This interim advice is being issued as a precautionary step following a Food Standards Agency survey that revealed relatively high levels of mercury in these species of fish. Large predatory fish such as shark, swordfish and marlin can contain relatively high levels of mercury in the form of methylmercury, which can harm the nervous system of an unborn child if the fish is eaten regularly by its mother. Infants and children may also be at greater risk from mercury poisoning because they eat more food relative to their body size in comparison with adults.
Occasional consumption of shark, swordfish or marlin as part of a balanced diet by any other adults is unlikely to result in harmful effects. However, on a precautionary basis, they are advised against eating more than one portion each week of either shark or swordfish or marlin.
The Food Standards Agency surveyed 336 fresh, frozen and processed sea fish and shellfish for mercury content, including trout, salmon, tuna, halibut, hoki, seabass, lobster, mussels and prawns. Levels of mercury in fish other than shark, swordfish and marlin did not give cause for concern. Previous UK surveys have not found mercury at levels that cause concern in the UK's most frequently consumed fish - cod, haddock and plaice.
1506 tonnes of shark and swordfish were consumed in the UK in 2001 compared with 244,366 tonnes of cod and haddock.
The Food Standards Agency's general advice on fish consumption is to eat two portions of fish a week, one of which should be oily, as part of a balanced and varied diet. This advice is based on the recommendation of the Committee on Medical Aspects of Food Policy (COMA) which found that this level of fish consumption resulted in a significant reduction in the risk of heart attacks. On average, UK consumers eat only three quarters of a portion of white fish and one quarter of a portion of oily fish a week.
The independent expert Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) will consider the survey results and any implications for consumers at its June meeting. When the COT has completed its considerations, any further advice will be issued if necessary.
This survey complements earlier surveys of the more commonly consumed fish. The results of the new survey showed that levels in shark, swordfish and marlin are relatively high (a mean of around 1.5 mg/kg). These results have been combined with data on average portion size for these fish for different age groups, taking account of exposure to mercury from the rest of the diet. The resulting dietary exposure for adult consumers of fish is close to the safety guideline (PTWI*) for methylmercury - the predominant form of mercury in fish. Children would exceed the safety guideline.
The World Health Organization considered that the PTWI for methylmercury is applicable to the general population, but pregnant women and infants may be at greater risk owing to concerns regarding the effects of methylmercury on the developing nervous system.
Methylmercury is known to be neurotoxic to humans as well as to animals. Effects may include paraesthesia (a sensation of pricking, tingling or creeping on the skin), malaise and blurred vision. Concern for the developing fetus and infant relates to possible neurobehavioural effects such as deficits in motor skills, attention, language, visual-spatial skills and memory.