Northern Sweden. 36 out of the 56 spawners tagged at the fish ladder in Stornorrfors initiated downstream migration during the time of the study. 25 out of the 36 salmons immediately cancelled their migration and fell back downstream after tagging, while the remaining 11 first spawned upriver and returned downstream as kelts. All fish that migrate downstream passed through the turbines at Stornorrfors, no individual was found to use the alternative passage route via the fish ladder. 23 (64%) individuals had their last registration at the powerhouse intake and no further registrations downstream. Out of the remaining 13 (36%) that passed through the turbines, an additional 6 (17%) was lost before reaching the last downstream receiver at the river mouth. In the end, only 7 (19%) individuals that initiated downstream migration managed to get to the coast. With this low survival rate, it is apparent that management actions to improve the situation for downstream migrating kelts in the River Umeälven are necessary. However, we could not identify a key area where such measures should be implemented, further studies should have a more heterogeneous sample including more large sized individuals and higher resolution telemetry.