Prey selection by the three-spined stickleback (Gasterosteus aculeatus L.).

(1) The diet of the three-spined stickleback (Gasterosteus aculeatus L.) feeding on a range of denstities of waterfleas (Daphnia magna Strauss) and mayfly larvae (Cloëon dipterum L.) was studied in laboratory experiments. (2) Sticklebacks almost invariably caught Daphnia at the first attempt but the success rate at catching Cloëon larvae decreased when the total prey density increased. (3) The relative abundance of the prey is a better predictor of the diet composition of the stickleback than the absolute density of either prey type. (4) Daphnia were always preferred above Cloëon but this preference was not constant over the range of prey density combinations studied. The preference was found to change according to two principles: (i) preference for the preferred prey, Daphnia, increases when the total prey density increases; (ii) preference for the relatively scarce prey increases when the total prey density increases. (5) The results are discussed in the light of optimal foraging theory, whereby the predators' disproportionate concentration on the relatively scarce prey-type (counter-switching), when the total prey density is high, is related to possible changes in the profitability of the prey types.