Global outcry followed when pictures were published of the whales struggling to swim through ice-encrusted waters in cramped enclosures

The last whales held in a notorious facility dubbed the "whale jail" in Russia's Far East were released on Sunday, the institute overseeing the operation said.

The fate of the whales, which were captured to perform in aquariums, sparked international outrage earlier this year after pictures appeared in the media of them struggling to swim through ice-encrusted waters in cramped enclosures.

Moscow eventually bowed to pressure to release the 93 beluga whales and 11 orcas, which were held for more than a year in the secretive facility in Srednyaya Bay near the far eastern town of Nakhodka.

The whales have since been released in batches, with the last of the orcas freed in August.

The All-Russian Fisheries and Oceanography Institute said the operation to free the last belugas started five days ago.

"In the region of Primorsky Krai, the operation to release the marine mammals into their natural habit has been completed," it said in a statement on Sunday.

Russian NGO Sakhalin Watch, which campaigned for the whales to be freed, said the final 21 belugas were released from two ships on Sunday.

The organisation said it was delighted that "the release of all the beluga whales has taken place and that the 'whale jail' has finally freed its last prisoners!"

Killer whales are transported to a tank on a truck to be released from the "whale jail" earlier this year

The operation began around 9:00 am (2300 GMT Saturday) and went on until 5:00 pm, Sakhalin Watch said.

Many scientists and activists have criticised the Oceanography Institute for keeping the details of the release secret, not taking any observers on the trip and freeing only a small group of animals at a time rather than all of them together, which would boost their survival odds.

Sakhalin Watch said the coastguard forced its members to leave the bay where the whales were being released.

Russia is the only country that captures wild orcas and belugas to sell to aquariums, a controversial practice made possible by legal loopholes.

Many of the whales held at the facility in Srednyaya Bay were to be sent to aquariums in China.

A Change.org petition for the whales to be released collected more than 1.5 million signatures, including that of Hollywood star Leonardo DiCaprio.

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© 2019 AFP