River Rehabilitation for Conservation of Fish Biodiversity in Monsoonal Asia

Freshwater biodiversity is under threat worldwide, but the intensity of threat in the Oriental biogeographic region of tropical Asia is exceptional. Asia is the most densely populated region on Earth. Many rivers in that region are grossly polluted, and significant portions of their drainage basins and floodplains have been deforested or otherwise degraded. Flow regulation has been practiced for centuries, and thousands of dams have been constructed, with the result that most of the rivers are now dammed, often at several points along their course. Irrigation, hydropower, and flood security are among the perceived benefits. Recent water engineering projects in Asia have been exceptionally aggressive; they include the world’s largest and tallest dams in China and a water transfer scheme intended to link India’s major rivers. Some of these projects, i.e., those on the Mekong, have international ramifications that have yet to be fully played out. Overexploitation has exacerbated the effects of habitat alterations on riverine biodiversity, particularly that of fishes. Some fishery stocks have collapsed, and many fish and other vertebrate species are threatened with extinction. The pressure from growing impoverished human populations, increasingly concentrated in cities, has forced governments to focus on economic development rather than environmental protection and conservation. Although legislation has been introduced to control water pollution, which is a danger to human health. it is not explicitly intended to protect biodiversity. Where legislation has been enforced, it can be effective against point-source polluters, but it has not significantly reduced the huge quantities of organic pollution from agricultural and domestic sources that contaminate rivers such as the Ganges and Yangtze. River scientists in Asia appear to have had little influence on policy makers or the implementation of water development projects. Human demands from agriculture and industry dominate water allocation policies, and in-stream flow needs for ecosystems have yet to be widely addressed. Restoration of Asian rivers to their original state is impractical given the constraints prevailing in the region, but some degree of rehabilitation will be possible if relevant legislation and scientific information are promptly applied. Opportunities do exist: enforcement of environmental legislation in China has been strengthened, leading to the suspension of major dam projects. The 2003 introduction of an annual fishing moratorium along the Yangtze River, as well as breeding and restocking programs for endangered fishes in the Yangtze and Mekong, offer the chance to leverage other initiatives that enhance river health and preserve biodiversity, particularly that of fish species. Preliminary data indicate that degraded rivers still retain some biodiversity that can be the focus of rehabilitation efforts. To strengthen these efforts, it is important to identify which ecological features enhance biodiversity and which ones make rivers more vulnerable to human impacts.