Why do some fish contain mercury?
Posted by kathryn in Fish
Mercury is a heavy metal which occurs naturally in the environment. However it’s also present in our world as a result of pollution. Mercury is used in the manufacture of a number of common products, including:
- car parts
- fluorescent light bulbs
- medical products
- dental amalgams
When these products are not disposed of carefully, they end up in landfill. As the products degrade, mercury is exposed and free to pollute both land and waterways. Mercury is highly toxic and most industrialised countries have made efforts to limit its use and ensure safe disposal. There have been campaigns in some countries to ban its use entirely.
Mercury does nasty things to our bodies. It damages the central nervous system, endocrine system, kidneys and other organs. Given this it’s wise to limit your exposure to this heavy metal. Fortunately, here in Australia most of us don’t come into contact with enough mercury to cause serious health damage. When toxic problems do occur it’s usually because of industrial exposure.
How does it get into fish?
Mercury is found in our waterways as a result of pollution and landfill run-off. Anaerobic bacteria which are found in lakes, rivers, soils, wetlands and the oceans then convert mercury into an organic form called methylmercury. As well as being highly toxic, methylmercury accumulates in organisms, working its way up the marine food chain by a process called biomagnification.
The anaerobic bacteria that work on mercury are consumed by plankton, which are then eaten by small fish, which are in turn consumed by larger and larger fish.
Each larger fish then absorbs the body burden of mercury from the smaller fishes it consumes. Therefore the danger to your health comes from eating a lot of the larger fish, those at the top of the marine food chain. These fish contain more concentrated levels of mercury, which may be ten times that found in smaller fish.
Higher mercury containing fish include shark (or ling), swordfish, barramundi, gemfish, orange roughy and Southern bluefin tuna.
Photo by Joseph Wu Origami