Print PageBecause of the high binding affinity between mercury (Hg) and selenium (Se), environmental availability of Se must be considered when assessing Hg bioaccumulation in aquatic ecosystems. Inverse relationships between environmental Se availability and Hg contents of lake fish have been noted in natural experiments and intervention studies involving anthropogenically increased Se. However, no prior study has examined the influence of naturally occurring Se on bioaccumulation of Hg in freshwater fish.
The purpose of this study was to examine relationships between environmental Se and Hg bioaccumulation in stream fish from the western US. Since tissue Se contents reflect dietary Se intakes, we used fish Se as an indicator of environmental Se availability. We hypothesized that formation of insoluble mercury selenide (HgSe) in tissues of prey animals would limit digestive absorption of Hg by predators. Therefore, we expected increased availability of Se in the food web to be accompanied by diminished Hg bioaccumulation in fish tissue. Fifteen species of fish were collected from 130 sites across the western United States (total n = 659) and their whole body Hg and Se contents were analyzed and evaluated in relation to their weight and length.
Results/Conclusions
We found significant inverse relationships between Hg and Se in 3 piscivorous species (Northern pike, Northern pikeminnow, and walleye) and 4 non-piscivorous species (common carp, white sucker, shorthead redhorse, and black bullhead). Mercury contents of 56% of the 659 fish exceeded the wildlife protective Hg threshold and 12% exceeded the MeHg water quality criterion for the protection of humans. However, Hg occurred in molar excess of Se in only 13 individual fish, which were all Northern pikeminnows. Molar Hg:Se ratios increased significantly with increasing weight and length in all piscivores studied (Northern pike, Northern pikeminnow, walleye, and smallmouth bass) as well as in 5 non-piscivores (common carp, white sucker, shorthead redhorse, largescale sucker, and brown trout).
Since Hg toxicity risks are proportional to Hg:Se molar ratios, Se should be analyzed whenever Hg risk assessments are performed. Our findings indicate that the molar Hg contents of certain piscivorous fish species could rise to exceed those of Se in locations affected by Hg point sources. In summary, the findings of this study support our hypothesis that increased environmental Se availability is an important determinant of the amount of Hg that will bioaccumulate in fresh water fish from rivers and streams.
