Sawfish are extinct in nearly two dozen countries but in Western Australia, Murdoch University researchers have unlocked critical breeding and reproductive knowledge about the locally found species - the green sawfish.
Using DNA collected over a decade, a team from the Harry Butler Institute used genetic techniques to reconstruct a series of family trees for green sawfish in the Ashburton River in the state's northwest.
They found more than 50 female green sawfish were reusing the same sites in the 680km-long river to give birth.
"These findings are really important for informing conservation efforts, as they demonstrate that some female green sawfish are actually returning to, and reusing, the same site for pupping," sawfish expert Jack Ingelbrecht said on Tuesday.
Jack Ingelbrecht (c) says conservation efforts should focus on protecting key waterways. (Supplied Murdoch University/AAP PHOTOS)
"This suggests that conservation efforts for green sawfish could be more effective if targeted towards protecting key waterways like the Ashburton River."
The green sawfish is one of the largest fish on the planet, known to grow longer than six metres, and it's critically endangered.
Dr Ingelbrecht's study identified some 90 half-sibling pairs, including high proportions of both maternal and paternal half-siblings.
This provided clarity about sex-biased dispersal.
"This is where individuals of one sex are more prone to returning to their birthplace for reproduction, whereas members of the opposite sex are prone to disperse," he said.
"The presence of so many maternal and paternal half-siblings suggests that dispersal is limited in both female and male green sawfish."
Dr Ingelbrecht said the study showed there was an elevated risk of localised extinction for the species because depleted populations were unlikely to be replenished from other areas.
Green sawfish have a shark-like body and a flattened head with a narrow blade-like snout studded with 24 to 28 pairs of rostral teeth.
They are greenish-brown or olive in colour on the upper surfaces and pale whitish below and inhabit coastal marine and estuarine waters.
Protected in Australia, the species has historically been targeted or taken as incidental bycatch in commercial and recreational fisheries.
The rostrum or beak with the large teeth is easily entangled in nets.
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