Optimizing fish sampling for fish - mercury bioaccumulation factors

Fish Bioaccumulation Factors (BAFs; ratios of mercury (Hg) in fish (Hg_fish) and water (Hg_water)) are used to develop Total Maximum Daily Load and water quality criteria for Hg-impaired waters. Both applications require representative Hg_fish estimates and, thus, are sensitive to sampling and data-treatment methods. Data collected by fixed protocol from 11 streams in 5 states distributed across the US were used to assess the effects of Hg_fish normalization/standardization methods and fish sample numbers on BAF estimates. Fish length, followed by weight, was most correlated to adult top-predator Hg_fish. Site-specific BAFs based on length-normalized and standardized Hg_fish estimates demonstrated up to 50% less variability than those based on non-normalized Hg_fish. Permutation analysis indicated that length-normalized and standardized Hg_fish estimates based on at least 8 trout or 5 bass resulted in mean Hg_fish coefficients of variation less than 20%. These results are intended to support regulatory mercury monitoring and load-reduction program improvements.