Diet and Growth of Brown Trout in Southeastern Minnesota: Seasonal Patterns Across Two Years

Growth rates and productivity of brown trout (Salmo trutta) varies considerably among coldwater streams across the Driftless Area. Although water temperature is widely considered the most important regulator of fish growth, past research in this region has suggested that diet composition and prey availability may play a significant role in determining growth. In addition, the unique geological and hydrological characteristics of the region create conditions where winter season dynamics may be more important for trout populations than previously thought. Although trout diets during the summer season have been studied in the past, the composition and importance of winter diets are relatively unknown. We collected stomach samples from brown trout in six different streams during early summer, late summer, early winter and late winter between November 2010 and September 2012 to investigate seasonal patterns in trout diet. Aquatic invertebrates were identified to genus or family level to identify the relative importance of various taxa to brown trout. Growth was measured directly from marked and recaptured fish collected simultaneously with diets. Trout diets varied both seasonally and between streams. Winter diets were dominated by benthos, including larval stages of aquatic invertebrate taxa, whereas summer stomachs contained higher proportions of terrestrial drift and adult stages of aquatic invertebrates. Growth rates varied by individual and by stream, but growth was observed in most streams during spring, summer and winter.