Food-safety experts focus on three types of risks from seafood: bacteria, natural toxins, and chemicals.
Pathogens in Seafood
Vibrio vulnificus is the most dangerous microbiological hazard in seafood, and Norwalk virus is the most common. And unlike other pathogens that target people with weakened immune systems, Vibrio parahaemolyticus can make just about anybody sick.
Seafood can also carry Salmonella, Staphylococcus, and Norovirus. The good news is that all of these pathogens can generally be avoided through safe handling (to avoid cross-contamination) and thorough cooking. And just like with meat and poultry, in the grocery store you should be aware of possible cross-contamination of cooked and raw seafood if displayed in the same case. Make sure the raw fish is on a level lower or separated from the cooked fish so that any raw fish juices don’t flow onto the cooked items.
Toxins in Seafood
Beyond microbial contamination, there are risks from seafood that persist even if the food is thoroughly cooked. Some toxins, for example, begin to grow in seafood as soon as the fish begins to decay. If the fish isn’t kept sufficiently cold from the moment it’s caught, the toxins can form—and they can’t be cooked away. Non-pathogenic bacteria can convert the natural histidine in some fish to toxic histamine.
That can be a problem with fresh (not canned) tuna and mahi mahi. Ciguatera is a neurotoxin that occurs in certain fish harvested from reef areas. That toxin is actually produced by dinoflagellate plankton that are eaten by the fish. To minimize your risk, use the tips from earlier in this chapter to check whether fish has been kept properly chilled. To cut your risk of ciguatera poisoning specifically, avoid locally caught grouper, amberjack, and red snapper in tropical areas. It’s especially a problem in Florida, the Caribbean, and Hawaii.
Chemicals in Seafood
Finally, certain chemicals in seafood, like PCBs, dioxins, and mercury, pose risks to some or all of us.
Methyl mercury, for example, is an organic compound that causes adverse health effects at lower doses. That mercury compound occurs naturally, but most is produced from inorganic mercury that enters the water supply from coal and industrial emissions. Bacteria combine mercury and carbon to form methyl mercury. As small fish eat much smaller organisms, that nasty form of mercury contaminates fish muscle. When a predator fish, like a shark, eats many small mercury-contaminated fish over the course of its life, the methyl mercury concentrates in its tissues. When we eat that fish, the mercury accumulates in our own tissues.
Most adults won’t suffer “mercury poisoning” from eating seafood, but pregnant women who eat swordfish, shark, tuna, and other fish with high mercury levels may be unintentionally harming their unborn children.
Other environmental pollution can show up in certain seafood. Dioxins, furans, and PCBs are chemicals that are suspected endocrine disrupters, which means that they can mimic or interfere with the effects of hormones. Dioxins are also potent carcinogens— in fact, one type, TCDD, has the dubious distinction of being the most potent animal carcinogen ever tested. PCBs are a class of industrial chemicals that are highly stable, heat-resistant, and nonflammable. PCBs and dioxins contaminate the food chain as a result of pollution. They spill into the water during industrial accidents or from old dumpsites, and they go up in smoke when, for example, a fire destroys an old building.
Fish and other wildlife and livestock ingest PCBs and dioxins as they eat. Freshwater fish can carry the highest levels of dioxins and PCBs. Marine fish that spend most of their lives in deep oceans are less likely to carry dioxins and PCBs than freshwater varieties. But some marine fish, like rockfish, striped bass, snapper, and redfish, may also be tainted with dioxin because they spend part of their lives breeding close to the shore. Lower-fat seafood carries less of the harmful chemicals, but don’t eat the tomalley (the soft, green substance found in the body cavity) of lobsters and crabs, which can carry a big dose.
Other relevant links:
• You’re better off with wild salmon. See: Is Farmed Salmon Safe?
• Common bacteria, toxins, viruses, and parasites in food. See: What’s Bugging You? Check Out this Table of Potential Food Safety Risks.
• Aquaculture is here to stay. See: Food Safety: Should We Avoid All Farm-Raised Seafood?