Collecting kaimoana from the sea is a much-loved tradition for many New Zealanders. Unfortunately shellfish are a high risk food because they can happily live in contaminated water and pick up and store whatever pathogens (bacteria, viruses, fungi and other micro-organisms that cause illness in people), biotoxins or pollution that are present.
Shellfish are often eaten raw or lightly cooked which does not kill pathogens. No amount of cooking will destroy biotoxins or other chemical contaminants they may have accumulated. The most effective way to ensure you don’t get sick from eating shellfish is to collect them from areas where the seawater is clean and not contaminated in any way (see below). Proper handling, storage and cooking can further lower the risk of illness from pathogens.
This fact sheet outlines some risks with eating shellfish that has been gathered or harvested from the wild, (i.e. not purchased through retail outlets), and how to reduce your exposure to these risks.
Your risk of becoming ill from shellfish depends on a number factors including the type of shellfish you eat, any possible contamination, and your own immunity (amongst other things). Effects will be worse for people with low immunity i.e. young children, the frail elderly, pregnant women (and their unborn baby) or anyone who has a chronic illness. Because illness from contaminated shellfish can be serious, if you or any member of your family becomes ill after eating shellfish, see your doctor immediately.
Type of shellfish: Bivalve shellfish (with two shells) such as mussels, tuatua, toheroa, oysters, cockles, pipi and scallops are filter feeders, and pose a greater risk than other seafood. As they filter food particles from the seawater they also pick up and store biotoxins, micro-organisms, contaminants and pollution. In contrast, grazing shellfish eg, paua, pupu (catseyes) pose a much lower risk because they are not filter feeders. Other seafood where the guts (hua) are discarded before being eaten is also low risk. These include sea eggs (kina), crayfish, crabs and fish.
Type of contamination: Bivalve shellfish can accumulate
- biotoxins from phytoplankton (microscopic algae that are part of the staple diet of bivalve shellfish)
- >pathogens from water contaminated by animal or human sewage or farm run-off, or
- chemical contaminants such as heavy metals, fuel, paints and solvents.
Biotoxins: Toxic Shellfish Poisoning is caused by biotoxins that are generated by phytoplankton. You cannot tell if shellfish are toxic by looking at them. Three forms of biotoxin have been found in New Zealand shellfish. If you suffer any of these symptoms after eating shellfish, seek medical help immediately.
- Paralytic shellfish poisoning (PSP) is caused by a group of chemicals called the saxitoxins and gonyautoxins. Symptoms can occur within 12 hours of consumption. These include numbness and a tingling (prickly feeling) around the mouth, face, and extremities, followed by difficulty swallowing or breathing, headache, dizziness, and double vision. Overseas, people have died from eating shellfish contaminated with PSP
- Amnesic shellfish poisoning (ASP) is caused by Domoic Acid in shellfish. Symptoms are mainly gastrointestinal (vomiting, diarrhoea etc), especially at low levels, however about a quarter of cases experience neurological problems including memory loss that may be significant and permanent. Gastro-intestinal symptoms first appear within 24 hours and neurological difficulties within 48 hours
- Diarrhetic shellfish poisoning (DSP) is caused by Okadaic Acid and related compounds. Symptoms are diarrhoea, nausea, vomiting and abdominal pain. Acute symptoms occur within 12 hours and are usually of short duration.
For more information see Marine Biotoxin Alerts
Pathogens: Shellfish contaminated with sewage may contain pathogens such as norovirus, hepatitis A, Shigella, Vibrio and Salmonella. These can cause dysentery, gastro-enteritis or other illnesses within a few hours or days. Symptoms of hepatitis A may be delayed for a month or longer, with fever, malaise, anorexia, vomiting, nausea and jaundice. Complications from these pathogens may result in longer-term illness such as damage to your blood, liver and immune system.
Chemicals: Dangerous levels of contamination from heavy metals are very rare in New Zealand shellfish although the areas most likely to be contaminated are harbours near slipways and marinas, and near discharge outlets for sewage.
Shellfish from popular collecting sites are tested regularly for biotoxins. When the shellfish are contaminated with biotoxins, District Health Boards put up warning signs and may alert local newspapers, television, radio stations and local networks including Iwi and fishing/boating clubs. See the MPI Foodsmart website for Marine Biotoxin Alerts.
There are steps you can take to lower your risk of illness from contamination with sewage or chemical pollutants:
- avoid collecting shellfish from areas where pipes or culverts run down to the beach
- avoid collecting from areas where sewage or storm water is discharged, or areas with houses nearby
- after heavy rain, don’t collect near rivers or estuaries until the water has run clear for several days. Storms may flush sewage overflow or farm runoff downstream
- don’t collect where there are farm animals grazing nearby
- don’t collect from areas showing signs of industrial pollution
- avoid collecting near wharves or marinas where boats may have discharged sewage or chemicals such as anti-fouling paint or diesel.
Storing shellfish carefully can help reduce your risk of illness from pathogens.
- keep shellfish alive and cool, to keep them fresh
- use shellfish within two days of harvest
- don’t eat shellfish that have died during storage. Living mussels or scallops may respond by shutting their shells tightly when you tap them. Dead shellfish won’t respond and should be discarded. In contrast, live oysters will keep their shells closed
- do not cook or eat shellfish with broken shells
- prepare your shellfish carefully and avoid cross-contamination from pathogens – use clean hands, chopping boards, knives etc and keep them separate from other foods and utensils
- shellfish are often eaten raw or lightly steamed which is not effective at eliminating pathogens
- no amount of cooking will destroy biotoxins or other chemical contaminants they may have accumulated. Therefore, collect shellfish from areas where the seawater is clean and not contaminated. Thoroughly reheat leftover seafood dishes such as chowder to a minimum core temperature of 80oC for at least three minutes
- keep shellfish in the shade during harvest to keep them moist and cool
- in the fridge, cover them with a clean wet towel (not in an airtight bag or container)
- refrigerate shellfish as soon as possible after collection (within four hours), and store on ice if transporting in a chilly-bin (be careful as freezing will kill them)
- store them in the lower part of your fridge, below cooked food
- make sure your fridge is operating within the recommended 2-4oC temperature range
- if you are preparing shellfish to freeze, shuck (shell) them and freeze in small amounts in their cooking water or their natural juices. They will keep for up to a year but their quality deteriorates after three months
- thaw frozen shellfish in the fridge for 24 hours before you cook them.