Differential Interactions of Two Introduced Piscivorous Salmonids with a Native Cyprinid in Lentic Systems: Implications for Conservation of Roundtail Chub

Abstract

The effects of multiple nonnative piscivore species on native prey species in lentic systems are poorly understood. We studied the relative predation risks posed by two piscivorous salmonids (brown trout Salmo trutta and lake trout Salvelinus namaycush) to endemic roundtail chub Gila robusta in two lakes within the upper Colorado River basin. Gill nets were set in various habitat types to study habitat use by the three species before and after the onset of summer stratification. Roundtail chub and brown trout were mainly associated with shallow-water habitats, and this association did not change with thermal period. By contrast, lake trout habitat use changed with thermal period as fish moved from shallow areas in the spring to deepwater habitat after stratification. These habitat use patterns indicate that roundtail chub are more susceptible to predation by brown trout than by lake trout because both roundtail chub and brown trout occupy the littoral zone for a prolonged period. Diet data indicated that brown trout consumed littoral fish species (i.e., cyprinids), whereas lake trout primarily consumed opossum shrimp Mysis spp. and dipterans (true flies). Brown trout consumed proportionally more fish than did lake trout, began feeding on fish at smaller total lengths, and increased fish consumption during the period of thermal stratification. An important consideration for efforts to conserve lentic roundtail chub populations is the prevention of future introductions of littoral predators.

Received October 11, 2011; accepted October 20, 2011

ACKNOWLEDGMENTS

Funding was provided the U.S. Bureau of Reclamation, the Wyoming Game and Fish Department, and the State Wildlife Grant Program. We thank John Pokallus, Jennifer Harris, Jessica Julien, Kyle Blake, Daniel Leimbeck, Jared Nelson, and Erin Sobel for their help in the field and laboratory. We also thank Peter Cavalli, Kenneth Gerow, David Zafft, Mark McKinstry, and K. J. Reddy for their ongoing help during this study. The manuscript was greatly improved by comments from three anonymous reviewers.