A conservation project led by UNSW scientists is rebuilding lost underwater pastures within the city's harbour – and at the same time helping to recover an endangered seahorse population.
White's seahorse (Hippocampus whitei) can only be found on the east coast of Australia – from southeast Queensland to the NSW south coast. It's the second seahorse species in the world listed as endangered.
In recent decades, this shy little seahorse has lost one of its essential habitats, namely Posidonia australis – a native seagrass that once formed extensive meadows across Sydney's coastline. With urban sprawl, pollution, dredging and boating activities, Sydney's Posidonia meadows have disappeared almost completely in the past 50 years.
"As the seagrass meadows disappear, so too do the species that depend on them," says Professor Adriana Vergés from UNSW Sydney. She is member of the conservation project, Greener Pastures, as well as co-founder of the UNSW-led Operation Posidonia and a lead investigator at Project Restore – a multi-agency effort to rehabilitate the long-degraded habitats of Sydney Harbour.
"There is minimal Posidonia remaining in Sydney Harbour," Prof. Vergés says. "Historically, these meadows were extensive but coastal development and physical disturbances have wiped much of them out. We predict Posidonia will be locally extinct from Sydney Harbour within the next couple of decades unless we do something about it."
Seagrasses are true flowering plants with root systems that help stabilise sediment and prevent erosion, protecting the foreshore. Their meadows can trap carbon at much faster rates than terrestrial forests, which means they are important for mitigating climate change. And they also provide essential nurseries for fish and invertebrates.
A little fishy
Seahorses are unique fish in many ways. They swim vertically and use their tails to remain within their habitats, by grasping and wrapping around seagrass leaves, coral branches or sponge-fingers. This means they don't drift away as currents pass through and instead sway while anchored in place.
"With the Greener Pastures project, we are aiming to tackle one of the root causes of the decline in White's seahorse populations, which is the loss of habitat," says Prof. Vergés.
"We can breed and release them, but unless we restore their homes, their populations will keep falling."
"To hold on to that habitat is critically important," says Mitchell Brennan, program manager of the Sydney Seahorse Project at the Sydney Institute of Marine Science and PhD candidate at the University of Technology Sydney.
"Seahorses rely on highly complex habitats like seagrass meadows to camouflage, anchor and feed," Brennan says.
This is where seahorse hotels come in.
While waiting for grass to grow
Dr David Harasti from the NSW Department of Primary Industries and Regional Development originally created the concept of 'seahorse hotels'. Inspired by old crab traps that he found teeming with marine life, he designed the 1x1 metre steel mesh cages to mimic a tangled complexity that was closer to a natural habitat.
"When left on the seafloor these bespoke hotels soon become encrusted with sponges, corals and algae," Dr Harasti says.
It's this 'crust' that provides ideal anchoring points for the seahorses.
The growth also helps them camouflage from predators and attracts tiny crustaceans – their main food source. This means each hotel becomes a thriving mini ecosystem, providing seahorses with shelter, protection and nourishment.
"Within three to six months, the seahorses move in," Dr Harasti says.
At Cobblers Beach, one of the key project sites, 15 seahorse hotels are already home to a population of White's seahorses. The hotels were placed alongside natural remnant Posidonia meadows in late 2023, and extra shoots were transplanted there late last year.
The project team recorded a maximum of eight seahorses in the area for their 2024 survey. Early this year, they recorded 12 individuals – including several juveniles that were likely born onsite.
A harbourside retreat for expectant fathers
Unlike most fish, it's the male seahorses that give birth. Fully formed baby seahorses are released from the male's pouch around three weeks.
"We've seen multiple pregnant males and partners on the seahorse hotels in the past six months," Brennan says. "With some of these juveniles being sighted, hopefully the population starts to grow and become stronger and more self-sustaining."
The hotels appear to be benefitting the Posidonia replanted alongside them too, with the structures acting as a buffer from wave action, while mitigating sand inundation.
This means a broader ecological comeback.
"In areas where we've replanted seagrass, we're already seeing more fish – snapper, flathead, bream and even pyjama squid," says Brennan.
"It's about restoring the function of the whole system."
More than just a cute playland
Greener Pastures is a story of true collaboration.
"This all started when Operation Posidonia received a grant from the Environmental Trust and support from Mosman Council," Prof. Vergés says. "The local community is incredibly engaged. They're not just helping us plant seagrass – they're also learning why it matters and sharing this information with their friends and neighbours."
Community involvement is a defining feature of the original project. Since the project's early days in Port Stephens, volunteers helped collect thousands of storm-detached Posidonia shoots for replanting.
"Hands-on restoration helps people connect with ecosystems they might never have known about," Prof. Vergés says. "When people become part of the process, they gain a sense of ownership – that's where real stewardship begins.
"You can't protect what you don't know exists."
She also says that Posidonia, for all its massive ecological importance, isn't exactly a crowd-pleaser.
"Seagrasses do so much for us – they stabilise our shorelines, provide a nursery habitat for fish and invertebrates and sequester large amounts of carbon, but admittedly they kind of look a bit boring on the surface and not particularly interesting.
"The cuteness of the seahorses certainly helps bring attention to the habitat."
Prof. Vergés says her hope is to see both Posidonia meadows and White's seahorses thrive within the next 10 to 15 years – and maybe even lose their endangered status.
"I'd like to see a growing population of White's seahorse occupying this habitat – both of them recovering together."
Key Facts:
A two-for-one local effort aims to restore endangered native seagrasses to Sydney's urban waters along with one of their most charismatic inhabitants: the also endangered White's seahorse.
