Samples of fish tissue are tested for total mercury, selenium and arsenic. If the mean concentrations of these contaminants exceed levels in which the general population would have to limit consumption to fewer than three meals per month, a fish consumption advisory is issued for the water body.
- The advisory includes meal frequency advice for all affected fish for that water body.
- A minimum of 10 fish and five composites are required before we post an advisory.
- To assess the risk associated with each water body and species, the weighted mean for each contaminant is compared with established threshold values specific to each contaminant.
- Advisories are created based on the most recent data available.
The weighted mean approach
About the use of weighted averages in fish consumption advisories:
- A sample is “a finite part of a statistical population whose properties are studied to gain information about the whole.”
- A disproportionate amount of the population may be represented, in this case, number of fish per composite sample, if the number of fish per composite sample is either over- or underrepresented. Thus, it differs from the population in proportion.
- The application of a “weighted mean” seeks to increase the probability that the sample is more representative of the population at large.
- A minimum of 30 fish per species/size class is required before we list a body of water as impaired due to fish tissue contamination.
- The methodology recommends these 30 fish should, at minimum, be composed of five composite samples.
- For both the impairedwaters list and fish consumption advisories, use of a weighted mean gives equal weight to individual fish within a composite.
- By using the weighted mean approach, we try to avoid composites with fewer fish (and either significantly higher or lower mercury concentrations) from having a biased effect on the resulting mean statistic.
Eq. 1((x * N) + (y * N) + (z * N))/(x + y + z)
Where:
- x = Number of fish per composite 1.
- y = Number of fish per composite 2.
- z = Number of fish per composite 3.
- N = Total number of fish per species/size class.
Other agency resources
- Methylmercury Fish Tissue Criterion(EPA).
- Methylmercury(FDA).