Age Structure and Growth Rates of Brown Trout in Southeast Minnesota Streams

Stream environments and their associated aquatic communities may be particularly sensitive to the ecosystem-altering effects of climate change. Southeast Minnesota streams are home to several species of cold-adapted, stenothermic insects that are sensitive to thermal changes. These insects are capable of producing multiple generations in winter and provide an important food source for brown trout during an otherwise food-limited time of year. Climate-induced changes in the abundance and distribution of these insects could have secondary effects on brown trout. We determined the relative importance of diet composition (especially cold-adapted insects) to overwinter growth and survival of brown trout in southeast Minnesota streams. We hypothesized that if trout growth and survival varied as a function of cold-adapted insect abundance, these parameters would increase as insects became more available. Using scales collected from 12 streams during winter 2010-2011, we quantified seasonal age and growth dynamics of trout. We aged scale samples and determined back-calculated length at age and incremental growth to compare growth and age structure among streams and sampling periods. We used regression analysis to examine the relationship between insect populations and trout growth rates. Aquatic insect abundance was positively related to trout growth rate. Winter diet explained some of the variation in growth rates and age structure between streams, which mirrored historical trends. It is evident that climate change is impacting biotic communities in southeast Minnesota streams, demanding contemporary, conservation-driven management strategies.