Swimming in circles alongside a fishing boat, two otters wait to catch fish in a river in southern Bangladesh. As the animals squeak in the water, fishermen lower a net into the river, in the heart of the Sundarbans, the world’s largest mangrove forest. Then, one by one, the short-haired otters dive under the water with a splash, chasing a school of fish close to the banks of the river. Otter fishing is a centuries-old tradition in Bangladesh, where fishermen have been using trained otters to lure fish into their nets – a rare technique passed on from father to son that relies on coordination between man and otter. Fishing is usually done at night, and the otters can help fishermen catch as many as 26 pounds of fish, crabs and shrimp. But the partnership between man and otter is on the verge of extinction. It’s already died out in other parts of Asia as fish populations decline, wildlife experts say. Short-haired otters are an endangered species in Bangladesh and experts say that otter fishing may play a key role in their conservation.