Vetren (Bulgaria) (AFP) - Europe's last wild sturgeons got a rare boost this summer when the conservationist group WWF Bulgaria released more than 50,000 babies of these prehistoric fish into the lower Danube, marking the end of a three-year project co-funded by the European Union.

These so-called giant "living fossils" date back to the time of the dinosaurs but are now teetering on the brink of extinction.

"The sturgeons are a very ancient species, which have survived to our time from the era of the dinosaurs almost unchanged... But they are now critically endangered," WWF Bulgaria country manager Vesselina Kavrakova told AFP.

Six varieties of sturgeons once lived in the Danube, the expert said. Now only four of these species are left.

"The Beluga, Russian and Stellate sturgeons are listed in international classifications as critically endangered and the smallest species, the Sterlets -- the ones we bred -- are listed as vulnerable but their condition is estimated as very bad," she added.

The lower part of the river in Bulgaria and neighbouring Romania is home to the EU's last still viable populations of sturgeons, which live mostly in the Black sea but swim upriver to spawn.

They can grow to up to six meters in length and live for 100 years.

Extensive poaching for the sturgeons' tasty meat and expensive caviar has led to their dwindling stocks, though local fisherman Rumen Ivanov insisted he had not heard of anybody catching a sturgeon "in ages".

To counter the trend, experts have appealed to the governments of Bulgaria and Romania to extend their sturgeon fishing bans due to expire next year.

- Dangerous journey -

The sun has barely risen in the quiet village of Vetren but a dozen fish experts and fishermen are already feverishly preparing the release of the last batch of 2,000 wild sturgeons.

Bred from Sterlet caviar in artificial ponds some 400 kilometres (250 miles) away, they arrive in a big water tank and undergo a slow process known as "tempering" to help them get used to the water temperature in the river.

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Once this is over, a splash of water and cheerful applause accompany the 10-centimetre (4-inch) minions as they embark on their journey into the wild.

The hope is that many will survive long enough to reach maturity and throw their own caviar into the Danube within another three or four years.

The Sterlets are the only sturgeon species that do not migrate to the Black sea. As a result, WWF fish experts and local fishermen will try to check on their flock by catching them in fishing nets with smaller openings.

"Every one of the little fish that we release is individually marked with a minute metal wire with a number, implanted in the fish's left front flipper. If it is caught again, we will know that it is one of our fish," explained WWF's sturgeon project officer Stoyan Mihov.

"This way we will be able to monitor how they migrate, where they go, how they grow -- all this valuable information that is lacking now. In order to protect a species, the first thing that you need to do is to understand its biology and peculiarities."

The WWF said it was encouraged when its experts found an extremely rare two-month-old wild baby Beluga last summer in the Danube.

Two more babies born in the wild -- a sterlet and a stellate sturgeon -- were also caught by the WWF experts this summer and released back into the river.

"We keep our fingers crossed for as many as possible to survive and we hope to have more good news in a couple of years," Mihov said with a smile.