FEEDING-BEHAVIOR AND PREY CHOICE OF THE THREESPINE STICKLEBACK - THE INTERACTING EFFECTS OF PREY SIZE, FISH SIZE AND STOMACH FULLNESS

This study investigated changes in the feeding behaviour and prey choice of threespine stickleback, Gasterosteus aculeatus, of different body sizes, in response to increasing prey size and stomach fullness of the fish. Within the behavioural feeding sequence, a decision to attack and a decision to eat the prey were made by the fish. Smaller fish were more likely to stop in midwater during the attack and to manipulate the prey when handling it. Larger fish were more likely to attack and eat the prey. The probability of success, however, decreased with larger prey. Success also decreased with increasing stomach fullness when prey larger than the prey width:mouth width (PW:MW) ratio of 0·6 were encountered. Regardless of fish or prey size, there was a constant handling time of approximately 3 s during which the fish decided to eat. The time taken to handle and eat a prey decreased for larger fish and rose with larger prey. Spitting frequency increased with prey size and spitting was always required for prey greater than the 0·6 PW:MW ratio. This corresponded to a preference for prey oriented head first and ventral side up. Prey closest to the 0·6 PW:MW ratio were the largest prey that could be eaten with little change in the time cost over all levels of stomach fullness. These prey, therefore, gave the fish the best energy return per unit cost, except when the stomach was empty, as larger prey represented a bigger energy return for the same time cost giving them a greater effective profitability. The morphological relationship between a predator and prey determines the resulting feeding behaviour and prey choice, although this choice also depends on the predator's need to acquire food. © 1994 The Association for the Study of Animal Behaviour.