We develop a model to predict position choice of drift-feeding stream salmonids, assuming a fish chooses the position that maximizes its net energy intake rate. The fish's habitat is represented as a series of stream cross-profiles, each divided into vertical strips characterized by water depth and velocity. The fish may select a focal point in any of these strips, and include several neighbouring strips in its foraging area. The number of prey the fish encounters depends on its reaction distance to prey, water depth, and water velocity; the proportion of detected prey the fish is able to capture declines with water velocity. The fish's net energy intake rate is its gross energy intake rate from feeding minus the swimming cost calculated by using water velocity at the fish's focal point. There was a close match between the positions predicted by this model and those chosen by solitary Arctic grayling (Thymallus arcticus) in the pools of a mountain stream in Alaska.

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Field experiments in the pools of a mountain stream demonstrate that Arctic grayling (Thymallus arcticus) rank feeding positions according to desirability and that competition sorts fish so that the dominance rank of each individual matches the rank desirability of its position. Groups containing the same number of fish always occupied the same set of positions, and positions were added (in reverse order of desirability) as group size was increased; these results show that fish ranked positions. There was an almost perfect correlation between the dominance rank (measured as fish length) of each fish and the rank desirability of its position, suggesting that competition sorts fish among positions. This conclusion was strengthened by the results of sequential removal experiments in which the dominant fish was removed at the end of each day. After each removal the remaining fish almost always moved into the positions previously occupied by fish immediately above them in the dominance hierarchy.

We tested the assumptions and predictions of a foraging model for drift-feeding fish. We used three-dimensional videography to describe the foraging behavior of brown trout, Salmo trutta, mapped water depth and velocity in their foraging area, sampled invertebrate drift to determine length class specific drift densities, and captured trout to determine the size composition of their diet. The model overestimated the fish's prey capture rate and gross energy intake rate by a factor of two. Most of this error resulted from the fact that prey detection probabilities within the fish's foraging area averaged only half the expected value. This was the result of a rapid decrease in capture probability with increasing lateral distance from the fish's focal point. Some of the model's assumptions were accurate: equations for predicting reaction distance and minimum prey size supported reliable predictions of the shape and size of the fish's foraging area and the size composition of the diet. Other assumptions were incorrect: fish detected prey within the predicted reaction volume, not on its upstream surface as expected, fish intercepted prey more slowly than the expected maximum sustainable swimming speed, and fish captured about two-thirds of their prey downstream of their focal point, rather than upstream.

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