IDEAL FREE DISTRIBUTION AND THE MECHANISM OF PATCH PROFITABILITY ASSESSMENT IN 3-SPINED STICKLEBACKS (GASTEROSTEUS-ACULEATUS)

In this paper I tested four factors that might be involved in a fish's choice of a food patch: a) shoal size present in the patch b) food delivery rate c) conspecific's feeding rate d) excitement of conspecifics Single sticklebacks were presented with a choice between two display shoals feeding at different rates but were prevented from entering the food patch by a glass sheet. When size of display shoals was equal and food delivery rate different sticklebacks spent more time near the shoal with the higher food delivery rate. When the same two different food delivery rates were presented without display shoals, test fish spent about equal amounts of time in both halves of the test tank. In a third experiment different shoal sizes (5 vs 2) were presented while keeping food delivery rate constant (5 vs 5 prey items; thus individual payoffs were in the ratio 1:2.5). The test fish spent more time with the larger shoal during the prefeeding period but significantly reduced this time during the feeding period and increased it again after feeding had stopped indicating a trade-off between safety and energy intake. A final experiment investigated the influence of excitement in display shoals on the test fish's choice. The test fish was given a choice between two display shoals (20 hungry vs 20 satiated fish). During the prefeeding period, fish in the hungry shoal showed biting rates 10 times higher than that of fish in the satiated shoal (directed at the tank interior). The test fish showed no preference, indicating that biting rate/fish is not an important factor. However, the test fish turned to the hungry shoat immediately when a clay ball (which stimulated feeding movements) was introduced into each of the outer compartments. I conclude from these experiments that neither shoal size nor food delivery rate alone play a role in the initial choice among patches. Rather an individual's initial choice of patch appears to be influenced by the excitement that conspecifics show to food, which in nature may lead to selection of the more profitable patch.