Abstract
Mercury contamination in freshwater fish is widespread across North America, including the western United States. Atmospheric mercury from both natural and manmade emissions deposits into watersheds and, through methylation and biomagnification, accumulates in aquatic food webs. Highest mercury concentrations are found in predatory fish. The endangered Colorado pikeminnow (Ptychocheilus lucius) is a long-lived, top-level piscivore endemic to the Colorado River basin. Mercury exposure to Colorado pikeminnow and another native fish species, the roundtail chub (Gila robusta), was assessed by analyzing muscle tissues collected using a nonlethal technique. Mercury concentrations in Colorado pikeminnow > 400-mm long, captured from critical habitat throughout the species’ present range, exceeded the tissue threshold-effect level of 0.2 µg/g wet weight (WW) for whole body fish (0.31 µg/g WW in muscle) recommended to protect fish from injury. Mercury is a neurotoxin and endocrine disruptor, and impacts to fish may include reduced ability to avoid predators, secure food, and reproduce. The highest mercury concentrations were found in both Colorado pikeminnow and roundtail chub collected from the White River, a tributary to the Green River. Colorado pikeminnow from the White and Green rivers had the highest mean mercury concentrations and the lowest mean relative body conditions. Exposure to high mercury concentrations may act in concert with other threatening factors to compromise Colorado pikeminnow population viability and eventual recovery.
Highlights
Water quality and fish surveys conducted during the past 20 years revealed widespread mercury contamination, especially in freshwater systems of the northern hemisphere (Schmidt and Brumbaugh 1990; Brumbaugh et al 2001; Scudder et al 2009; Cladis et al 2014; Eagles-Smith et al 2014, 2015, 2016)
The mean concentration for 83 Colorado pikeminnow > 400-mm long was 0.66 μg/g wet weight (WW) or twice that of the toxicity guideline
For the 10 Colorado pikeminnow sampled from the White River, the mean concentration (1.1 μg/g WW was over three times that of the toxicity guideline (Table 2)
Summary
Water quality and fish surveys conducted during the past 20 years revealed widespread mercury contamination, especially in freshwater systems of the northern hemisphere (Schmidt and Brumbaugh 1990; Brumbaugh et al 2001; Scudder et al 2009; Cladis et al 2014; Eagles-Smith et al 2014, 2015, 2016). After inorganic mercury enters aquatic ecosystems, conversion to methyl mercury is a key mechanism affecting bioaccumulation in the aquatic food web (Cocca 2001; Engstrom 2007; Pasquale et al 2009; Tsui et al 2010; Driscoll et al 2013; Eagles-Smith et al 2016). Skeletal tissue concentrations vary with fish species, location, feeding habitats, and age (Sandheinrich and Wiener 2011). Fish at higher trophic levels usually contain the greatest mercury concentrations (Beckvar et al 2005; Peterson et al 2007; Sandheinrich and Wiener 2011). Environmental conditions (e.g., pH and temperature), sensitivities of individual species and life stages, and chemical and physical form of mercury, all affect toxicity (Wiener and Spry 1996). Several field and laboratory studies have demonstrated neurotoxic effects and impaired reproduction occur in the fish themselves at relevant dietary exposures similar to those found in the environment (Crump and Trudeau 2009)
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