A debt relief scheme backed by the film star will protect tuna stocks as well as other marine life

The Seychelles have created a vast marine reserve about the size of Britain under an innovative debt relief scheme backed by Leonardo DiCaprio.

In exchange for getting some of its national debt paid off, the island nation has agreed to protect 210,000 sq km (81,000 sq miles) of ocean, limiting tourism, oil drilling and commercial fishing to protect dolphins, turtles and other marine life.

Under the deal the Nature Conservancy, a US group backed by philanthropists including DiCaprio, buys $21 million of Seychelles sovereign debt so that its government can redirect its payments to the protection of its waters.

The Indian Ocean archipelago of fewer than 100,000 people relies on tourism and fishing for revenue but in recent years oil and gas companies have been