Beware the dangers of eating some fish and shellfish

Fish and seafood have many nutrients and incorporating them into your diet is important. That being said, certain precautions must also be taken when preparing fish. These guidelines will walk you through the potential risks so you can safely reap the benefits of these wonders of the sea.

Some raw fish preparations, particularly sushi, can harbour parasites. Dutch "green" herring and Scandinavian gravlax (pickled salmon) are also raw, but the pickling process used in herring and properly made gravlax eliminates worms and eggs.

Oily fish, like fresh herring and mackerel, must be cooked or processed soon after they are netted. If kept too long before cooking they are susceptible to bacterial growth, which can cause scombroid poisoning, characterized by a rash and stomach upset.

Shellfish from waters polluted by human waste bring a threat of viral hepatitis as well as bacterial infections that can cause severe gastrointestinal upset. Shellfish farms are re­quired to meet strict health standards to ensure that their products are safe, but bacterial contamination still occurs. The old rule of eating raw oysters only in the "R" months does have some validity since bacteria are more likely to survive in warmer waters. Thorough cooking destroys the bacteria that can contaminate oysters.

Coastal waters are, at times, tinged red by a species of algae (Karenia brevis) in a phenomenon known as "red tide." Shellfish from red tide areas should not be eaten because they concentrate a toxin produced by the algae. Eating contaminated shellfish brings on symptoms of poisoning within 30 minutes: facial numbness, breathing difficulty, muscle weakness, and sometimes partial paralysis.

Ciguatera poisoning, which is similar, is caused by a toxin produced by a species of plankton. The plankton are consumed by fish, which then pass the poison on to humans. In some cases, the effects of this toxin have lasted more than 20 years.

Large, long-lived fish, such as tuna, shark, king mackerel, and swordfish, may accumulate heavy-metal contaminants — especially mercury — which are toxic to the human nervous system and can be dangerous for unborn babies. Because of this potential hazard, women should either avoid these fish completely during pregnancy or eat them no more than once a month. In terms of canned tuna, albacore tends to be higher in mercury than light tuna.

Some consumer groups recommend that pregnant women eat no more than 170 grams (six ounces) of light tuna every four days — 10 days for albacore. Some species of fish caught in certain areas may show high levels of PCBs and other industrial pollutants; they are best avoided.

Pregnant women are advised not to eat striped bass, especially from the northeastern regions, which may accumulate oil residues. Check with your local health department before eating fish caught in local streams and lakes. They may contain pollutants that are harmful.