Wild cormorants have returned to Dianchi Lake in southwest China's Yunnan Province in a sign of the improved environment of what used to be one of the most polluted lakes in China.

Researchers have observed three wild cormorants in wetland around the lake, 30 years after the birds' disappearance from the area.

Their return is a result of improved biological diversity following a decade of government work to improve the lake's environment, said He Feng, one of the lake's environmental management staff.

"Lakeside farms and fishpond have been turned back into wetland and woodland," He said. The result is that the lakeside vegetation coverage has risen to about six times of what it was in 2006.

The quality of the lake's water has improved and despite being once rated as severely polluted, eutrophication has been alleviated and the fish population increased, according to He.

Cormorants have been trained by Chinese fishermen to catch fish since ancient times and are an icon of south China's river and lake scenery.Cormorant fishing is now mainly performed for tourists.