Add to this another 3,000 tons produced by burning fuel and household wastes, and you have an excessive amount of mercury that blankets the land and water. Once in the water, bacteria change it into methyl mercury, a very toxic form of the element. The fish absorb the poison, and it binds very tightly to the fish tissue. Cooking does not notably reduce the amount of mercury in the fish. Eating fish with high amount of methyl mercury can cause a serious illness and even death.
Natural toxins can also contaminate fish.
Certain types of algae produce toxins that accumulate in fish and can cause symptoms ranging from mild to very severe and even death. Paralytic shellfish poisoning can occur by consuming oysters or other mollusks that have eaten naturally-occurring toxic algae in red tides. Symptoms include trouble swallowing, lack of balance, lack of muscle coordination and eventually respiratory depression that can lead to death. Many other natural toxins can cause disease such as ciguatera toxin and scombroid fish poisoning. Infections such as Vibrio vulnificus and the Norwalk virus can also be acquired from oysters.