3.3 Assessment of risk
3.3.1 Risk to fish
Only 4 of the 454 prey fish analyzed had whole-body concentrations of Hg exceeding 200 ng/g wet weight, an estimated tissue threshold associated with sublethal and reproductive effects in fish. These include 2 longnose dace from Grand Portage Creek, 1 blacknose dace from Poplar Creek, and 1 central mudminnow from Snow Creek. Mercury concentrations in most (99%) of the prey fish analyzed would not be expected to adversely affect health or reproduction. Stream-specific, mean concentrations of total Hg exceeded 40 ng/g wet weight, an estimated dietary threshold associated with diminished reproduction of piscivorous fish, in six of the seven species of prey fish analyzed (Table 5). Only slimy sculpin from Grand Portage Creek (24 ng/g) and central mudminnows from Snow Creek (beaver pond; 35 ng/g) contained a mean concentration less than the 40 ng/g threshold value for reproductive effects on piscivorous fish.