During a 144-wk period three generations of brook trout (Salvelinus fontinalis) were continuously exposed to mean water concentrations of methylmercuric chloride (MMC) of 2.93, 0.93, 0.29, 0.09, 0.03, and <0.010 (control) μg Hg/liter. During the first 39 wk the highest concentration (2.93 μg Hg/liter) produced deformities and 88% mortality of first-generation adults. At an MMC concentration of 0.93 μg Hg/liter, second-generation trout developed deformities and all but one female died during the 108-wk exposure. No significant effects on survival, growth, or reproduction of second-generation trout were noted at any of the lower MMC concentrations. No toxic symptoms were observed in the third generation at the three lowest MMC concentrations. The maximum acceptable toxicant concentration (MATC) for brook trout exposed to MMC in this water with a hardness of 45 mg/liter (as CaCO3) and a pH of 7.5 fell between 0.93 and 0.29 μg Hg/liter. The mean 96-h LC50 for yearling (200 g) and 20-wk-old (12 g) juvenile brook trout exposed to MMC was 75.0 μg Hg/liter, and the application factor (MATC/96-h LC50) lies between 0.004 and 0.013. The accumulation rate of mercury by eight selected tissues of first-generation trout exposed to MMC was relatively rapid at all MMC concentrations tested. The 2-wk tissue Hg: water Hg concentration factors ranged from 1 × 103 to 12 × 103, depending on the tissue, whereas after 28–38 wk of exposure the maximum tissue Hg: water Hg concentration factors for both first- and second-generation trout ranged from 6.9 × 103 to 6.3 × 104. The blood, spleen, and kidney accumulated mercury most rapidly and contained the highest residues in both first- and second-generation trout followed by liver, gill, brain, gonad, and muscle in order of decreasing mercury residues. There was no significant elimination of mercury from the tissue of first- or second-generation fish, yet a "steady state" (micrograms mercury per gram = constant) was reached in all tissues after 20–28 wk of continuous water exposure. Monomethylmercury made up 90–95% of the total mercury present in muscle, the only tissue analyzed for this compound. Mean muscle residues in first-generation trout, dying after 16–28 wk of exposure to 2.93 μg Hg/liter and in second-generation trout, dying after 64–100 wk of exposure to 0.93 μg Hg/liter, were 23.5 and 9.5 μg Hg/g, respectively.
Brook trout passage performance through culverts