Evol Ecol Res 20: 569–596 (2019)     Full PDF if your library subscribes.

Predicting the effects of currents on the adaptive movement of juvenile salmonids

Richard A. Hinrichsen

Hinrichsen Environmental Services, Seattle, Washington, USA

Correspondence: R.A. Hinrichsen, Hinrichsen Environmental Services, 4837 NE 85th Street, Seattle, WA 98115, USA. email: rich@hinrichsenenvironmental.com


Background: Anadromous salmonids present a marvellous opportunity to study animal movement, with some juveniles in the Yukon and Amur rivers travelling more than 2000 km from their natal areas to the ocean. During their freshwater residence, juvenile salmonids, regardless of river of origin or migration distance, balance the pressures of feeding, predator avoidance, and migration to survive.

Questions: What are the choices of current and swimming velocities that stream-dwelling juvenile salmonids use to optimize lifetime reproductive success? How are these influenced by maximum current velocity in the stream or river that they inhabit?.

Mathematical methods: I developed a dynamic optimality model that treats current and swimming velocities as decision variables. The state variables are downstream river location and fish size. I solve the optimality model using optimal control theory and apply it to juvenile oceantype Chinook salmon in the Hanford Reach, Columbia River, Washington.

Results: Five fundamental behaviours or movement phases result from the optimality model: rapid upstream migration, appetitive (‘foraging’) upstream movement, station holding, appetitive downstream movement, and rapid downstream migration. These fundamental behaviours were not specified a priori, but emerge when optimizing lifetime reproductive success over the full range of possible behaviours. The appetitive and station holding behaviours are broadly characterized as foraging/predator avoidance. Rapid migration is favoured over foraging/predator avoidance whenever the magnitude of the marginal value of displacement exceeds the marginal predation risk of displacement. If, during foraging/predator avoidance, the maximum current velocity rises above the swimming speed that maximizes growth, station holding is optimal; otherwise, appetitive movement, which carries greater predation risk, might be optimal. The two types of downstream movement predicted by the optimality model (appetitive movement and rapid downstream migration) describe the movements of the ‘ocean-type’ and ‘stream-type’ races of Chinook salmon populations of the Columbia River. In the Hanford Reach application, optimal movements begin with station holding, then switch to downstream appetitive movement or rapid downstream migration, depending on the maximum current velocity. Juveniles accelerate as they migrate downstream. I describe an experiment to test the influence of current velocity on foraging behaviour and a field study to characterize juvenile upstream migrations.

Keywords: behavioural ecology, bioenergetics, freshwater ecology, migration, optimal control, predation, salmonids.

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