How juvenile Atlantic salmon, Salmo salar L., respond to falling water levels: Experiments in an artificial stream

The responses of juvenile Atlantic salmon, Salmo salar L., to an experimentally imposed drop in water level were studied in an artificial stream. In a pilot experiment, 20% of fish which had a feeding station in an area of riffle remained there throughout the period of low water levels. The remaining riffle-dwelling fish moved into areas of deeper water, although not necessarily to the pool nearest their feeding station. Out of the fish which left the erea, 89% moved in an upstream direction and 11% moved downstream. In a second experiment, which was designed to look in more detail at this response to de-watering, 95% of riffle-dwelling fish left when the water level dropped and moved into a pool, mostly within the 4-h period after water levels started to fall. Seventy-four per cent of fish which left moved upstream and the remainder moved downstream. There was a strong tendency to leave in an upstream direction as shallow areas began to dry out, and this tendency persisted irrespective of variability in fish size, prior social experience and the size of the home range prior to de-watering. It is suggested that the prior opportunity to explore alternative habitat may be an important determinant of response of riffle-dwelling salmon to a sudden drop in water level.