Scientists from the University of Hawaii at Manoa, and the University of Michigan have found the mechanism that causes certain fish species to exhibit higher levels of mercury than others. Researchers have known for years that predatory fish that feed at lower depths contain higher levels of the toxic metal; but the reason for this was not understood until now.
It was previously thought that most of the production of methylmercury, the organic compound of mercury that fish easily absorb was mainly produced near the surface; however measurements of mercury levels in both the water and the fish found higher levels of the toxic chemical below 165 feet. Methylmercury production has been observed in depths of as much as 2,000 feet.
Methylmercury is also produced near the surface of the ocean, but the higher availability of light can destroy as much as 80% of the substance at shallower depths through a photochemical reaction.
The mercury is first released into the atmosphere as exhaust fumes from sources such as power plants that burn coal and oil. Rainfall then conveys it to the ocean. It is then thought that anaerobic bacteria are the mechanism that turns the inorganic mercury into methylmercury, which then accumulates in fish.
Fish at the top of the food chain and at lower depths exhibit higher levels of contamination as there is more methylmercury at those depths, and they also ingest fish that are tainted with the toxic chemical.
The study also points out that it is not enough to reduce pollution locally in order to curtail mercury contamination. The study was conducted on nine different species of fish near Hawaii and are thought to have been tainted from sources as far away as India and China. University of Michigan Environmental Scientist Joel Blum, head of the study, said “this study reinforces the links between mercury emitted from Asian countries and the fish that we catch off Hawaii and consume in this country.”
The Environmental Protection Agency last revised mercury pollution guidelines in 2011, sharply limiting mercury emissions, among others. The United Nations Environment Programme has brokered the Minamata Convention on Mercury to curtail contamination by the toxic metal internationally. However, no pollution guidelines have resulted so far.
In addition, as global temperatures rise, lower oxygen levels are expected in the ocean, and it is thought this will boost the production of methylmercury, and its adverse effects on marine life, and the human food chain.
Despite the mechanism by which mercury accumulates in fish being discovered, we have much to do before we can claim victory over this problem.
By Milton Ruiz