Anti-predatory responses in juvenile brown trout (Salmo trutta) threatened by a simulated bird predator

The aim of this study was to study the anti-predatory behaviour of wild and hatchery-reared brown trout. Fish were held singly over a five-day period in a semi-natural stream channel and a bird dummy that either quickly passed 1.3 m above the fish (low threat) or dived towards the fish (high threat) was used to simulate predation risk. The strength of the threat had no overall significant effect on the anti-predatory responses. Wild trout always responded to threats in an adaptive way by different anti-predatory behaviours without any change in response over the five-day treatment period. The proportions of hatchery-reared trout that adaptively responded to threats were on average 69% on day one, but decreased significantly over time and by day five, they essentially lacked anti-predatory responses. The two most common anti-predatory responses in wild trout were either immobility (freeze) or hiding (refuging). Independent of strength of threat different wild individuals adopted different anti-predatory strategies, i.e. some mainly used a freeze behaviour and others refuging. Hatchery-reared trout exposed themselves significantly more outside shelter than wild trout during periods without access to food. In addition, their position above the bottom was significantly higher than wild ones