From the Food Standards Agency -
Mercury is found in all fish due to natural accumulation from the environment, so it can’t be eliminated entirely. Mercury is naturally higher in predatory fish (such as tuna) because the mercury builds up over time from the other fish that they eat. The amount of mercury is also influenced by the size and age of the fish. Therefore, an older, larger fish is likely to have higher levels of mercury than younger, smaller fish. Larger predatory fish (like swordfish or shark) have the highest mercury levels.
To accommodate this, in Great Britain we have a maximum level for mercury in tuna and other specified species of 1.0 mg/kg. For other fish (like cod), a lower maximum level for mercury applies. It’s important to note that the researchers have compared levels of mercury in tuna to the lower level for other fish species, rather than the maximum level for tuna. The report found that all 148 cans of tuna tested positive for mercury contamination, but in fact, the study shows a relatively small number of products were above the legal maximum level. Out of 30 products from the UK market, just one product was found which may have been over the legal level.
Guidance for vulnerable groups
The NHS guidance specifies recommended weekly portions of tinned or fresh tuna for those trying for a baby and those who are pregnant. These groups are advised to have no more than four cans of tuna a week or no more than two tuna steaks a week. This recommendation is based on a medium-sized can of tuna with a drained weight of around 140 g per can and a 140 g cooked steak. The 2018 Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) review of the most toxic form of mercury in the diets of very young children did not identify any concerns.