Contamination caused by rising mercury levels in the sea has been documented for what is claimed to be the first time by US scientists.

The study looked at the impact of mercury emissions, that it claims are mostly from Asia, and how they're entering the food chain through tuna and other seafood.

This study documents for the first time the formation of methylmercury in the North Pacific Ocean.

It shows that methylmercury is produced in mid-depth ocean waters by processes linked to the 'ocean rain'.

Algae, which is produced in sunlit waters near the surface, die quickly and 'rain' downward to greater water depths.

At depth, the settling algae is decomposed by bacteria and the interaction of this decomposition process in the presence of mercury results in the formation of methylmercury.

Many steps up the food chain later, predators like tuna receive methylmercury from the fish they consume.

It also found, through water sampling, that mercury levels in 2006 were approximately 30% than those measured in the mid-1990s.

Scientists also believe much of the mercury entering the North Pacific comes from the atmosphere and predict an additional 50% increase in mercury in the Pacific by 2050 if emission rates continue as projected.

Ken Salazar, US secretary of the interior, said: "This unprecedented study is critically important to the health and safety of the American people and our wildlife because it helps us understand the relationship between atmospheric emissions of mercury and concentrations of mercury in marine fish.

"We have always known mercury can pose a risk, now we need to reduce the mercury emissions so we can reduce the ocean mercury levels."

Scientists sampled Pacific Ocean water from 16 different sites between Honolulu, Hawaii and Kodiak, Alaska.

In addition, the scientists constructed a computer simulation that links atmospheric emissions, transport and deposition of mercury, and an ocean circulation model.

In the United States, about 40 percent of all human exposure to mercury is from tuna harvested in the Pacific Ocean, according to Elsie Sunderland, a coauthor of the study.

Luke Walsh