CANCELLED - The influence of river currents on the adaptive movements of juvenile salmonids

Background/Question/Methods

Anadromous salmonids present a marvelous opportunity to study animal movement, with some juveniles in the Yukon and Amur rivers traveling more than 2,000 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. There are two main questions I address: 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? 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 ocean-type Chinook salmon (Oncorhyncus tshawytscha) in the Hanford Reach, WA.

Results/Conclusions

Five fundamental behaviors 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 behaviors were not specified a priori, but emerge when optimizing lifetime reproductive success over the full range of possible behaviors. The appetitive and station holding behaviors are broadly characterized as foraging/predator avoidance. Rapid migration is favored 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.