Assessing Smallmouth Bass Trophic Position in a Hierarchical River Network Using Stable Isotope Methods

Smallmouth bass (Micropterus dolomieu) is a common, coolwater species in the rivers of Virginia. In response to climate change, the range of smallmouth bass is expected to expand higher into river watersheds where increasing sympatry with coldwater assemblages is anticipated. An understanding of smallmouth bass trophic ecology in these smaller stream communities is important to quantify their potential impact on native brook trout (Salvelinus fontinalis) populations and overall community structure.  We hypothesized that smallmouth bass prey selection would reflect changes in prey assemblages with stream order, resulting in a decline in trophic position in smaller streams and increased drift feeding on invertebrate prey. To test this hypothesis we analyzed gut contents and employed stable isotope methods to assess smallmouth bass prey selection and trophic position across stream order in a hierarchical river network. Results show prey selection and trophic position do not change as a function of stream order. Numerical abundance of prey items in gut contents was dominated by omnivorous crayfish, a high-to-intermediate trophic level invertebrate that could potentially obscure inference on piscivory from stable isotopes. These illustrates that stable isotope methods are best applied in combination with gut content data to compare smallmouth bass trophic ecology between systems.