The effects of abiotic factors and intraspecific versus interspecific competition on the diel activity patterns of Atlantic salmon (Salmo salar) fry

We conducted semi-natural experiments to measure the relative contribution of various environmental factors and intraspecific and interspecific competition with an exotic invader on the daytime and crepuscular activity levels of Atlantic salmon (Salmo salar) fry. We demonstrated that interspecific competition with the exotic rainbow trout (Oncorhynchus mykiss) significantly increases the daytime activity of Atlantic salmon. The effect of intraspecific competition on the daytime activity of salmon was half that of interspecific competition. This indicates that the effect of rainbow trout was a combination of increasing density and the identity of the competitor. We also demonstrated that the effect of rainbow trout was probably the result of territorial interference between species. Moreover, we showed that water temperature simultaneously played an important role in explaining daytime activity of Atlantic salmon. During twilight, we observed no effect of competition on salmon activity, but environmental cues other than temperature (e.g., invertebrate drift, cloud cover) became significant predictors of activity. Feeding and growth rates of Atlantic salmon were not affected by the different levels of competition. Nevertheless, the exotic species may have a major impact by exposing the native species to increased risks of daytime predation.