It is not much more than a chicken wire cage, but the nicely named "seahorse hotel" is behind a population resurgence of Australia's endangered white seahorse.

Know your seahorses Seahorses are the slowest swimming fish in the sea

Seahorses are the slowest swimming fish in the sea But they are masters of camouflage and can change their colour

But they are masters of camouflage and can change their colour It's the male who becomes pregnant and can do so eight times in a breeding season

It's the male who becomes pregnant and can do so eight times in a breeding season They give birth to between 100–300 babies, but the survival rate is very poor

They give birth to between 100–300 babies, but the survival rate is very poor Seahorses have excellent eyesight and can move their eyes independently

Seahorses have excellent eyesight and can move their eyes independently They use their eyes to ambush food, mainly micro-crustaceans

They use their eyes to ambush food, mainly micro-crustaceans But seahorses are also on the menu for 82 different species, from octopus to penguins

But seahorses are also on the menu for 82 different species, from octopus to penguins Habitat loss is the single biggest threat to seahorse survival

One of the largest concentrations of seahorses in the world was once found at Port Stephens on the New South Wales coast, until wild storm seasons from 2010 to 2013 destroyed habitat and almost destroyed the population.

"If you wanted to see seahorses, you'd come to Port Stephens. You'd hop in here and you'd see 30 to 40 seahorses on every single dive, it was fantastic," said Dr David Harasti from NSW Fisheries.

"But in just a three-year period the population declined by 90 per cent. One site where I used to get 400 animals, I saw 10 there. On another site the population of 200 went down to about 20.

"The decline was quite dramatic and because of that decline the species became the second endangered seahorse listed in the world."

Dr Harasti said several large East Coast Low storms shifted large volumes of sand in the port which smothered the seahorse habitat of soft corals and sponges.

A number of poorly located moorings and anchor damage from boats also contributed to the habitat destruction and left behind vast tracts of barren sand.

Seahorses rely on habitat, like this seahorse hotel, for camouflage and as a source of food. ( Supplied: Dr David Harasti )

Underwater hotels are born

Fortunately, Dr Harasti was struck with inspiration.

"I used to see old discarded crab traps underwater covered in sponges and algae and just by chance I'd see seahorses living on them," he said.

"So we decided to introduce our own artificial habitats and I've called them seahorse hotels.

"I've always got this whole concept underwater — if you build it they will come — so we've built the infrastructure for the seahorses, the hotels are there and now the seahorses are colonizing them."

It is still early days, but after just six months underwater, the seahorse hotels are yielding positive results.

"Where we used to have no seahorses. We've had around 20 or so animals living on the seahorse hotels," he said.

"Each time we survey a hotel there are adults on there, there are juveniles and we're finding they're very territorial, so we tag the seahorses and we find the same animals from three or four months ago still living on them which is great."

Seahorses rely on their habitat to camouflage themselves from predators, as a holdfast to prevent being washed away and as a source of food, predating on the micro-crustaceans that live on the habitats.

Seahorses mate for life. ( Supplied: Dr David Harasti )

Global replication

The success of the seahorse hotels is being closely monitored by Michael Simpson, who is conducting his PhD on the experiment with Sydney University.

Three different seahorse hotel designs are being trialled for their effectiveness, and so far all the hotels are proving an early success.

"It was a bit of a surprise how quickly seahorses appeared on there," Mr Simpson said.

"It's a good thing in the sense that you may not need additional complexity in a seahorse hotel to promote seahorse numbers.

"It may be as simple as putting a cage out."

The goal of his research is to design a cheap and easy model for building seahorse hotels that could be used in sites all over the world where populations are declining.

Mr Simpson is also trialling the effectiveness of swimming nets as an artificial habitat in Sydney Harbour, another population centre of White Seahorses that is on the decline.

Seahorse populations in Port Stephens declined by 90 percent after large storms smothered habitat with sand. ( Supplied: Dr David Harasti )