Domestication and growth hormone alter antipredator behaviour and growth patterns in juvenile brown trout, Salmo trutta

In the hatchery environment, there is no selection by predators against risky foraging behaviour and excessive aggressiveness. Consequently, hatchery-reared trout should have a higher competitive ability and a less pronounced antipredator response than wild trout. Growth hormone increases the energy demand and thereby the feeding motivation of an animal. Therefore, growth hormone injections should reduce the antipredator response and promote competitive ability. Both hatchery selection and growth hormone injection consistently reduced antipredator behavioural responses in juvenile brown trout (Salmo trutta) in the presence of a trout predator. However, neither hatchery selection nor growth hormone had any effect on dominance or quantity of contested food consumed, whereas both hatchery selection and growth hormone injection promoted growth. RNA levels were increased by growth hormone, and hatchery-reared trout had lower RNA levels than wild trout. The presence of the predator increased RNA levels in the trout independent of strain and treatment. Our study suggests that antipredator behaviour, growth rate, and resource allocation patterns in brown trout change rapidly as a consequence of hatchery selection and that similar changes in antipredator behaviour can be induced by increasing GH levels. Thus, released or escaped hatchery-reared fish or fish with manipulated growth hormone levels with altered behaviour patterns may pose a potential threat to wild populations.