Mercury pollution is a major global environmental problem, with small-scale gold mining and coal burning the two biggest sources, but fish can recover quickly when the pollution stops
15 December 2021
Fish populations appear to recover rapidly from mercury pollution once humans stop adding it to their environment.
A 15-year study of a lake in Canada found that eight years after the metal’s supply ceased, concentrations of methylmercury – a highly toxic substance made from mercury by bacteria in aquatic ecosystems – fell by 76 per cent in northern pike (Esox lucius) and 38 per cent in lake whitefish (Coregonus clupeaformis).
“I can’t imagine a much faster recovery,” says Paul Blanchfield at government agency Fisheries and Oceans Canada, who led the research. The team are not suggesting the fish excrete the mercury quickly – the experiment in fact shows they hang on to it for a long time – but that quick turnover of generations sees concentrations fall fast when new pollution stops.
Mercury pollution is still a major global environmental problem, with small-scale gold mining and coal burning being the two biggest sources. Transported in the atmosphere and rained down on lakes and oceans, the metal’s accumulation in freshwater and marine species has raised concerns over the human health impact of eating fish.
Yet little is known from observations about how fast mercury levels decline once the pollution stops. To find out, the team ran a study in the Experimental Lakes Area in Ontario, Canada, a remote set of lakes set aside for science. The researchers added mercury to a lake there for seven years, on a par with average amounts found in mercury-polluted waterways of North America. They used mercury with a distinctive isotope so the pollutant could be distinguished from any that fell in rain.
After the team stopped adding mercury, the lake’s top predator, the pike, had the highest amount of methylmercury. But concentrations in the species fell roughly twice as fast as in the other main large-bodied species, the whitefish. The faster recovery seems to be explained by a quicker turnaround of new fish among the pike, which, on average, were much younger than the whitefish.
The rapid declines in mercury are good news for communities reliant on selling fish and countries that consume a lot of fish, says Blanchfield. But he cautions that only a “tiny fraction” of the mercury the team also added to the forest and wetland around the lake entered the water and this may reach it later, highlighting the persistent nature of the pollutant.
John Munthe at IVL Swedish Environmental Research Institute says the study was a “unique scientific experiment most likely never to be repeated again”. He says the results are robust and demonstrate “what we have always assumed but only being partly or indirectly being able to provide evidence for”. They provide a strong argument to boost efforts to cut mercury emissions, he adds.
Separately, new research shows that mercury is accumulating at “remarkably” high rates in the hadal zone, the deepest part of the ocean that extends down as far as 11 kilometres. The paper, published in the journal PNAS, shows that the average accumulation rate in the sediment of deep-sea trenches since 1950 is 30 per cent higher than it was between 1900 and 1950.
Journal reference: Nature, DOI: 10.1038/s41586-021-04222-7
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