Lentic habitat use of roundtail chub Gila robusta and overlap with two nonnative piscivores, brown trout Salmo trutta and lake trout Salvelinus namaycush

Roundtail chub Gila robusta were historically widespread in the Colorado River drainage, but are now found in only 45% of their historic range and in low abundances. Range-wide declines in roundtail chub abundance are due to anthropogenic impacts from dams and water diversions and to additions of nonnative fishes to the system. This Colorado River endemic is commonly known as a "Big River" fish occurring in lotic environments, but isolated populations of roundtail chub are found in six natural lakes in the upper basin. These lakes contain the only lentic populations of roundtail chub, and little is known about their life history in these systems. The purpose of this study was to determine habitat use of roundtail chub in lentic systems and to explore potential impacts from two introduced species of predators, brown trout Salmo trutta and lake trout Salvelinus namaycush in two of the lakes where roundtail chub occur. Roundtail chub were commonly sampled in littoral habitats, occurred seldom in pelagic habitats, and were absent from deep benthic habitats. Brown trout tended to overlap with roundtail chub in the littoral zone both before and after thermal stratification, while lake trout tended to overlap with roundtail chub only before stratification. Brown trout consumed higher proportions of minnows than lake trout, especially after thermal stratification. Stable isotopic signatures (delta13C and delta15N) of brown trout and lake trout revealed that brown trout assimilated a greater proportion of their diet from fish that did lake trout, but instances of roundtail chub consumption by these predators were rare among fish sampled.