FIRST BEHAVIORAL-ADJUSTMENTS IN RELATION TO HABITAT USE AFTER EMERGENCE IN RUNNING WATER SALMONIDS

In rheotactic salmonids, the short period (a few days) after emergence from the gravel is critically important for individual survival. In a llopatric or sympatric situations, fry have to select an habitat provi ding shelters against predators or downstream displacement by water cu rrent, or food as invertebrates drift. Experiments done during the las t few years with different salmonid species such as Brown trout (Salmo trutta), Atlantic salmon (Salmo salar) and Grayling (Thymallus thymal lus) showed post-emergent behavioural variability corresponding to var ied environmental conditions. Different forms of fry dispersal appeare d through downstream movement or displacement towards the banks and/or in the vicinity of the redd area, to more or less benthic microhabita ts well adjusted to the size and swimming capacity of the different sp ecies. The presence of conspecifics influenced microdistribution throu gh competition for the best stations for prey capture (invertebrates d rift). The presence of predators could delay emergence or cause reburi al of fry in the gravel, but more often increase downstream movement.