Winter diets and growth of brown trout Salmo trutta in groundwater-dominated streams

Principal Investigator: Bruce Vondracek.

PhD Student: William French

Status: Project completed November 2013

Summer interactions of stream-dwelling Salmonidae within food webs have been frequently studied; however, relatively little is known regarding winter food webs. Groundwater may increase the availability of potential prey and create more favorable foraging conditions for trout by maintaining relatively warm winter temperatures. Our objective was to augment our collection of diet and growth data of brown trout in 12 streams in southeast Minnesota with an additional analysis of carbon and nitrogen stable isotopes to more accurately and effectively depict trophic relationships. We collected tissues with differing turnover rates (mucus and fins) to track changes in diet from autumn through winter. A pilot study (accepted for publication by the Journal of Freshwater ecology) was conducted in Badger Creek (Houston County) during the winter of 2011-2012 to test the feasibility of stable isotope analysis from the published literature under winter field conditions. Samples were analyzed at the University of California Davis Stable Isotope Facility. Although relative food web position varied both temporally and by stream, brown trout (Salmo trutta) showed an increased reliance on terrestrially derived resources during the winter and there was a positive relationship between stream drainage area and brown trout trophic level. Our results suggest that allochthonous inputs may be of greater importance to stream food webs during periods of reduced autochthonous production, and ecosystem size has the potential to impact food webs in groundwater buffered streams.