Winter Foraging and Growth of Brown Trout in Southeastern Minnesota Streams

                Growth rates and abundances of brown trout vary widely among streams in the Driftless Ecoregion and previous research has suggested prey availability and composition in diet as a possible mechanism.  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.  Additionally, several taxa of aquatic invertebrates; known as ultra-cold stenotherm (UCS) species, are rare during the summer but often highly abundant during the winter season.  We collected diet samples from brown trout in 12 different streams on three occasions during the winter (November-March) of 2010-2011.  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, or back calculated length at age and incremental growth measurements from scale samples collected simultaneously with diets.  Standardized sampling of aquatic invertebrates was also conducted to estimate prey availability in each of the 12 streams.  Trout diets varied both seasonally and between streams, with different invertebrate taxa dominating diets at various times and locations.  Diets varied by stream and sampling date, but were dominated by Chironomidae, Brachycentrus, Gammarus, and Gastropoda.  Growth rates varied by individual and by stream, but the majority of fish grew over the winter season.  Growth rates were positively related to increasing groundwater influence.