Methylmercury in fish from Owyhee Reservoir in southeast Oregon: scientific uncertainty and fish advisories

Data collected during 1987–1994 showed elevated levels of mercury (Hg) in fish tissue from the Owyhee Reservoir in southeastern Oregon. Sixty-five percent of the samples analyzed had total Hg levels exceeding the US Environmental Protection Agencyʹs (EPA) health screening value of 0.6 mg/kg. Eighteen out of 89 (20%) fish tissue samples also had total Hg levels greater than the US Food and Drug Administrationʹs (FDA) mercury action level of 1.0 mg/kg. The overall mean Hg content for all fish collected from the reservoir was 0.75 mg/kg wet weight (wet wt.). Fish muscle taken from largemouth bass (Micropterus salmoides), smallmouth bass (Micropterus dolomieu) and channel catfish (Ictalurus punctatus) had the highest mean Hg levels of 0.92, 0.87 and 0.82 mg/kg, respectively. In contrast, rainbow trout (Salmo gairdneri) had the lowest mean Hg content of 0.37 mg/kg. Increases in total Hg concentrations were found to be positively correlated with size for rainbow trout and yellow perch. A weak but significant correlation was also observed between total mercury content and age for smallmouth bass. Based on these data, in 1994 the Oregon Health Division (OHD) issued a fish consumption advisory for the Owyhee Reservoir using a conservative risk-based approach. The process of defining and communicating these consumption limits is the subject of this paper.