Studies of the conditioned reflex in lower vertebrates

A defensive conditioned reflex was established in groups of fry of hatchery-reared and wild-born chum salmon, Onchorhynchus keta (Walbaum), using a plastic model of predatory fish as conditioned stimulus and an electric shock as unconditioned stimulus. Efficiency of elaboration, stability, and retention of the conditioned reflex were improved with age. In a group in which the majority of fish had been well conditioned, the poorly-reinforced minority followed the prevailing defensive movement of the majority group. The reflex to the fish model was readily generalized i.e., extended to other objects. Wild fry showed naturally-acquired, defensive behavior against the fish-model in the laboratory. Some artificially reinforced groups were marked on their fins and released upstream in a closed, experimental river-area, being recaptured downstream. The relatively high rate of survival in these groups proved the effectiveness of the elaborated defensive reflex against natural predators.