Dynamic changes in prey choice by stickleback during simultaneous encounter with large prey

When a three-spined stickleback Gasterosteus aculeatus encountered prey sim ultaneously the probability of hanging and the median pursuit time were gre ater than when prey were encountered sequentially. During simultaneous prey encounter fish did not choose to attack the more profitable prey but inste ad the nearer prey was handled first except when the difference between the two prey sizes was large. No difference was found in the level of total en ergetic intake by the fish regardless of prey size pairing. Fish that handl ed and ate the first prey of a pair in <5 s attacked the second prey with a high probability of success, demonstrating an opportunistic feeding strate gy. Importantly however, the fish did not choose to maximize long term ener gy intake rate by eating both prey, but rather short-term considerations ov er the course of feeding took precedence. With an empty stomach, the probab ility of a fish eating (P-cat) the fil st prey handled was high regardless of prey size. As stomach fullness increased, the P-cat the first prey handl ed decreased if it was the larger prey. Hence, the fish were unselective wh en the stomach was empty but thereafter there was a shift in preference tow ards the smaller prey. The decision of which prey to attack and eat appeare d to be based on short-term energy considerations and the level of stomach fullness. This study demonstrates that feeding on a short-term scale is a c rucial factor to take account of when analysing fish feeding during simulta neous prey encounter. (C) 1999 The Fisheries Society of the British Isles.