INTERBREEDING WITH DOMESTIC STRAIN INCREASES FORAGING UNDER THREAT OF PREDATION IN JUVENILE STEELHEAD TROUT (ONCORHYNCHUS-MYKISS) - AN EXPERIMENTAL-STUDY

The foraging behaviour of laboratory-reared juvenile steelhead trout (Oncorhynchus mykiss) and steelhead/domesticated rainbow trout hybrids were compared. In 10 replicate experiments, 10 fish from each strain were allowed to choose between foraging in a safe area or an area containing a predator. The hybrid trout were significantly more willing to risk exposure to the predator than were the steelhead. It was possible that differences in the relative willingness to risk exposure may have reflected differences in their susceptibility to predation. A second experiment measured the susceptibility of these two strains to the predator by simulating standardized encounters between predator and prey. Both strains suffered identical mortality rates and therefore were considered to be equally susceptible to the predator. This experiment confirmed that the hybrid trout were significantly more willing to take risks than the wild steelhead. These results indicate that interbreeding between escaped hatchery and wild fish may have a potentially damaging effect on the wild population.