Substrate discrimination and preference in foraging fish

Increasing substrate structural complexity has been linked to higher densities of various refuge-seeking aquatic macroinvertebrates, many of which constitute major prey species in a variety of freshwater communities. We investigated (1) whether the three-spined stickleback, Gasterosteus aculeatus, a predator of such macroinvertebrates and an established model organism in studies of animal behaviour, is capable of discriminating between structurally complex and simple substrates, and (2) whether they show a substrate preference when foraging. Hunger-motivated sticklebacks presented with a choice of simple and complex substrate types preferentially foraged over complex substrates. This was seen when the prey density was equal on both substrate types, and when it was greater on the complex substrate. Where prey density was greater on the simple substrate, fish showed no preference for either. The preference for complex substrates existed in both the allocation of time spent foraging and in the direction and frequency of feeding strikes. No substrate preferences were seen in fish that were satiated, or when prey were absent from both substrates. This ruled out refuge as an explanation for the observed preference. We discuss the results in the context of the relative usefulness of substrate discrimination in effective patch foraging in fish.