Effects of water discharge and temperature on the seaward migration of anadromous brown trout, Salmo trutta, smolts

The smolt run of anadromous brown trout (Salmo trutta) in a Norwegian river was studied for three consecutive years. The main run occurred in a period of 7-10 days in the middle or second half of May. Support was found for th e hypothesis that high water discharge and temperature triggered the run. F ew smolts descended when the discharge was low (<50 m(3) . s(-1)) and the w ater temperature was below 4<degrees>C. The maximum number were caught when the discharge was moderate (70-150 m(3) . s(-1)) and the water temperature high (6-8 degreesC). The relative importance of these factors varied from year to year. The first-year discharge explained 38% of the variation in th e number of smolts migrating. Discharge and temperature together explained 61% in 1992 and 28% in the second and third year, respectively.