Influences on brown trout and brook trout population dynamics in a Michigan River

Understanding the influences of local and regional processes on the dynamics of self-sustaining trout populations would help fishery biologists better manage trout populations and protect rivers supporting trout. We explored hypotheses behind long-term temporal variation in density, growth, and survival of brown trout Salmo trutta and brook trout Salvelinus fontinalis using data collected over several decades on Michigan's Au Sable River. Regression models developed for these species emphasized the influence of year-class strength on older age-classes, year-class strength being positively related to spawner abundance for both species and negatively related to high spring streamflow conditions for brown trout, Age-class density was also positively associated with high levels of large woody debris (LWD) in streams. Annual growth increments of brown trout and brook trout were often negatively related to increased age-class density and LWD and positively affected by elevated total phosphorus levels, cool summers, and warm winters. Annual survival of trout from age 0 to age 4 was negatively related to intra- and interspecific age-class density, and in three of seven models, positively associated with levels of LWD. Our findings emphasize the importance of year-class strength to trout population dynamics as well as the need to include collection of regional- and local-scale habitat data in studies of trout population dynamics.