TIMING OF SEA-RUN BROWN TROUT (SALMO-TRUTTA) SMOLT MIGRATION - EFFECTS OF CLIMATIC VARIATION

By trapping seaward migrating smolts of brown trout (Salmo trutta), we analyzed the timing of the run in a small stream in southwestern Sweden (58-degrees-N) during 1984-90 in relation to environmental factors. Ninety percent of the smolts were captured during a period of 28.7 d (SD 5.9), with median time ranging from April 26 to May 17. Using polynomial multiple regression, we found a positive relationship between the probability of migration per day and the number of degree-days, change in water level, temperature change during the preceding week, and mean annual fish length. Forty-seven percent of the variation was explained. The predicted probabilities were used to calculate the population fraction migrating per day for the seasons 1984-90. There was a close agreement between the observed and expected daily fraction of migrants. The model, with parameter values based on the observations from 1984-90, was also used to predict the daily migration in 1991. The result indicated that the model accurately predicts the smolt run (R2 = 0.40).