Paroma Basu, National Geographic News 12 Mar 08;
Invasive fish carrying industrial chemicals likely triggered the recent die-off of 110 critically endangered reptiles known as gharials in a central Indian river sanctuary, scientists announced last week.
Since December officials have found the crocodile-like animals washed ashore dead along the banks of the largely pristine Chambal River, one of the few unpolluted rivers in India.
But the bulk of reptile fatalities occurred along a 22-mile (35-kilometer) stretch near the Chambal's confluence with the Yamuna, considered to be of the dirtiest rivers in the world.
Researchers therefore think an unidentified substance might be seeping into the Chambal and affecting the gharials' food supply.
Autopsies of the animals revealed evidence that they perished from gout, a painful metabolic disease, after ingesting polluted fish.
"Gharials that are already infected with the toxin will continue to die," said Ravi Singh, secretary general and CEO of the India branch of WWF.
The international conservation group is coordinating efforts by the government, other animal-welfare groups, veterinarians, and state departments to conduct an investigation and contain the crisis.
"If this mass die-off has truly stemmed from ongoing pressures on the habitat, people should know that there's no short-term fix."
Kidney Lesions
Conservation groups say that no more than 1,400 gharials are left in the wild, living in pockets of India and Nepal.
More than 300 of these individuals live in the National Chambal Sanctuary along the Chambal River, which contains the largest of the world's three breeding populations.
When the reptiles began to wash up dead in December, post-mortem examinations revealed chemical-laced lesions on the animals' kidneys.
These lesions probably caused the onset of debilitating gout, said Paolo Martelli, one of four veterinarians with the World Conservation Union sent in to examine the dead reptiles.
Gout is painful inflammation caused by a buildup of microscopic crystals of uric acid in the joints. Kidneys normally remove most uric acid from the blood.
The doctors also found surprising amounts of fat in the animals' tissues, Martelli added, which could be explained by the growing abundance of a cichlid fish from Africa known as tilapia in Indian rivers.
The species was introduced in the region a few years ago to boost Indian aquaculture. It has since grown so plentiful that gharials now feed on it almost exclusively.
As tilapia move from polluted rivers into the Chambal, they ingest and store chemicals in their tissues. Gharials eating the abundant fish therefore accumulate even larger amounts of potentially harmful substances in their body fat.
"Cold winter temperatures coupled with excess food could have also made the [cold-blooded] gharials more susceptible to the toxin," said Martelli, who is based in Hong Kong.
"As the temperatures warm up, the animals will improve. But next winter may again be a delicate time for the gharials."
Two Indian laboratories are still trying to determine exactly what kind of chemical is to blame.
Experts think that the agent is either an industrial chemical being released into the Yamuna by a new facility or one that was used by an older plant that shut down and illegally dumped all its waste in the river.
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