Nearly 70 percent of the fish resources in the Pearl River, the third longest river in China, are under threat as a result of excessive damming, overfishing and water pollution, according to a study.

Estimates suggest that more than 260 fish species will face extinction in the next five to 10 years because of existing and planned dam projects along the Pearl River and its tributaries. Such work has disrupted the migration routes taken by some fish species and changed the environments of many spawning places, warned Li Xinhui, a researcher with the Pearl River Fisheries Research Institute under the Chinese Academy of Fishery Sciences.

According to samples collected near the biggest spawning site along the Pearl River, both the number of individual fish and the number of fish species have declined significantly during the past three years, Li was quoted as saying by Guangzhou Daily at the weekend.

Anadromous fishes are forced to spawn before arriving at the best sites, usually upstream, because traditional grounds are blocked by hydro projects. As a result, their young have slimmer chances of surviving largely because their migration routes back to the ocean are not long enough to allow them to mature.

Li also noted that the numbers of many types of common freshwater fish have decreased as result of changes in the aquatic environment. The four major species of freshwater fish in China - black carp, grass carp, silver carp and crucian carp - have become more and more uncommon as their spawning rates have fallen, while the numbers of alien species have risen.

Rare and endemic fish, such as Dabry's sturgeon, a unique species that has been living in the Yangtze River for more than 140 million years, and the Hilsa herring, have disappeared in recent years from the Pearl River as a result of over-fishing and because of deteriorating water quality. Researchers who hope to bring them back to the river are now faced with a daunting task.

The Yangtze River, the longest river in China, is also no longer as abundant in fish or in animal species as it once was. The blame here too rests with the construction of a spate of dam projects and with a deterioration in water quality, threats that have been little mitigated in recent times.

During the past two decades, the extent of the upper reaches of the Yangtze River occupied by rare and endangered fish has been reduced from thousands of kilometers to just 200 km.

The reserve, which stretches from Sichuan province's Yibin city to Chongqing municipality, is the last place where researchers can find several endangered and endemic species such as the Chinese paddlefish, Dabry's sturgeon and the Chinese suckerfish.

Experts have warned that if greater efforts are not made to protect these endangered species, they could become extinct, just as happened to the Yangtze River dolphin, a 20 million-year-old species nicknamed "the goddess of the Yangtze".

In order to maintain the diversity of aquatic species in China, Li suggested that dam projects make accommodations that allow fish to migrate and spawn. One means toward that end may lie in the construction of separate channels that fish can take upstream to their spawning sites.