Optimal swimming speeds and forward-assisted propulsion: energy-conservingbehaviours of upriver-migrating adult salmon

Anadromous salmon migrations are energetically expensive. Long-distance mig rants should be efficient in their use of energy and minimize swimming cost s wherever possible. We explore swimming strategies and energy-saving tacti cs employed by three long-distance-migrating sockeye salmon (Oncorhynchus n erka) stocks in the Fraser River watershed, British Columbia. We used stere ovideography and bank-side observations to estimate swimming speeds (from t ailbeat frequency) and ground speeds (using distance traveled and duration) for individuals at several sites. Salmon were highly efficient at migratio n (i.e., ground speeds equaled or exceeded swimming speeds) through reaches with relatively low encountered currents (<0.25 m.s(-1)). We speculate tha t salmon exploit small reverse-flow vortices to achieve this feat. With low encountered currents, most salmon migrated according to an optimal swimmin g speed model: migrants minimized transport costs per unit distance travele d. Generally, salmon were less efficient at migration with fast currents, a lthough the Chilko stock were superoptimal migrants, possibly owing to uniq ue morphology and (or) behaviours. The risk of significant delays is enhanc ed when fast currents are encountered. Under these conditions, relatively f ast swimming speeds could minimize travel time, despite high costs. Migrant s may be balancing energetic costs of migration against the fitness costs o f spawning delays.