Trophic Ecology of Roundtail Chub, Gila robusta, Along a Gradient of Invasion

Authors: Jane Rogosch, Julian Olden

Year: 2018

Abstract

Like many other species in the southwestern United States, Roundtail Chub, Gila robusta, is threatened by invasive species, water diversions and abstractions, and land-use change. Due to precipitous declines in range size and population number in recent decades, Roundtail Chub has been a candidate for listing under the Endangered Species Act (2009 USFWS: FWS–R2–ES–2009–0004; MO 92210530083– B2). Invasive species are likely the most significant threat to this species in the Bill Williams River basin, where the range of invasive species continues to expand while native species ranges contract. Using the gradient of invasive species prevalence as a natural experiment, our objective was to determine how negative interactions with invasive fishes such as competition or predation affect the trophic status and quality of resource use for Roundtail Chub. We identified 12 sites in the upper Bill Williams watershed that support native only, invasive dominant, and mixed native and invasive fish assemblages (n = 4 sites per assemblage type). We collected fish fin tissues, macroinvertebrate primary consumers, and algal and detrital basal resources at each site. Carbon and Nitrogen stable isotope analysis of these tissues were used to establish isotopic trophic relationships along a gradient of invasion. We used Bayesian approaches to compare the three assemblage types and examine trophic overlap using niche metrics (R package SIBER) and to examine resource use and potential predation using isotope mixing models (R package mixSIAR). Trophic area, total food chain length (nitrogen range), and basal resource use (carbon range) did not differ among the three assemblage types: native, mixed, and invasive assemblages. However, our hypothesis that competition affected resource use of Roundtail Chub populations was supported by examining niche overlap and changes in resource use between Roundtail Chub and Green Sunfish, Lepomis cyanellus. When the two species co-occurred in mixed assemblages, Roundtail Chub and Green Sunfish had 59% niche overlap. Bayesian mixing models indicated both species shifted diets when they co-occurred. Roundtail Chub and Green Sunfish had an 18% and 13% shift, respectively, away from diets dominated by riffle invertebrates (e.g. Ephemeroptera). Furthermore, mixing models that included small Roundtail Chub (< 100 mm) as a potential resource for known predatory fishes in the community estimated that Roundtail Chub made up a quarter (0.25) of Black Bullhead, Ameiurus melas, diets (95% Credible Interval (CI): 0.01 – 0.60) and over a quarter (0.29) of Green Sunfish diets (95% CI: 0.05 – 0.65). Therefore, Green Sunfish could be significant intraguild predators on young-of-year Roundtail Chub. Indicators of competition and predation from stable isotope analysis along a gradient of invasion supports other research in the Colorado River Basin, namely that invasive fishes can reduce the fitness and long-term survival and persistence of native fishes, including Roundtail Chub. Invasive species removal efforts in the current range of Roundtail Chub or removal efforts before Roundtail Chub are reintroduced into former ranges should be prioritized in light of these findings.