Off it swims with two others, all trailing satellite receivers wired to their dorsal fins _ launching a pilot study expected to yield valuable information about a species of sturgeon hearty enough to have survived from prehistoric times but now on the brink of extinction due to the insatiable appetite of the wealthy for caviar.

If the three beluga sturgeon escape poachers' nets and data are successfully retrieved, the Pew Institute for Ocean Science of the University of Miami will tag more of the fish for the first ever comprehensive study of the Caspian beluga population.

A worldwide study released by the Pew Institute last year showed that most major sturgeon fisheries are now catching 85 percent fewer fish than at their peak in the late 1970s. It called for a total ban on fishing for most endangered species and reducing fishing pressure on others.

Beluga, whose roe is reputedly the world's most expensive delicacy, is the most threatened species of sturgeon. And the population in the Caspian - which provides 90 percent of beluga caviar - "got hammered very fast," said Phaedra Doukakis, a Pew Institute research scientist.

"The peak (in fishing) and decline was very rapid," she says.

There is no reliable estimate of how many Caspian beluga remain. According to the Pew Institute, in 2001 they numbered around 375,000, of which 55,000 were adults.

In the same year, scientists from Russia and Iran - countries that benefit most from the beluga caviar trade - came up with a figure of 9.3 million and the following year with more than 11 million, figures that Doukakis dismissed as "a fantasy." The data were widely criticized by scientists and no such estimates have been submitted since.

Akhat Nimatov, director of a state-run sturgeon hatchery in the Kazakh city of Atyrau that works with the Pew Institute, says the Caspian beluga population has declined by 70 percent over the past 15 years, with fishermen catching on average one egg-bearing female to seven males.

Beluga, the largest fish in the Caspian, can live over 100 years and grow to over 6 meters (19.7 feet). But few now survive longer than 20 years.

A female beluga is ready to produce eggs by the age of 15-18, becoming the fisherman's most-sought catch. Male sturgeons are either thrown back into the sea or sold for their flesh.

The U.N. Convention on International Trade in Endangered Species of Wild Fauna and Flora - which sets sturgeon fishing quotas each year - imposed a ban on catching sturgeon and exporting caviar from the Caspian this year after littoral states failed to submit a convincing plan to protect the fish. The Persian species concentrated in Iranian waters was exempted from the ban because it is not endangered.

Doukakis said the ban would not help because CITES has no tools to implement the rules on the ground and because "there seem to be enough outlets for illegal trade or a big enough domestic market."

The caviar trade is so lucrative that it makes poaching hard to resist and control. One female beluga produces up to 17 percent of her total weight in caviar. A kilogram (2.2 pounds) of beluga caviar costs an average US$6,000 (euro4,700) in Europe and North America.

The damage from legal fishing is also significant. The Caspian sturgeon was under heavy fishing pressure during the Soviet era, with its command economy and ever-growing fishing targets. But the 1991 collapse didn't make things better as the emergence of three new states along the Caspian shore - Kazakhstan, Azerbaijan and Turkmenistan - has meant less regulation.

Another serious concern for sturgeon stocks is environmental damage from the northern Caspian oil boom.

The Soviet collapse gave Western companies access to the sea's vast oil and gas reserves, the third-largest untapped reserves in the world. That increases the risk of ecological disasters in this part of the Caspian, where sturgeon migrate in the summer to fatten in the shallow waters and spawn in Kazakhstan's and Russia's Ural and Volga Rivers.

The northern Caspian is the site of the largest oil project in the world: an international consortium, Agip KCO, is preparing to start commercial oil production at the giant Kashagan field, in Kazakh territorial waters.

Agip KCO is the main sponsor of the Pew Institute's Caspian sturgeon research with a grant of "a few hundred thousand dollars," said Doukakis.

The satellite tags, which have been in use for about 10 years for tracking other fish species, will be gathering data on the beluga's migration habits and information on the depth and light level where it travels.

The microphone-shaped, 18-centimeter (7-inch) tags - which cost US$3,400 (euro2,600) each - are attached at the Atyrau hatchery, where beluga that have been caught in the Caspian are brought to lay eggs and then freed again.

The tags are programmed to drop off the fish, one after another, at three-month intervals. The tag will then float to the surface and send information to a satellite.

The data could help define the beluga's preferred areas in the sea, which could then be turned into special protected zones, says Erickson. It will also define beluga habitat in relation to oil drilling.

"I'm thrilled," says Doukakis after the three beluga - which had been on a barge towed by the Neputune - were freed 10 kilometers (6 miles) off the Kazakh Caspian coast. "Now we have to sit and wait."

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