Ecology: Predicting the recovery of mercury-contaminated fish populations

Reducing mercury pollution to lake ecosystems can lower methylmercury contamination in fish within just a few years, research published in Nature indicates. The findings suggest that efforts to control mercury emissions can reduce the risk of human exposure through fish consumption.

Mercury released as a result of human activity can find its way into aquatic ecosystems, where it is converted into methylmercury, a potent neurotoxin that accumulates in fish and poses a health threat to humans. Our understanding of how effective controls on mercury emissions might be at removing this contaminant from the food chain is limited. To directly assess the effects of mercury control measures on the recovery of fish contamination, Paul Blanchfield and colleagues carried out a 15-year whole-ecosystem experiment.

For the first seven years, the researchers supplied specific isotopes of mercury (to directly monitor the added mercury) to a remote, undisturbed lake and its watershed in Canada. During this period, the authors recorded an increase in the uptake of this isotopically labelled mercury as methylmercury in the fish within the aquatic ecosystem. Methylmercury concentrations increased by 45–57% in invertebrates (plankton, for example) and small fish (such as yellow perch), and by more than 40% in large fish, such as pike and whitefish populations. Thereafter, mercury additions ceased and the effects on the food chain were observed for eight years. Labelled methylmercury quickly diminished in the smaller fish, with concentrations dropping by at least 85% by the end of the study period. This triggered a subsequent reduction of labelled methylmercury in the larger fish, with concentrations declining by 76% in pike and 38% in whitefish populations.

The rapid reductions in methylmercury contamination observed in this experiment demonstrates the potential for mercury emission controls to improve the safety of fish for human consumption, the authors conclude.