Mercury pollution hits ocean fish

Mercury spilled into the air from polluting industrial centres travels thousands of miles before raining into the oceans to poison fish, according to scientists.

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Toxic metal in edible fish is likely to rise in the coming decades, scientists predict

Mercury spilled into the air from polluting industrial centres travels thousands of miles before raining into the oceans to poison fish, according to scientists.

The findings suggest that levels of the toxic metal in edible fish is likely to rise in the coming decades.

Even small amounts of mercury can damage nerves and be harmful to the heart and digestive and immune systems.

Mercury exposure in the womb can impair a baby's brain development, affecting thinking, memory and motor skills.

US scientists suspect that mercury in the tissues of deep-living North Pacific fish caught near Hawaii originated in Asian industrial centres.

After being blown downwind for thousands of miles, the mercury was deposited into the ocean in rainfall.

Bacteria then processed it into methylmercury, the organic form of the metal that accumulates in the flesh of fish.

"The implications are that if we're going to effectively reduce the mercury concentrations in open-ocean fish, we're going to have to reduce global emissions of mercury, including emissions from places like China and India," said lead researcher Professor Joel Blum, from the University of Michigan.

"Cleaning up our own shorelines is not going to be enough. This is a global atmospheric problem."

Coal-burning power plants, common in rapidly industrialising Asian countries, are a major source of mercury pollution.

Most human exposure to methylmercury is through the consumption of large predatory marine fish such as swordfish and tuna.

The toxin builds up in the tissues of creatures at the top of the food chain which eat smaller species whose bodies contain mercury.

Bacteria in the sea convert inorganic mercury into methylmercury. It used to be thought this had to occur near the surface, but Prof Blum's team found evidence of the process continuing to a depth of about 2,000 feet.

Up to 80% of the methylmercury found in the fish studied originated at these levels - probably as a result of oxygen-shunning microbes attaching to sinking particles of dead plant and animal matter containing inorganic mercury.

The findings, published in the journal Nature Geoscience, are important because mercury levels at intermediate ocean depths are predicted to rise in the coming decades, say the scientists. One estimate forecasts a doubling by the middle of the century.

At the same time, oxygen-depleted ocean regions which occur at depths greater than 1,300 feet are expanding, partly driven by climate change.

"The implication is that predictions for increased mercury in deeper water will result in higher levels in fish," said Prof Blum.

The scientists analysed tissue samples from nine species of marine fish feeding at different depths near Hawaii.

Atomic measurements of mercury indicated where the metal was being converted in the open ocean.

The atomic composition of mercury found in the fish tissues was a "nearly perfect match" with the chemical signature of atmospheric mercury known to travel long distances, Prof Blum added.

"These results strongly support the hypothesis that long-range transport of mercury deposited to the ocean surface is ultimately what's ending up in these fish," he said.