Feeding and growth response of roach, Rutilus rutilus, to alarm substance

Feeding and growth responses of roach from three size classes to alarm subs tance (Schreckstoff) were quantified in laboratory experiments. Larger fish (60.0-80.0 mm in length) reacted stronger to treatment than two smaller si zed groups (35.0-45.0 and 46.0-55.0 mm) lowering feeding rate by 80 and 40 and 50%, respectively. The reduction in feeding rate of larger fish caused decrease in growth rate in length and weight, while the lowered consumption of smaller fish caused only reduction in growth rate in weight. Condition factor of exposed to alarm substance small sized roach was lower than that of the control individuals and roach from other two size classes, both, tre ated and untreated. The difference in growth response to a danger of predat ion has its roots probably in different metabolism and growth rates of smal l and large fish. Small fish have higher metabolic rate and less lipid rese rves than larger ones, therefore they are probably forced to feed to be abl e to grow. Also, small sized roach is more vulnerable to predation than lar ge sized fish, thus growing fast seem to be crucial for survival in a risky environment. Study shows that small roach trade off their safety against f ood, feeding in risky environment to sustain fast growth. This ability of f ast outgrowing of a dangerous, vulnerable to predators, size increases surv ival of juveniles in dangerous environment.