Conservationists have warned Russia against its plan to release 10 captive killer whales into the Sea of Japan, rather than where they were originally taken from.

Celebrities including Oscar-winning actor Leonardo DiCaprio, have backed a petition that has had almost 1.5 million signatures, urging the mammals be released from their cramped conditions.

The orcas are being held, along with 87 beluga whales, in a bay near the port of Nakhodka, some 800 miles (1,290km) south of the Sea of Okhotsk, where four Russian companies captured them last summer.

Image: A view of enclosures with nearly 100 whales being held captive

The aim was to sell them on to marine parks or aquariums in China.

Their plight sparked an international outcry and last month, under increasing pressure, Russia signed a deal with a group of international scientists to release them.


But this week, experts in the country said the whales would be freed from their holding pens, into the Sea of Japan - but the plan is being criticised, with claims it could harm them.

Jean-Michel Cousteau, the son of celebrated French marine expert and broadcaster Jacques Cousteau, claims: "It puts the well-being of these orcas at undue risk and compromises their long-term survival.

Image: Jean-Michel Cousteau claims Russia's plans compromises their long-term survival

In a statement released by the Whale Sanctuary Project, he said they "cannot support or participate in this plan", adding that the animals "may not survive" because it could mean they are unable to re-adapt to life in the ocean, and that they would return to the pens for food.

Cousteau supports the idea of transporting the whales in seawater tanks to near where they were caught, where he says there are associated orcas and belugas and appropriate and available food sources.

But the Russian scientists say they believe the orcas could suffer injury and stress if taken back to the Sea of Okhotsk - and that such a strategy would be complex and time-consuming.

Image: Almost 100 whales are being held captive in Russia's far eastern Primorye region

Following heavy criticism, the Kremlin ordered local authorities in the Russian Far East to intervene and Russia's FSB security service has brought charges against the companies for breaking fishing laws.

The Kremlin has said Russia has no direct ban on catching whales, but that they can only legally be caught in specific circumstances, such as for scientific and educational purposes.