Tracking Atlantic salmon smolts, Salmo salar L., through Loch Voil, Scotland

Twenty-two salmon smolts, Salmo salar L., carrying miniature sonic tags were tracked individually for periods of up to 175 h in Loch Voil, Scotland, during May 1979 and 1980. Activity was predominantly nocturnal, 80% occurring between 21.00 and 06.00 hours, and was apparently undirected. Average velocities during this active interval were 0.6 body lengths per second (bl s⁻¹), with 98 and 93% of the time spent moving at less than 2 and less than 1 bl s⁻¹, respectively. The rates of downstream displacement were 0.04 bl s⁻¹ in 1979 and 0.01 bl s⁻¹ in 1980. The direction of displacement of smolts and of movement of water at a depth of 1 m was positively correlated ( P <0.001) and smolt displacement was biassed slightly ahead of water movement. Mean step lengths were 141 and 200 m in 1979 and 1980, respectively. Rates of downstream passage of 327 ICES plate-tagged smolts released 16.8 km upstream of the fish trap at Clunie dam, Loch Tummel, during the spring migrations of 1975 and 1976 averaged 0.13 bl s⁻¹ in each year: net surface water movement was about 3.7 times this rate during the same intervals. These data are consistent with the model of passive smolt migration postulated by Tytler et al. (1978) and suggest that the active component required to ensure passage through a loch (Thorpe & Morgan, 1978) is very small.