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.