Assessing the potential for Smallmouth Bass population establishment in Grand Canyon

ABSTRACT Objective The expansion of nonnative Smallmouth Bass Micropterus dolomieu into the Grand Canyon ecosystem downstream of Glen Canyon Dam is widely seen as a potential threat to native fishes, particularly the Humpback Chub Gila cypha, which is listed as threatened under the Endangered Species Act. This concern stems from observations in the Green and upper Colorado rivers, where dam-modified habitats and reservoir introductions have allowed Smallmouth Bass to become established and impact native fish species. Our objective was to compare habitat conditions between a Smallmouth Bass population center in the Green River—where Humpback Chub are rare—and two major Humpback Chub population centers in the Colorado River in Grand Canyon, where Smallmouth Bass invasion is a conservation concern, to help assess invasion risk and inform management actions downstream of Glen Canyon Dam. Methods We developed a conceptual model of Smallmouth Bass recruitment potential emphasizing the roles of temperature, turbidity, and the timing of these conditions, based on literature for Smallmouth Bass and Largemouth Bass Micropterus nigricans. Using U.S. Geological Survey gauge station data, we summarized daily temperature and turbidity patterns near a known Smallmouth Bass population center in the middle Green River (near Jensen, Utah) and compared those with conditions in the Colorado River in Glen Canyon and Grand Canyon downstream from Glen Canyon Dam. We then used our conceptual model of Smallmouth Bass early life history, along with observed turbidity and temperature patterns, to explore how irregular recruitment could affect Smallmouth Bass population dynamics and individual growth potential using an age-structured population model and a bioenergetics model. Results Although warm reservoir releases due to low water storage in Lake Powell has made temperature regimes in Glen and Grand canyons more similar to those of the middle Green River, the Colorado River ecosystem in Grand Canyon has a very different turbidity regime from that of the middle Green River. The Colorado River in Grand Canyon becomes very turbid during late summer and fall, when tributaries experience flash floods during the North American monsoon season. These periods of high turbidity coincide with critical early life stages of Smallmouth Bass, likely limiting foraging efficiency, growth, and overwinter survival. Our population model indicates that rapidly growing and sustained Smallmouth Bass populations require successful recruitment at least once every 2 years, but empirical data suggest that these conditions occur only every 5–7 years in Grand Canyon. This would make long-term establishment of self-sustaining Smallmouth Bass populations in Grand Canyon unlikely or at least highly uncertain. Conclusions Despite the presence of nonnative predators, Humpback Chub populations in Grand Canyon have expanded during the past 10–15 years, very likely due to favorably warm river temperature. While Smallmouth Bass in Glen or Marble canyons (between Glen Canyon Dam and the Little Colorado River) may pose a downstream dispersal risk (and potentially a threat to Humpback Chub populations), high turbidity conditions downstream of the Little Colorado River reduce the likelihood of persistent local Smallmouth Bass recruitment. These results suggest that Smallmouth Bass invasion risk to Humpback Chub is not uniform across the Colorado River basin and should be evaluated in the context of the different site-specific water quality and flow regime conditions, other known risks to Humpback Chub, and Humpback Chub population size in different parts of the basin.

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