Stream-dwelling, juvenile Atlantic salmon, Salmo salar L., feed mainly on drifting invertebrates, usually by swimming upstream from a stationary position to intercept individual prey items. Laboratory experiments tested the prediction that individual salmon should reduce the distance over which they would travel (attack distance) to intercept drifting food items as the energy cost of swimming increases with increasing current velocity. Attack distance varied inversely with current velocity as expected. The fish's average speed of upstream movement relative to the substrate remained constant and the duration of individual attacks therefore declined as current velocity increased. Calculated reaction distances and a second ecperiment using tethered prey drifting at speeds independent of current velocity confirmed that these relationships were due to fish actually delaying attacks on perceived prey for longer periods as current velocity increased. Using estimated metabolic rates for burst swimming, it appears that energy expenditure per attack varies little with current velocity. Therefore, by reducing their reaction and attack distances in response to increasing current velocity, the fish reduced their energy cost of travel per attack.

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Journal Animal Behaviour
Godin, J.-G.J, & Rangeley, R.W. (Robert W.). (1989). Living in the fast lane: effects of cost of locomotion on foraging behaviour in juvenile Atlantic salmon. Animal Behaviour, 37(PART 6), 943–954. doi:10.1016/0003-3472(89)90139-5