Sexual dimorphism in the migratory dynamics of a land-locked population of brown trout (Salmo trutta) in central Sweden : a study at three temporal scales

Sammanfattning: In this study I use a 56 years long dataset to study the migratory behaviours of a land-locked population of Brown trout (Salmo trutta) in the river Dammån, positioned in central Sweden. I found that there was a mean female-to-male sex ratio in the spawning-run of approximately 2.31 with an average 50% run date on the 29th of July. I principally investigated if there were any trends in 3 different temporal scales: diel, seasonal and interannual. The model investigating diel effect was not significant, likely due to the light summer nights at this latitude. I found that there was a seasonal trend, where the probability of an observation being female increased by 54.33% during the season, indicative of a protandric trait where males gain from an early arrival to the spawning-grounds. The interannual model displayed a decline of 13.85% in the proportion of females migrating over the 56 years investigated.