Optimizing fish sampling for fish-mercury bioaccumulation factors

•Watershed total maximum daily loads for Hg are estimated from fish Hg Bioaccumulation Factors (BAFs).•Fish mercury BAFs are ratios of concentrations of Hg in fish (Hgfish) and in water (Hgwater).•Hgfish estimates are sensitive to sampling and data-treatment methods.•Length normalization and standardization reduced variability in Hgfish estimates up to 50%.•Sampling at least 8 trout or 5 bass resulted in mean Hgfish coefficients of variation less than 20%.

Fish Bioaccumulation Factors (BAFs; ratios of mercury (Hg) in fish (Hgfish) and water (Hgwater)) are used to develop total maximum daily load and water quality criteria for Hg-impaired waters. Both applications require representative Hgfish 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 Hgfish normalization/standardization methods and fish-sample numbers on BAF estimates. Fish length, followed by weight, was most correlated to adult top-predator Hgfish. Site-specific BAFs based on length-normalized and standardized Hgfish estimates demonstrated up to 50% less variability than those based on non-normalized Hgfish. Permutation analysis indicated that length-normalized and standardized Hgfish estimates based on at least 8 trout or 5 bass resulted in mean Hgfish coefficients of variation less than 20%. These results are intended to support regulatory mercury monitoring and load-reduction program improvements.