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for the Colorado pikeminnow. Although total mercury was not detected in 6 of the 8 samples <br /> (lab reporting level unknown),the maximum concentration measured was 0.10 µg/L,which is 10 <br /> times the chronic aquatic toxicity standard of 0.01 µg/L; the level of concern was listed as High, <br /> but clearly more sampling is needed. Chronic toxicity is the development of negative effects as <br /> the result of long term exposure to a toxicant or other stressor. It can manifest as direct lethality <br /> but more commonly refers to sub-lethal endpoints such as decreased growth,reduced <br /> reproduction, or behavioral changes such as impacted swimming performance. <br /> Total mercury concentrations of 0.20 and 0.1 µg/L were also measured in the 1990s in the <br /> Yampa River at the Maybell and Craig stations,respectively, although the median values for the <br /> datasets were below the detection limit(assumed to be zero) (USGS 2015). Despite occasional <br /> high water mercury concentrations, most values were low enough that the Yampa and White <br /> Rivers are not listed as impaired for mercury on the EPA 303(d) list(CDPHE 2012b) (all median <br /> values were below the detection limit of 0.018 µg/L at the Craig station,unknown limit at the <br /> Maybell station). Water mercury concentrations are not currently measured in the Yampa or <br /> White Rivers within endangered fish critical habitat. <br /> As explained more fully in the Effects of the Action section below, and provided as reference <br /> here, mercury in whole body fish < 0.2 micrograms per gram (µg/g)wet weight(WW)is an <br /> approximate threshold below which mercury tissue concentrations can be considered protective <br /> of juvenile and adult fish (see Beckvar et al. 2005 and further discussion in Effects of the Action <br /> section). Using the Model B regression equation (slope = 0.9048, intercept= -0.2387)developed <br /> by Peterson et al. (2005) for the northern pikeminnow (Ptychocheilus oregonensis),which is <br /> very similar physiologically to the Colorado pikeminnow,this translates to a value of 0.31 µg/g <br /> WW in muscle tissue. Muscle tissue is often sampled as muscle plugsa small, circular, <br /> shallow sample of muscle tissue taken from a live fish without significant injury. Osmundson <br /> and Lusk(2012) found a range of 0.39 to 0.58 µg/g WW mercury in Yampa River pikeminnow <br /> muscle tissue,with a mean of 0.49. Colorado pikeminnow that were captured in the 1960's from <br /> the Yampa River and more recently tested had slightly higher mercury concentrations (all <br /> archival pikeminnow averaged 0.65 µg/g WW mercury in muscle tissue) (Osmundson and Lusk <br /> 2012). Additionally, muscle tissue samples,taken from 4 adult pikeminnow (length 20-26 <br /> inches) in the Yampa River in 2006,had levels of mercury between 0.42 and 0.68 µg/g WW, <br /> with a mean of 0.56 µg/g (CDPHE 2015). <br /> Within the White River, Osmundson and Lusk(2012) found that mercury concentrations in <br /> pikeminnow muscle plugs were higher there than within any other occupied critical habitat unit, <br /> with muscle plug concentrations for these fish ranging from 0.43 to 1.83 µg/g WW (Osmundson <br /> and Lusk 2012). Roundtail chub (Gila robusta)were also tested in the White River as a part of <br /> the same study and were found to have elevated mercury levels as well (Osmundson and Lusk <br /> 2012). Whole body mercury concentrations in four adult pikeminnow(502-760 mm in length) <br /> taken from the White River immediately below Kinney Reservoir in 1986 ranged from 0.31 to <br /> 0.96 µg/g (after conversion to wet weight from dry weight(Krueger 1988)). Using the <br /> conversion factor derived from Peterson et al. (2005),the 1986 Colorado pikeminnow samples <br /> from the White River then ranged from concentrations of 0.50 to 1.75 µg/g WW mercury in <br /> muscle tissue (quite elevated). Osmundson and Lusk(2102) state that the White, Green, <br /> 39 <br />