Summary

Description

In 2002 the U.S. Fish and Wildlife Service Service Southern Nevada Field Office initiated a study to identify environmental contaminant impacts to native fish of the Muddy River, Clark County, Nevada. Potential sources of pollution include a coalfired power plant, dairy cattle operation, agriculture, and irrigation return flows. The Muddy River Ecosystem provides habitat for at least eight rare aquatic species including the endangered Moapa dace Moapa coriacea. Between February 2002 and April 2003, over five hundred fish were surveyed for external health. Overall fish condition was good and incidence of external lesions, parasites, and physical anomalies were low in representative fish species. A total of sixty nonnative fish were collected at six stations throughout the Muddy River and submitted for inorganic trace metal analysis. Three of the fish were also analyzed for organic organochlorine compounds. Species sampled include one common carp Cyprinus carpio, two bluegill Lepomis macrochirus, nineteen blue tilapia Oreochromis aurea, and 38 black bullhead catfish Ictalurus melas. Chemical residues recovered in whole body fish were compared to National Contaminant Biomonitoring Program NCBP data as well as threshold toxicity values in published literature. Of the nineteen trace elements tested, only molybdenum was not detected in any fish sample. Most concentrations were within background levels, however, mercury range nondetect 0.28 parts per million wet weight ppm ww, 0 0.097 and selenium range 0.21 1.2 ppm ww, 0 0.654 are potential contaminants of concern. While levels detected appear to be below thresholds associated with acute toxicity, sublethal chronic effects to the longterm health and survival of endemic fish is unknown. Mercury is a known endocrine disruptor and of particular concern as an environmental toxin due to its ability to bioaccumulate in the food chain. All organochlorine compounds OCs tested were below the lower detection limit, with the exception of p,p DDE maximum value 0.029 ppm ww, which was within the expected range of concentrations based on historical DDT use. Our study confirms that Muddy River fish are being exposed to and accumulating certain trace metals. In the absence of species specific toxicity information for native fish, only general inferences can be made about the possible effects of trace metals detected based on comparisons to threshold toxic affect levels for other fish species. Results indicate mercury and selenium may pose a slight risk to native fish even at low levels. If trace metal concentrations in surrogate nonnative fish are a good estimator of levels in native fish, our study did not find evidence of gross contamination. We conclude that contaminants pose a relatively small risk to native fish compared to documented threats from habitat modification and predation by nonnative species. To achieve maximum benefit for recovery of endemic fish, we recommend resource managers place a higher priority on removal of exotic species and habitat restoration while continuing to monitor the abundance, health and distribution of native fish.