More than 350 seahorses released into Sydney Harbour

Scientists say they have completed the world's largest release of seahorses into Sydney Harbour to boost marine population.

White's Seahorses are native to the waters around Australia's east coast and were classified as endangered on the International Union for Conservation of Nature Red List in 2017 due to loss of habitat.

Sydney Harbour's seahorse population nearly halved between 2008 - 2015, but now more than 350 newly bred seahorses have been released and are making themselves at home.

The release is thanks to the Sydney Seahorse Project, a collaboration between the Sydney Institute of Marine Science, the University of Technology Sydney and the New South Wales (NSW) Department of Primary Industries.

The conservation group said the seahorse population on New South Wales's north coast had fallen by about 95%.

Earlier this year, the Sydney Institute of Marine Science captured three pregnant males and helped to raise their 380 babies in captivity to improve their chances of survival.

The babies have been released with tags on so researchers can monitor their growth and reproductive success in the wild.

Local divers have also been asked to help monitor the population by sharing any photos they take of the seahorses.

Project manager Mitchell Brennan said it was "fantastic" and added: "These are endangered White's Seahorses that are going out to restock the wild population here."

The young creatures have been provided with seahorse hotels made from biodegradable metal that provides much needed habitat.

The hotels were installed at least a month before the release, to allow for algae, sponges and other marine life to form around them and are designed to mimic discarded crab traps, a common home for seahorses.

Over time the metal breaks down and disappears, leaving behind a semi-natural reef, benefiting the harbour's broader ecosystem too.

This is what the seahorse hotel looks like after a few months

Results from earlier releases look promising.

A year after the 2020 release in Chowder Bay, part of Sydney Harbour National Park, 20 percent of the released population remained on the hotels and 10 percent were pregnant in the wild.

"It's a really positive first sign," Mr Brennan said.