Fish has always been considered part of a healthy and nutrient-rich diet since it has low saturated fat content and is high in omega-3 fatty acids, proteins, and other nutrients. Along with these nutrients, fish, shellfish, or any other consumable fish variety, have certain amounts of mercury also in them, and this can lead to mercury poisoning. It does not mean that consuming all types of fish can lead to toxicity, but consuming certain varieties — those that have more mercury content like swordfish, shark, tuna, mackerels, and tilefish — can have risks. The amount of fish consumed also matters in deciding the risks. Symptoms of toxicity can occur from overconsumption of fish varieties with higher levels of mercury in them. Thus, limiting the amount to two meals in a week (about 12 ounces) has been suggested by FDA and EPA to avoid mercury poisoning and to get the good nutrition that eating fish provides.
 

The nutrition we gain from fish contributes to good health in major ways, especially in growing children, but this nutrition rich food can have risks of toxicity if it is overconsumed. Mercury is released into the atmosphere from industrial pollutants that are let out into the air and into streams near factories. From air, the mercury settles into streams and lake water as methyl mercury, which is consumed by the fish feeding in that water. Finally, based on the feeding area, pattern, and amount of fish, the levels of methyl mercury built up in the fish vary. Some accumulate more while others accumulate less mercury in them and thus are related to high or low risks of toxicity for the consumer.

The nutrients we get from fish are important for the appropriate development of the body and brain, especially for women and children, but it has attached risks. Mercury poisoning occurs from overconsumption of fish with high levels of methyl mercury accumulated in them. Mercury trapped in the human body gets converted to a kind of neurotoxin that is associated with symptoms like delayed or hampered development of the brain and nerves.
This toxicity can show several other symptoms such as the following.

Symptoms in Developing Children:

To avoid risks of mercury poisoning, pregnant women are advised to avoid eating larger varieties of fish and consume only the small types. While consuming fish, appropriate steps should be taken to balance nutrition and the risks of toxicity to prevent symptoms associated with mercury poisoning.