Overfishing is Emptying World's Rivers, Lakes, Experts Warn

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The authors say overall harvest from the world's lakes and rivers has quadrupled since 1950. The current catch is estimated at around 8.5 tons (8.7 million metric tons) annually. That figure doesn't include fish taken by anglers, which often go unrecorded.

Two-thirds of the total catch is taken in Asia, with China alone home to some 12 million fishermen.

Average yearly fish consumption in the Mekong Basin in Southeast Asia is estimated at 123 pounds (56 kilograms) per person. Items on the menu include some of world's largest and rarest river fish, including the Mekong giant catfish, freshwater whipray, and giant barb.

Zeb Hogan, a fish biologist at the University of Wisconsin at Madison and a National Geographic Society emerging explorer, has been collecting data on these fish in Cambodia and Thailand.

Hogan says catches of the legendary Mekong giant catfish, which can grow to 660 pounds (300 kilograms) and 9 feet (2.7 meters) in length, have fallen drastically in recent years—from 60 in 1995 to just 4 in 2005.

"There may be a season soon when no fish are caught," he said.

The researcher says there is currently no comprehensive conservation strategy to save these fish from extinction.

He says priorities should include sustainable catch limits for rare Mekong species and the creation of freshwater protected areas.

"Environmental education is also very important," Hogan added. "Few people know about the endangered animals of the Mekong or how important fish are to the livelihoods of people living within the basin.

"Had a coordinated effort been undertaken a decade ago, the situation would not be so dire."

Diminishing Returns

The report says big fish are particularly vulnerable to being captured in nets, and the global trend in overfished waters is towards fish of ever diminishing size.

"Large adult fish are the broodstock that sustains the population," said David Allan, lead author of the report and a conservation biology professor at the University of Michigan in Ann Arbor.

"[Fertility] increases with body size, so large females are especially important reproductively."

And as populations of bigger fish dwindle, smaller species take their place. The Queme River in Benin, West Africa, has seen large predatory fish like the Nile perch replaced by small species of cichlids and catfish.

Allan says fishermen lower their targets accordingly to fill their nets, using smaller mesh sizes to catch the fish that are available.

Depleted fish stocks can have serious repercussions on freshwater ecosystems. The researchers point to the example of waters in western Canada and Alaska where the rotting bodies of Pacific salmon, which die after breeding, provide vital nutrients.

"As salmon populations have decreased because of overfishing and other causes, declines have also occurred in lake productivity and juvenile salmon recruitment," the authors state.

They also warn that overfishing has the potential for severe impacts on human health, especially in developing nations.

For example, increased incidence of schistosomiasis in Africa has been linked to declines of fish species that eat the snails carrying the disease-causing parasites.

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