Scientists at the University of Liege, in Belgium, examined the effects of methylmercury, the main form of mercury found in the blood of marine mammals and people with lots of fish in their diet.

They applied increasing doses of the substance to white blood cells, which provide the body’s defence against disease and infection.

The levels used matched those found in an analysis of blood taken from harbour seals caught in the North Sea.

Tests found that methylmercury harms T-lymphocytes, which are key cells in a seal’s immune system. Similar results were also found for human lymphocytes.

Krishna Das, the lead author of the study, said: “Mercury is known to bioaccumulate and to magnify in marine mammals, which is a cause of great concern in terms of their general health.

“In particular, the immune system is known to be susceptible to long-term mercury exposure.”

However, the researchers acknowledged that their approach could not be expected to mimic exactly what happened when mercury was ingested by seals and human beings through eating fish.

They said: “Although the in vitro approach utilised in this investigation represents an extreme reductionism relative to the complex situation in the intact organism, it provides an insight in the specific effects of mercury pollution.”

They added that immune system damage may already have taken a toll on the harbour seal populations.

Outbreaks of phocine distemper virus – an illness that affects mainly seals – killed thousands of harbour seals in 1998 and 2002. The outbreaks were linked to the effects of pollution on the seals’ immune systems.

Kate Martin

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