Environmental Determinants of Spawning Location, and Density and Size of Age-0 Brown Trout Salmo trutta in a Small Boreal Stream

Brown trout Salmo trutta populations are common in small boreal streams in Scandinavia. Populations are often small and isolated. In this chapter, I summarized studies conducted for 8 years on a population of small-sized brown trout living in a small Norwegian boreal stream exposed to harsh winters (>5 months of ice cover) and low water levels during summer. Subsequently, I evaluated factors influencing local use of spawning areas and the distribution of age-0 individuals during autumn in a contiguous 1.4 km portion of the stream. Finally, I analyzed the local and temporal variation in individual length of the age-0 individuals during late fall and related distribution of age-0 brown trout to habitat qualities, including fish density and environmental factors varying at the catchment scale (i.e., discharge and temperature) or at the local scale (i.e., habitat characteristics summarized using principal component analysis and presence/absence of competitors). Spawning occurred throughout the stream, indicating that female brown trout were able to find small patches of suitable spawning habitat in most study sections. Age-0 brown trout were also found in all sections of the stream, with large variation among sections. General linear mixed effects models, with year as a random effect, indicated that variation in the second principal component was the main factor determining density of both mature and age-0 brown trout. However, the effect was in opposite direction for the two groups. Variation in length of age-0 brown trout was substantial, and associated with most of the measured parameters. There was a significant negative density effect, and this effect was exacerbated by the presence of alpine bullhead Cottus poecilopus. Both mature brown trout and age-0 juveniles utilized the full extent of the study area, leading to substantial phenotypic variation.