Downstream Migration of Landlocked Salmon (Salmo salar) Smolts Through Multiple Dams, River Klarälven, Sweden

Hydroelectric plants (HEPs), with their dams and turbines, interrupt connectivity in water bodies and constitute barriers and sources of mortality for upstream-migrating spawners and downstream-migrating smolts and kelts. In watersheds with multiple dams the cumulative losses of migrating juvenile salmon can be high, contributing to population declines or extirpations. Although there are numerous studies of migrating anadromous Atlantic salmon, relatively little is known about landlocked stocks. In the River Klarälven, we studied smolt migration of wild landlocked Atlantic salmon (Salmo salar L.) along a 180 km long river segment, which includes eight dams, with regulated and unregulated stretches. Salmon smolts tagged with acoustic transmitters were released at different locations, and followed as they passed some 40 receivers, strategically placed along the entire river segment. We found that only 16% of the smolts survived passage through all eight dams. The direct loss of smolts passing HEPs was 79%, whereas only 5% were indirectly lost in unregulated stretches between HEPs. Migration speed was 77% slower along regulated stretches than along unregulated stretches. The observed lower migration speed at regulated stretches was dependent on fish size, with large fish moving slower than small fish. Discharge affected migration willingness and migration speed, but not losses. As previously shown for anadromous populations, our study of landlocked salmon demonstrates similar negative effects of multiple passages of HEPs by downstream migrating smolts. Together with earlier migration studies, this highlights the need for a holistic approach in the management and conservation of migratory fish in regulated rivers, which includes safe passage for both upstream- and downstream-migrating fish.