CHANGING HANDLING TIMES DURING FEEDING AND CONSEQUENCES FOR PREY SIZESELECTION OF 0+ ZOOPLANKTIVOROUS FISH

The interrelationship of fish size, prey size and handling time within a 15-min feeding period was studied in three size groups of 0 + roach , Rutilus rutilus, and bleak, Alburnus alburnus. Four size classes of cladoceran prey were used to measure changes in feeding rate and handl ing time from initial rapid feeding to sustained feeding. Observed dif ferences in increase of handling time between prey size classes led to a change in the prey profitability ranking of those size classes with in the first 2 min of the experiments. A 2-min feeding peri od is inte rpreted as reflecting an intermediate motivational status between extr eme hunger and satiation. The use of average handling times for this p eriod revealed a substantial change in prey profitability estimates co mpared to previous studies which used handling times based on short-te rm (a few seconds up to 1 min) feeding. It is not the largest prey ite ms a fish can handle and swallow that are most profitable, but prey of intermediate size. By this approach a closer fit between expectations derived from optimal foraging theory and empirical data on prey size selection of 0 + zooplanktivorous fish is qualitatively achieved. Opti mal prey size was found to be close to the mouth gape width in small f ish of 15 mm standard length, decreasing to 50% of mouth gape width in fish of 40 mm standard length.