ALAS, poor Harald. Wired up to a satellite transmitter, he had much to teach science about the life of the great sturgeons of the Danube River and Black Sea.

His probable demise is a cautionary tale of the multiplying threats to the great sturgeons, sought since Roman times for the wealth they yield in meat and caviar.

Consider: A living creature from the age of the dinosaurs, a fish that can grow as long as a minibus, lives longer than most men, sniffs its way to its birthplace to spawn and can yield a fortune in caviar.

When in 2009 a team of Romanian and Norwegian researchers attached a satellite transmitter to Harald's 2.9 meter body, they hoped the data beamed back would suggest ways to halt the rapid drop in sturgeons' numbers. But now the Beluga sturgeon is missing, presumed a victim of poachers.

Sturgeon have thrived in the Danube for 200 million years, migrating from feeding grounds in the Black Sea to Germany 2,000 kilometers upstream. Archeologists have found wooden sturgeon traps in the ruins of Roman forts on the Danube, along with sturgeon bones dated to the 3rd century AD.

In the 1970s and '80s Romania built giant dams across the Iron Gates gorge, cutting off half the sturgeons' spawning grounds.

Fishermen, unrestrained after the collapse of order in eastern Europe in 1989, caught them in huge numbers before they could reproduce. Pollution has put them under further pressure, although water treatment plants have lessened the flow of filth.

Now there's a new threat: a European Union plan to deepen Danube shipping channels that environmentalists fear could eliminate the last shallows where the sturgeon deposit their eggs. This would doom the fish to extinction in its last European stronghold.

"Right now it's teetering on the edge of extinction," says Andreas Beckmann, director of the Danube-Carpathian program of the Worldwide Fund for Nature, or WWF. "That one project, depending on how it's done, could push it over the edge."

Under the plan, engineers would block partially several side channels of the Danube and divert water to the main fairway, enabling year-round shipping through what are now low-water bottlenecks. Concrete would reinforce the banks of some islands.

European and Romanian officials insist the project would not harm fish in the wild, free-flowing waters of the Lower Danube. But construction has been delayed for a year to allow more monitoring of the channels.

"If the data shows there is some influence, we will decide together whether to stop the project," says Serban Cucu, a senior Transport Ministry official in Bucharest.

Sturgeon, which can live a century or more in both salt and fresh water, are genetically wired to reproduce only where they themselves were born. Using four nostrils, they sniff out their birthplace, says researcher Radu Suciu.

After the Iron Gates went up, fish west of the two dams effectively were rendered infertile. The reproduction rate fell by half, according to the Danube Delta National Institute in Tulcea, at the mouth of the delta.

Even 40 years later, older fish congregate at the foot of the dam in spawning season.

This month, conservationists, governments and the UN Food and Agriculture Organization agreed to explore building a fish ladder for the sturgeon to crawl around the Iron Gates dams. But unlike salmon, sturgeon cannot jump and would have to use powerful underside muscles to climb nearly 40 meters through a chain of pools.

Also, experiments have begun to breed sturgeon in fish farms, safe from poachers who kill them for their roe, which is processed into expensive caviar.

International trade in sturgeon was banned in 2001, and in 2006 Romania outlawed sturgeon fishing, followed by Serbia, Ukraine, Moldova and lately Bulgaria.

But the threats have not disappeared. Harald, named for the king of Norway because that country sponsors sturgeon research, was 12 years old and weighed 80 kilograms when he was caught and taken to an experimental farm. His sperm was harvested to artificially fertilize eggs.

After a month he was tagged with a transmitter and released back into the Danube in May 2009, carrying the hopes of scientists.

"He was healthy and strong," says Suciu.

He swam downstream to the Danube Delta and into the Black Sea. Abhorring light, he stayed in murky depths of 10 to 50 meters.

Scientists pieced together movements from 11,000 messages transmitted over five days after the tag surfaced six months later.

Harald had foraged for herring, sprats, mackerel and other small fish for several weeks. Then in October he swam north. Suddenly, on November 2, he stopped moving. For three days he stayed 65 meters down on the bottom, immobile.

On the night of November 6, sometime after 2am, Harald rose swiftly to the surface and went in a straight line 11 kilometers to Ukraine's Crimean coast. He remained offshore for two days and on land for another two. The transmitter's final messages showed movement along a railway line.

Much of Harald's data was lost during transmission, but scientists surmised his fate: he had been snared, hauled up by boat, taken ashore by a hook or net, then hauled up in the dead of night and taken ashore; the rail trip was his trip to processing or market.

Endangered Chinese sturgeon

The Chinese sturgeon - also tracked with microchips - is one of the world's largest among 27 sturgeon species. It can grow as long as 5 meters and weigh around 450 kilograms.

It is the largest animal in the heavily polluted Yangtze River where man, bridges and machines have drastically reduced the population and placed it on the endangered list.

The sturgeon lived with the dinosaurs 140 million years ago and today is practically unchanged.

The Shanghai Yangtze Estuary Nature Reserve for the Chinese Sturgeon is a base aiming to saving the fish through breeding.

It contains several hundred sturgeons and every year it releases hundreds of young - with implanted micro chips - into the Yangtze River but like Harald in the Danube, most go "missing" on their way into the East China Sea.