Freshwater sculpins and salmonids coexist in many streams throughout the Northern hemisphere, and often constitute an important component of stream ecosystems. Alpine bullhead Cottus poecilopus Heckel have been known to predate eggs and fry of brown trout Salmo trutta L., and also to function as a competitor to older brown trout for habitat and prey items. This study was designed to examine possible behavioural differences in activity level and positioning between a sympatric and an allopatric population of brown trout, when subjected to sight and smell of alpine bullhead. Sympatric and allopatric brown trout was collected from the same river system, and subjected to smell and sight of alpine bullhead under experimental conditions. Activity levels and positioning in three dimensions were recorded and analyzed, with the aid of digital video cameras.
When exposed to stimuli from alpine bullhead, neither brown trout population displayed changes in their positioning and activity level, statistically different from control experiments. Nevertheless, differences in behaviour between the brown trout populations were discovered. Allopatric brown trout changed their vertical position more than sympatric brown trout, in response to changes in their environment. In the same situations, there was larger variation between allopatric brown trout when it came to changes in horizontal and vertical positioning, than between the sympatric brown trout. The results from this study indicate that there is low level of aggression between alpine bullhead and brown trout, and that the behavioural differences discovered could be due to different degrees of intraspecific competition in the brown trout populations.