Foraging of a Small Planktivore (Pseudorasbora parva: Cyprinidae) and its Behavioral Flexibility in an Artificial Stream

Abstract

In nature, prey animals are not exposed to a single stressor but rather to a wide array of threats and dangers. Thus, fleeing prey need flexible behaviors that increase fitness and chances of survival. This study sought to understand how a small planktivore, Pseudorasbora parva, modifies its foraging behavior in a complex environment where multiple dangers exist. Fish were reared in a flume consisting of two types of microhabitats; pools representing a profitable site with a higher predation risk, and riffles representing a safer but otherwise unfavorable site due to low foraging success and flow velocity stress. The results showed that P. parva exhibited a high degree of flexibility in behavior reflecting the conflicting conditions of high velocity and predation risk. There was a trade-off between foraging rate and predation risk because prey escaped to the riffle when the pool which was the preferred habitat, was occupied by the predator. Microhabitat selection, feeding behavior, shoaling and swimming distance also reflected its adjustable responses with regard to variation of the risk of predation. The fish, under pressure of predation and high flow velocity, became more slender in body shape that might increase the swimming performance suited to avoidance behavior and minimize the swimming energy expenditure. This phenotypic modification may be a result of trade-off between morphological defense and swimming cost for fish inhabiting streams. These results suggest that small preys are able to demonstrate various compensatory behaviors in response to multiple conflicting conditions.