Winter Diets and Dynamics of Brown Trout in Groundwater Dominated Streams

Trout streams in the Driftless Ecoregion of southeast Minnesota provide valuable economic, recreational, and esthetic benefits.  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.  Although trout diets during the summer season have been studied in the past, the composition and importance of winter diets are relatively unknown.  Additionally, prey items are rarely identified beyond order or family, which can mask the contributions of particular species to brown trout.  The goal of our study was to document the composition of brown trout diets throughout the winter, and investigate possible implications for trout dynamics (i.e. growth, abundance, and survival) across a range of stream conditions in southeast Minnesota.  We collected diet samples from trout in 12 different streams on three occasions during the winter (December-March) of 2010-2011.  Aquatic invertebrates were identified to genus or species 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.  We performed bioenergetic modeling with our diet data to examine the energetic consequences of varying diets for brown trout.  Trout diets varied both seasonally and between streams, with different invertebrate taxa dominating diets at various times and locations.  Growth rates, abundances and survival of brown trout varied among streams, and empty stomachs were extremely rare.