Assessment of Mercury and Selenium Accumulation in Upper Colorado River Basin Fish Completed

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Native fish within the Upper Colorado River Basin encounter a suite of stressors including invasive species, habitat degradation, and flow alterations. Less attention has been given to contaminants as possible stressors, except for mercury and selenium, which are included in population recovery plans for some managed fish species of the Upper Colorado River Basin. Although these contaminants are known to be elevated in some fish species of the Upper Colorado River Basin, a broader spatial and temporal assessment was needed to understand the potential threats that these contaminants may have on the long-term viability of managed fish populations.

To meet this need, U.S. Geological Survey (USGS) scientists compiled 50 years (1962–2011) of mercury and selenium tissue concentrations for native and nonnative fish (2,324 individuals) collected during previous studies within the Upper Colorado River Basin. These studies were done to provide an understanding of concentrations among different species, sampling years, and locations in context with fish health benchmarks to determine potential health risks. The scientists used a statistical technique (linear mixed effects models) to estimate the average total mercury and selenium for each fish species while accounting for fish size, location, and year sampled. This technique allowed for comparison of contaminant concentrations among different studies.

Mercury and selenium concentrations varied by species and by location within the watershed. Concentrations of mercury and selenium were above health benchmarks in 13 and 48 percent of individual fish, respectively, and some species of management concern, such as the federally endangered Colorado pikeminnow (Ptychocheilus lucius) and the candidate roundtail chub (Gila robusta), accumulated mercury above health benchmarks. The Colorado pikeminnow and the razorback sucker (Xyrauchen texanus) had selenium concentrations that exceeded the selenium benchmarks. Most mercury exceedances were in White-Yampa River fish, whereas most selenium exceedances were in the Gunnison tributary fish.

The study identified data and knowledge gaps that can be used to guide research and monitoring to address the effects of mercury and selenium on species populations and recovery. For example, there is still a lack of mercury and selenium accumulation data for many native species in the Upper Colorado River Basin, including the razorback sucker, as well as for many nonnative fish. As such, a subsequent study on the razorback sucker and Colorado pikeminnow investigated selenium mobilization, partitioning, and trophic transfer along approximately 60 river miles of the selenium-impaired segment of the Lower Gunnison River. Results indicated that periods of selenium mobilization to surface waters have a larger-than-expected effect on selenium concentrations in food webs, that dissolved selenium concentrations are insufficient for predicting selenium concentrations in riverine food webs, and that ecosystem managers may underestimate risks by modeling selenium exposure based on those metrics.

This research demonstrates how a critical review of existing data can identify gaps and guide more targeted research. The retrospective assessment can inform future fish collection strategies to reveal if these contaminants pose additional risks to fish health today.

Funding for this study was provided by the Bureau of Land Management, Trapper Mine Corporation, Colorado River Round Table, Colorado Species Trust Fund, and the USGS Environmental Health Program (Toxic Substances Hydrology and Contaminant Biology), which is part of the Ecosystems Mission Area.