Baby bull sharks rely on saltmarsh - not mangroves - for food and survival, University of Newcastle research has found.
The finding has surprised scientists, who had assumed young sharks relied on mangrove habitats for nutrition through the crustaceans and fish that feed off them.
The sharks obtain key nutrients from saltmarsh - an ecosystem under threat from humans and mangroves.
The study examined feeding grounds for juvenile bull sharks in the Clarence River in the Northern Rivers region of NSW.
Tissue samples were analysed from 55 juvenile bull sharks that commercial fishers had caught.
Dr Vincent Raoult, a study co-author, said the juvenile bull sharks move towards the mouth of the estuary at around the age of four.
They prey on fish and other species that mostly rely on threatened saltmarsh habitat.
Dr Raoult said saltmarsh had declined significantly around the world.
"It's called the saltmarsh squeeze," he said.
"Saltmarsh is being absorbed by mangroves as sea level rises and, on the higher side of land, it's typically being taken over by humans and reclaimed as land."
As well as urbanisation of coastal areas, climate change is also contributing to the disappearance of saltmarsh.
"It's estimated we've already lost 50 per cent of our saltmarshes across the world," Dr Raoult said.
He said the United Nations has a "huge focus on rehabilitating saltmarsh" in its decade of restoration.
"Historically, saltmarsh has been viewed as stinky, smelly, useless and good for draining," he said, adding "we do a lot of work with saltmarsh in the Hunter".
"Newcastle and the Hunter in particular is a really unique spot because there is so much saltmarsh and mangroves," he said.
It exists in places such as Kooragang Island, Fullerton Cove, Ash Island and Hexham Swamp.
The Newcastle Coal Infrastructure Group is rehabilitating saltmarsh as part of its requirements.
"There's all the saltmarsh in the Hunter Wetlands area at Shortland. There's been a lot of remediation work in that area as well."
Previous research found that 2133 hectares of saltmarsh in the Hunter estuary was reduced by 67 per cent from the 1950s to 1994, while mangroves increased.
Dr Raoult said a lot of species relied on saltmarsh, including targeted fishery types such as bream, mullet, prawns and blue swimmer crabs.
"Saltmarsh consistently comes up as a really important food source."
The study's lead author, Dr Yuri Niella of Macquarie University, said saltmarsh must be protected from further threat.
Dr Niella said restoration of saltmarsh habitats would benefit bull sharks but also increase "economic profit from an improvement in fishery catches".
"Understanding the importance of estuarine habitats for the animals living in these areas can help inform targeted management decisions to achieve the best outcomes for both people and animals."
