Migrational Behavior and Seaward Movement of Wild Subyearling Fall Chinook Salmon in the Snake River
behavior, chinook, salmon, Snake River, reservoirs, Lower Snake River, seaward migration, migration, Oncorhynchus tshawytscha, offshore, active, regression, tagging, temperature effect, mark-recapture
Journal or Book Title
North American Journal of Fisheries Management
Flow augmentation increases flow and decreases temperature in reservoirs in the lower Snake River during the seaward migration of wild subyearling fall chinook salmon Oncorhynchus tshawytscha. A study of the migrational behavior and seaward movement of wild subyearling fall chinook salmon in the Snake River was necessary to help understand the efficacy of flow augmentation. We studied fall chinook salmon in the Snake River during 1992-2001. After analyzing mark-recapture data, we deduced that fall chinook salmon passed through at least four migrational phases, including (1) discontinuous downstream dispersal along the shorelines of the free-flowing river, (2) abrupt and mostly continuous downstream dispersal offshore in the free-flowing river, (3) passive, discontinuous downstream dispersal offshore in the first reservoir encountered en route to the sea, and (4) active and mostly continuous seaward migration. We used ordinary-least-squares multiple regression to test the effects of flow (m3/s), temperature (°C), and three other factors on the rate of seaward movement (km/d) from initial tagging in the free-flowing river to the first dam encountered en route to the sea (period 1) and from passage at this first dam to passage at the next dam downstream (period 2). We found that flow and temperature influenced the rate of seaward movement during period 1 (N = 2,808; flow model R2 = 0.65, P < 0.0001; temperature model R2 = 0.726, P < 0.0001). We failed to find evidence for flow and temperature effects on the rate of seaward movement during period 2, possibly because of limitations on our study. We conclude that flow augmentation increases the rate of seaward movement of fall chinook salmon during period 1, provided that augmentation occurs when the fish have moved offshore in the free-flowing river and are behaviorally disposed to being displaced downstream. The cooling effect of summer flow augmentation likely prevents fish that successfully smolted during period 1 from reverting to parr during period 2, but research is needed to confirm this hypothesis.