In 2003 the Food Standards Agency looked at the mercury in fish. -
"The guidance was issued by the Food Standards Agency after a survey last year into levels of mercury in commonly eaten fish was reviewed by the independent Committee on Toxicity, which advises the Government. Pregnant women and children are already urged not to eat any shark, sword- fish or marlin because of they contain levels of mercury, which can harm the nervous system of babies.

The survey found the levels of mercury in tinned and fresh tuna were less than a thousandth of a gram. Shark, swordfish and marlin had about six times the levels of mercury of tinned tuna and about three times the levels of mercury in fresh tuna."

Probably best to avoid it or just use it as an occasional treat.