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Giant fish of the Mekong in battle for survival


The Mekong, twelfth longest river in the world, is the habitat of some of the world's

largest freshwater fish: the giant catfish, giant carp, and giant stingray.

This giant carp, caught in the Tonle Sap in December 2002, measured 1.72 meters long and weighed 102 kilograms.

They are not popular eating fish, but their numbers have declined due to accidental

net capture.

Cambodia remains one of the last strongholds of the giant fish. This is in part due

to the unique annual cycle of flooding and draining between the Tonle Sap lake and

the Mekong, which is responsible for Cambodia's huge fish resources. There are at

least 1200 species of fish in the Mekong Basin and only 10 to 20 percent of them

form the basis of the fisheries trade.

Cambodians are not surprised by stories of giant fish in the Mekong; for centuries

the Naga, a serpent that dwells in the river, has been an integral part of their

folklore.

Environmental threats to the Mekong have focused new attention on aquatic biodiversity.

"Conservation in Cambodia is almost entirely terrestrial," says Zeb Hogan,

an American fish conservationist who is working with the National Geographic Society

to learn more about the giant fish of the Mekong. "We need to see what is in

the rivers in order to protect them."

The giant catfish (Pangasianodon gigas) is still regularly caught two kilometers

north of the Japanese Friendship Bridge in Phnom Penh, where the main river channel

is narrow and a prime spot for nets.

Fish in their thousands, but tiddlers compared with the Mekong's giants.

The giant catfish can weigh as much as 300 kilograms and may be up to three meters

long. The main season these leviathans are caught is between October and December.

The giant catfish is not considered good eating but has acquired a god-like status

(Khmers call it trey reach, the royal fish). It is believed the fish uses either

lunar or tidal forces to navigate to its spawning grounds, passing through Phnom

Penh on its way.

Between October and March, when the migratory fish species leave the Tonle Sap and

swim into the Mekong and upstream, bagnet fisheries are established in the Tonle

Sap. They consist of two to six nets spread across the river. Each net can be 100

to 120 meters long with a 25-meter-wide entrance.

The conservation of the giant species is seen as a microcosm of aquatic conservation

in Cambodia: if such large animals cannot be protected then there is little hope

for smaller creatures. Nao Thuok, Deputy Director of Fisheries at the Ministry of

Agriculture, Forestry and Fisheries, says, "As the largest fish in the Mekong,

conservation of the catfish is important as an example for all fish."

"The catfish and giant carp (Catlocarpio siamensis) symbolize the ecological

integrity of the Mekong River," says Hogan. "The catfish migrates out of

the Tonle Sap and into the mainstream Mekong in the rainy season; this is common

with most Cambodian Mekong fish and as such the protection of this migratory pathway

benefits a huge array of fish.

Zeb Hogan tags a Mekong fish.

The catfish is no longer caught in other countries through which the Mekong passes.

Hogan says between five and ten are caught annually in Cambodia. He estimates that

the population in the Mekong basin has decreased over the last 50 years by 90 percent.

"The giant catfish is one of the most vulnerable species in the Mekong River

basin," says Hogan.

The giant catfish was registered as critically endangered by the World Conservation

Union this year in 2003.

To raise awareness about the fish's plight a local wildlife NGO, Save Cambodia's

Wildlife, Sang Kros Satprey, recently released a book entitled Samnang and the Giant

Catfish.

Little hard research has been done on some of the other larger fish. "The lack

of research is what is hampering our conservation efforts," Thuok says.

Hogan hopes this will change: "For the 2004 project we hope to include the freshwater

stingray and other giant fish species such as the freshwater sawfish, freshwater

sharks and the infamous dog-eating catfish." Since 2000 Hogan has been running

a buy-and-release research project for the giant catfish that includes a tagging

program that is providing insights into the migratory habits of the fish.

This giant stingray was caught in the Mekong in Prey Veng province in December 2002. Measuring 4.2 metres by 2 metres, it was too heavy to weigh, but was tagged and released. "It's the largest fish I've ever seen in the Mekong," says Zeb Hogan of the National Geographic Society.

The giant stingray (Himantura chaopraya) was only discovered in 1987. Considering

its four-meter-by-two-meter size it seems odd that it was not discovered earlier.

Its preference for deep parts of the river is thought to be the reason it has existed

for so long without scientific discovery. Freshwater stingrays inhabit other big

rivers, including the Orinoco and the Amazon.

The depth of the river is thought to be the key to the survival of large fish in

conjunction with a diverse and complex basin of tributaries. "The survival of

these species is the richness of the food in association with the deep pools,"

Thuok says.

Samnang and the Giant Catfish states that "the deep pools of the Mekong, some

of which may be over 60 meters in depth, are important dry-season habitat for fish.

Fishermen report that at least 53 fish species live in deep pools."

In Kratie and Steung Treng provinces, in the Mekong 58 deep pools have been identified

in a MRC report. Thouk says the total has now increased to 97 deep pools, the deepest

being 80 meters. They serve as a dry season refuge for fish, especially the larger

ones. But even these pools are vulnerable to human impact.

There are projects under way to blast the river's reefs for commercial navigation

as agreed in the Commercial Navigation Treaty between Laos, China, Burma and Thailand.

As the Phnom Penh Post reported last year, neither Vietnam, nor Cambodia was consulted

about the agreement. Such blasting could affect the flow of the river and thus these

crucial deep pools. Such a change in flow could disrupt the migratory patterns of

the fish because they are adapted to seasonality.

Hydro-electric dams would also have a large impact on the aquatic biodiversity. Prime

Minister Hun Sen in a speech to the Second International Symposium on the Management

of Large Rivers for Fisheries, in February 2003, speculated on the possible impacts

the commercial navigation plan and damming would have on Cambodia: "The Tonle

Sap could dry up; ending the famous river fishing industry and causing widespread

flooding, and eventually the home of endangered fish would be destroyed."

The largest scaleless freshwater fish in the world: a giant catfish caught, tagged and released on October 21, 2002, in the Tonle Sap.

Local examples of the ecological impact of dams indicate some ominous trends. A Mekong

River Commission report on the impact of the Yali dam on the Sesan River in Ratanakkiri,

a tributary of the Mekong, says that due to increased silt the deep pools are shrinking.

One specific case study shows that in Voeun Say district within the last three years

one pool that was between seven and eight meters deep has shrunk to less than one

meter.

"There is no emphasis on the aquatic conservation of the Mekong," Hogan

says.

The river has been seen more as a commodity producer than a habitat by the countries

involved in its development and management. Its source was only discovered in 1995

in the snowy plateaus of Tibet, one year after the first bridge was completed.

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