The response of bleak to predator odour of unfed and recently fed pike

Feeding and growth responses of bleak Alburnus alburnus to the odour of unfed and recently fed pike Esox lucius were quantified in laboratory experiments. Bleak reacted more strongly to the latter than to the former and feeding rates were lowered by 35 and 20%, respectively. The decreases in feeding rates were not an effect of the absence of fish from the feeding place, but of a lowered feeding efficiency due mainly to diminished time spent foraging. The lowered consumption rate of bleak exposed to the odour of recently fed pike caused a decrease in their individual growth rate. The stronger reaction to the recently fed pike stimulus supports the view that alarm substance is a marker for predator odour. In the absence of any further signal from the predator, the bleak became habituated to the predator odour and showed the reaction for only 5 days.