Nestled deep amongst Sydney's soaring blue and grey glass skyscrapers next to the roaring Western Distributor, hundreds of shark eggs are being laid.
These conical-shaped parcels are part of the world's first shark re-wilding program, which is aiming to recover an endangered species to the tropical waters of Raja Ampat, in West Papua, Indonesia.
9news.com.au went behind the scenes of Sea Life Sydney Aquarium, the first Australian institution involved in the project, to learn about the "nibbling" business of shark breeding.
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The nibbling business of shark breeding
In an observation pool within a 1.3 million-litre tank, "Kaya", a two-metre zebra shark, also known as the Indo-Pacific leopard shark, rests.
On both of her pectoral fins are two perfectly round "nibble marks".
"That's Kaya, she's just had a vet check and bloods taken," Laura Simmons, Regional Curator for Sea Life Australia and New Zealand, told 9news.com.au.
"It's her breeding season. Those nibble marks, they're made by Gohan, our male, as part of their courtship.
"When the sharks are ready to breed, the males start to nibble and suck on the fins of the female, who will occasionally even flip into 'tonic immobility', a rest state.
"The biting and sucking on the fins is a critical part of courtship for this, and many, shark species. It's important for the male to be able to get into position to mate.
"It could also help stimulate the female's interest."
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It may sound like brutal business, but Simmons said female sharks are resilient animals "with thick skins".
Sea Life Sydney Aquarium is playing a pivotal role in the conservation initiative, formally known as the Stegostoma tigrinum Augmentation and Recovery Project, or "StAR Project".
Its current population of three adult zebra sharks - which includes, one male called Gohan and two females, Kaya and Zimba - were identified as genetically suitable to support the breed-for-release program.
Simmons explained shark breeding is seasonal and has to be timed exactly, as each of the females "comes on" during different times of the year.
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"Kaya's egg-laying season is approaching, whereas Zimba started her season August-September.
"She's on around egg 150, which is a good result as zebra sharks can lay hundreds of eggs each season.
"All those eggs you see are Zimba's," Simmons added, indicating to a row of blue, black and white milk cartons.
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Wandering over Simmons inspects the conical-shaped pouches.
The hard work begins the moment the eggs are laid.
Staff have to monitor each one closely, to determine if the pouch has been fertilised and to track yolk development.
Detailed records are kept as to who lay the eggs, where they were laid, the date, and condition of the egg at the time.
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"Incubation takes around 130 days to as many as 150," Simmons said.
"And if, say, 20 per cent of those eggs have a yolk and are fertilised, only a small per cent of those will develop fully to hatch.
"In the first 30 days we will know if it's possibly viable, and then it will get shipped once it has reached 10-15 weeks of development.
"It's a long process with specific timing."
The StAR Project is a multi-national, collaborative initiative led by ReShark.
Since the program's launch in August 2022, five pups have hatched from the eggs shipped up to the StAR hatcheries in the protected waters within Raja Ampat, a "booming hub for marine diversity."
Of these, four came from Sea Life Sydney Aquarium's breeding program.
Why now, and why zebra sharks?
Zebra sharks are an endangered species that have been pushed to the brink of extinction through illegal and unregulated fishing.
The sharks, which reach lengths between 2 - 2.2 metres in the wild, have long fins which are highly-desirable for shark fin soup. Additionally, they are often caught as by-catch in nets.
This endangered listing, along with two other factors, make the species an ideal candidate for the program.
"With the zebra shark, it was a perfect match, so to speak," Simmons said.
"Basically, it is a species that does need help, a species we have lots of hands-on experience with in public aquariums and also breeds successfully in them, but the clincher is they are egg-layers.
"Being able to move eggs around is much less difficult than packing an adult shark to transport safely to Indonesia.
"Logistically, that is very complicated."
Simmons added that "a lot has gone into" making sure the StAR Project has long-term viability.
The program is subject to strict International Union for Conservation of Nature (IUCN) guidelines to ensure the genetics of the population being released is "correct", and the wider ecosystem wont be jeopardised.
"We don't just want to do this for the sake of the sharks, we have to do it the right way for the wild environment," Simmons said.
"Also for this project to work, there had to be established marine-protected areas. So, when we release the pups we know they are going into an area that doesn't have the same fishing pressures and other threats.
"I think that's what touches me the most, it's not just that we're seeing the recovery of a big shark species, but that we are giving back to an entire community of people a species that belongs in their waters - especially such a beautiful species."
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