What's the issue? Eating fish has a lot of health benefits, it is a great source of lean protein and omega 3 fatty acids that may improve brain function and help balance our diets rich in omega-6. However we've simultaneously been told to reduce our fish consumption due to levels of mercury in the food chain. Some species contain higher levels then others, for example you could swap tuna and swordfish for salmon and shrimp which have respectively lower amounts.
Where has the Mercury come from? Mercury occurs naturally in the environment from sea volcanoes to forest fires and the weathering of rocks. However as Mercury is an element and therefore not biodegradable, it persists in the environment forever. There are different types of Mercury and they have many uses in every day life, it is the methyl mercury form that had lead to warnings over fish consumption.
Humans have artificially raised levels of mercury through mining and other industrial applications that disrupt the natural ecosystem. Methylmercury is a neurotoxin that can affect brain and nervous development in children. Mercury is present in all fish and converted into the Methyl form by bacteria. The levels magnify as you move up the food chain therefore those further down will contain lower levels. Cooking does not remove any appreciable amounts.
What are safe levels? 1 part per million according to the FDA. Shark, swordfish, mackerel and tilefish and certain tuna usually have higher than this level. Shellfish, pollack, canned light tuna and catfish usually have less.
Benefits of fish oils Omega-3 fatty acids can protect against heart disease and stroke. Humans are unable to synthesise these fatty acids therefore they must be found in the diet. Unlike Mercury they are important for healthy brain and neuro development.
Therefore despite the risks it is still well worth ensuring an adequate fish intake when positioned against the health benefits. Just try to ensure you buy from reliable sources and eat species known to contain lower levels as suggested above. Environmentalchemistry.com provide a good table at the bottom of the page with a list of many different types of fish and their mercury levels.