Nineteen northwestern pond turtles reared at the Oregon Zoo were returned to the wild.

Footage shows staff releasing the reptiles in ponds along the Columbia River Gorge in Washington, where the turtles were seen swimming further into the murky depths. The Oregon Zoo team said a team of conservationists will continue monitoring them.

The hatchlings were collected from wild sites and have been raised at the zoo's conservation lab for a year.

Jen Osburn Eliot of the Oregon Zoo said: 'We've been caring for these turtles in the lab since last year. They're big enough now to have a much better chance of surviving in the wild.'

Before being freed, zookeepers first acclimated the turtles by letting them spend time outdoors. They are then released into the Columbia River Gorge once they weigh around 50 grammes.

The method, called 'head-starting', is carried out to improve the turtles' survivability in the wild. Scientists estimate that 95 per cent of released turtles survive each year in the Gorge.

The northwestern pond turtle is classified as an endangered species in Washington, and a sensitive species in Oregon.

Among their biggest threats is the invasive American bullfrog, which has pushed pond turtles and other small aquatic species toward extinction.

There were fewer than 100 northwester pond turtles left in Washington 20 years ago. Since then, over 1,600 turtles have been released through zoo-led efforts.

The Western Pond Turtle Recovery Project is a collaboration among multiple partners including the Oregon Zoo, Woodland Park Zoo, government agencies, and conservation groups.