Diet Selectivity of Brook trout (Salvelinus fontinalis) in Buttermilk Brook, Warren County, NY By Lyndon Watkins Abstract Buttermilk Brook, located within Warrensburg Warren County, NY is a first order high gradient Stream which flows directly into the Hudson River. Buttermilk Brook is a unique steppe pool stream consisting of a granite base and large areas devoid of substrate. Brook trout populations within Buttermilk Brook are unpressured by people and their abundance is high. As opportunistic insectivores they feed on both aquatic and terrestrial invertebrates. In order to better understand foraging behaviors in small stream brook trout populations, macroinvertebrate community composition within the stream was compared to gut contents of brook trout inhabiting the stream. Coupled with the uniqueness of Buttermilk brook's steppe pools and large shallow expanses with no substrate (only one riffle occurred within the 500m sample area) makes this brook an ideal sample area. Three points along a 30 meter stretch were sampled to obtain gradient, water quality and stream velocity. Fish (n=90) were captured by angling (October - December of 2015) and stomach contents were collected with gastric lavage. Four samples of macroinvertebrates were collected from the same reach with a Surber sampler inserted 15 cm into the substrate. All macroinvertebrate samples were identified in the lab to family. Using Ivlev's Electivity Index, which ranks diet preference in proportion to what is present in the habitat on a scale of -1 to 1, it was shown that brook trout selected for terrestrial invertebrates with indices of 1. Aquatic invertebrates varied with midge larvae (Chironomidae) (.89), Predatory caddisflies (Hydrobiosidae) (.85), and Net spinning caddisflies (Hydropsychidae) (.80) as the most selected. Habitat samples showed that Caddisflies were few in number within the stream and that Stoneflies were the most abundant. Brook trout readily sought out caddisflies and other aquatic invertebrates while ignoring others. stoneflies and annelids. though abundant in the habitat samples were barely present or nonexistent within the stomach samples. This study suggests that Brook Trout eat everything that fall into the water similar to other studies that also show perfect indices of 1. The Data suggest that brook trout exhibit adaptability when foraging for food in a challenging environment.