A Diet Model for Planktivores that Follow Density‐Independent Rules for Prey Selection

This is the authors' abstract. We don't add key points for this paper.

Whether prey are encountered simultaneously or one at a time is an important distinction. We have applied the concept of simultaneous encounters to models of prey selection by planktivorous fish. When the maximum distance from which a prey item can be detected is determined by the subtended angle a prey item makes the fish's eye (apparent size), selection for the apparently largest among simultaneously encountered prey results in prey preferences that are independent of the prey density or relative density. This occurs regardless of how many prey are likely to be encountered simultaneously, and the resulting diet is the same as that predicted by the simpler reactive field volume model. Furthermore, the assumptions of this apparent—size model can be greatly modified, to enhance realism, without changing our conclusion. In this context we allow prey selection and detection to become stochastic, and to depend upon aspects of prey consciousness other than prey length. The similar model proposed by O'Brien et al. (1976) as an alternative to Werner and Hall's (1974) optimal—diet explanation of size selection by bluegill sunfish yields a different conclusion. We find that there are, in all, five reasons for our different conclusions. In light of our results, we interpret the results of Werner and Hall (1974) and two more recent attempts to differentiate between apparent—size models and optimal—diet theory (Gibson 1980, Gardner 1981). We conclude that these studies do not provide strong support for the predictions of optimal—diet theory; field evidence does, however, suggest that fish can alter their diet in an adaptive fashion.