WWF has released 11,000 threatened sturgeons in the Danube river to restock Europe's last wild sturgeon populations, the conservationist group said on Tuesday.

A total of 5,000 sterlet sturgeons were released on Tuesday near Belene island close to the border between Bulgaria and Romania under an EU-funded programme, it said.

Six thousand fish were released last week under the same project.

The aim is to restock the Danube with a total of 50,000 sturgeons.

"Of the 25 sturgeon species that inhabit the northern hemisphere, six live in the Danube, with Bulgaria and Romania holding the only still viable populations of wild sturgeons in the European Union," said WWF's Stoyan Mihov.

Both Bulgaria and Romania have imposed a ban on sturgeon fishing but there is extensive poaching.

The 5,000 fish released have magnetic tags in their left front flipper to allow environmentalists to track their migration routes and breeding areas.

"When we know where they are, we can protect them better," Mihov added.

The sterlets are the smallest species of sturgeon and the only ones that do not migrate into the Black Sea.