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Mercury concentrations in dead red-crowned cranes in southeastern Hokkaido have been found to be 10 times higher on average and as much as 300 times the maximum generally found in birds, researchers said Monday.

It is the first time a high concentration of mercury has been found in the large “tancho” white-feathered cranes, which have been designated a special national natural treasure.

Hiroki Teraoka, a professor of veterinary toxicology at Rakuno Gakuen University who led the study, said one crane probably died of mercury poisoning because the level of the toxin in its system was 300 times greater than normal.

“Mercury could be a potential threat to the continued existence of the red-crowned crane species,” he said.

Teraoka’s group studied about 100 cranes that died of sickness or accidents around Kushiro swamp between 1988 and 2004.

Animals usually absorb mercury and other heavy metals through the food chain, but the origin of the “mercury poisoning” affecting the red-crowned cranes in Hokkaido remains unclear.

No mercury pollution has been detected recently in the soil and river system around the swamp, an area where the red-crowned cranes studied by Teraoka’s group were found dead. However, mercury deposits lie beneath northeastern Hokkaido.

If absorbed over a short time, mercury can weaken an animal’s motor functions and immune system, and cause reproductive disorders.

Because mercury can also affect vision, Teraoka said he intends to study the possible correlation between cranes carrying high mercury levels that flew into power lines or hit automobiles.

About 20 cranes are killed in such accidents every year.

While 1 ppm of mercury is commonly found in bird carcasses — including pigeons and crows — 17 to 34 ppm of mercury were found in the kidneys and livers of cranes dissected for the study.

In one crane found in 1992, however, 340 ppm of mercury was found — nearly the highest level to be found in land-based birds. The mercury levels in four others, found in 1994 and 1999, exceeded 100 ppm.

Some experts say human infants can exhibit developmental retardation if their mothers have 10 to 20 ppm of mercury in their bodies, but it remains unclear how much mercury is needed to impact the health of cranes.

“Mercury poisoning is a serious issue that can impact on the next generation of a species,” said Hiroyuki Masatomi, a professor emeritus of avian ecology at Senshu University’s Hokkaido College.

“Red-crown cranes are few in number and their reproductive ability is not very strong. They could face extinction in one breath if they cannot produce chicks,” he said.

Red-crowned cranes, mainly found in Japan, Russia and China, are designated as an endangered species by the International Union for the Conservation of Nature and Natural Resources.

About 1,000 of the cranes inhabit Hokkaido.

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