Optimal prey size for stream resident brown trout (Salmo trutta): tests of predictive models

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Abstract
The time required to handle different-sized prey (crickets) was measured in an artificial stream for eight wild brown trout (Salmo trutta L.) in two size classes (mean total lengths, 186 and 214 mm). Handling times (HTs) scaled by mouth size were described by an exponential equation: HT = 1 + 0.84e2.35(ps/ms) (ps, prey size; ms, predator (mouth) size). Cost curves based on handling time/prey weight were used to predict optimal prey lengths of 22 mm for small trout and 24 mm for large trout. A second model based on J. W. J. Wankowski's empirical results predicted slightly smaller optima. Physical constraints provided estimated minimum prey lengths of 2.8 and 3.2 mm for large and small fish, respectively; maximum prey lengths were 89 and 97 mm, respectively. We compared the predicted optimal prey size with the size distribution of invertebrates in drift and brown trout stomachs sampled in a second-order stream from July to September 1982. The most abundant prey sizes in the study stream were near the minimum size that can be effectively handled by brown trout. Prey of the predicted optimum size were rare, but feeding was size selective in spite of a limited food resource. The growth rates of these stream-dwelling brown trout were slower than the brown trout in other streams in this region. This may reflect diets consisting largely of suboptimal-sized prey.