Timing of Sea-Run Brown Trout (Salmo trutta) Smolt Migration: Effects of Climatic Variation

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Abstract
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°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).