Recent studies reveal that despite a decrease in seawater concentration of methylmercury, the amount of mercury found in fish varies among different species. This joint research, conducted by the Indian Institute of Technology, Hyderabad (IIT-H), Harvard University, and Fisheries and Oceans Canada, indicates that some species of fish have less mercury than before, while others have a high concentration of mercury, including tuna, salmon and swordfish. More alarmingly, the researchers warn that human exposure to the toxin, known as methylmercury, through fish consumption is likely to increase due to climate change.
Understanding Mercury and its Effects on Health
Mercury is a naturally occurring element found in air, water and soil. However, exposure to mercury, even in small amounts, can lead to serious health problems. Specifically, mercury poses a significant threat to the development of young children and fetuses. It can have toxic effects on various systems of the body, such as nervous, digestive, immune systems, as well as on lungs, kidneys, skin and eyes. The World Health Organisation (WHO) classifies mercury as one of the top ten chemicals of major public health concern.
Methylmercury, an organic compound, is the primary form of mercury humans are exposed to when consuming fish and shellfish. This leads to increased vulnerability to Minamata disease – a disorder resulting from methylmercury poisoning. First documented in Minamata Bay, Japan, the disease is characterised by peripheral sensory loss, tremors, dysarthria, ataxia, and both hearing and visual loss. It’s vital to note the differentiation between methylmercury and ethylmercury, the latter being used as a preservative in some vaccines without posing a health risk.
Factors Causing Variation in Mercury Accumulation in Fish
There are mainly three factors contributing to the variation in mercury accumulation in different species of fish: overfishing, variations in seawater temperature, and changes in the amount of mercury present in seawater due to pollution.
Overfishing can lead to dietary changes among marine animals. For instance, cod had lower mercury levels as they largely fed on lobsters and large invertebrates low in mercury, contrasted with dogfish whose diet included more squid and cephalopods high in mercury.
Seawater temperatures also play a role in altering mercury accumulation. Fish metabolism is temperature-dependent, meaning that an increase in ocean temperature results in a higher fish metabolism. Consequently, more energy obtained from food is spent on maintenance rather than growth, leading to a higher concentration of methylmercury in predatory fish.
Changes in the amounts of mercury found in seawater as a result of pollution also significantly contribute to the varying mercury concentrations in different fish species.
Table: Mercury Levels in Commonly Consumed Seafood
| Fish type | Mercury Level |
|---|---|
| Cod | Low |
| Dogfish | High |
| Tuna | High |
| Salmon | High |
The Minamata Convention on Mercury
Addressing the concerns surrounding mercury-related health risks is the Minamata Convention on Mercury. This global treaty is designed to shield human health and the environment from the adverse effects of mercury and its compounds. Established during the fifth session of the Intergovernmental Negotiating Committee in Geneva, Switzerland in 2013, one of the key obligations under the Convention is to control the anthropogenic releases of mercury throughout its lifecycle. The Convention also tackles issues related to interim storage and disposal of mercury waste, sites contaminated by mercury, as well as health concerns. More than 140 countries, including India, have ratified the convention.