Estimating population densities of salmonids in deep water in streams

Densities of salmon, trout and grayling were assessed in Scottish streams by systematic counting by underwater observers, where the water was greater than 1 m deep, and by using electrofishing equipment in adjacent shallower areas. Tests of the efficiencies of the two methods are described. Applications have included the estimation of production of trout, grayling and young salmon at a site on the River Tay and predicting the salmon smolt run of the River Tilt, and these applications are briefly described. The most suitable period for visual counting was summer, as at water temperatures of less than 15° C fish were hiding among stones in the stream bed. Wet weather hampered both electrofishing and visual counting.