Australia has some of the last remaining populations of sawfish[Link will open in a new window]. But sawfish numbers are so low that sampling them to get a better understanding of their movement, life history and population size can be difficult.
Our researchers recently hit the jackpot when surveying largetooth sawfish (Pristis pristis) in the Archer River in Far North Queensland. To their excitement, they recorded 47 largetooth sawfish. This offers a glimmer of hope for one of the most endangered groups of species on the planet.
Why these sawfish findings are so important
Worldwide, there are five species of sawfish. All are classified as Critically Endangered by the International Union for Conservation of Nature Red List. Of the 90 countries where sawfish occurred, 43 have lost one or more sawfish species. In at least 20 geographical areas, all sawfish species are now extinct.
Dr Richard Pillanscsiropeople link[Link will open in a new window] has been working on sawfish for 20 years. His team recorded more largetooth sawfish in the three-week survey than they’ve recorded in the past 20 years combined.
“The catch rates of sawfish are second only to the Fitzroy River in Western Australia, which is recognised as a global stronghold of the species,” Richard said.
“To find such good numbers of sawfish in the Archer River is really encouraging. It provides an exciting opportunity to learn more about the species.”
Rich and his team conducted the surveys in close collaboration with APN Rangers. The local name for sawfish in the Wik Mungkan language is ‘Kiikal Keeth’.
The rangers participated in the sawfish survey and worked with our researchers to capture, measure and tag sawfish. They also worked together to measure the salinity and temperature of the water.
Following the surveys, our researchers visited the Aurukun State School to share their findings with the students and community elders.
What we know about sawfish
The largetooth sawfish is the most wide-ranging of the sawfishes. It has a unique ability to live in both freshwater and seawater. It has distinct, geographically separated, subpopulations in the tropical Western Atlantic, Eastern Atlantic, Eastern Pacific, and Indo-West Pacific.
Juveniles are born live at the mouths of large rivers and spend about seven years in rivers and estuaries before moving into marine environments.
Female largetooth sawfish return to the river they were born in to have their pups. This attachment to their birth river is called natal philopatry. It results in genetically distinct populations of sawfish that cannot be replenished by other populations.
Sawfish are highly vulnerable to extinction and very slow to recover from population declines. This is a due to a range of factors such as their population structuring, slow growth rates, late age to reach maturity, small litter size and longevity (sawfish live for more than 35 years).
The primary causes of sawfish decline are bycatch in net fisheries and habitat destruction. Sawfish use a toothed rostrum to catch their prey of fish and prawns, which tangles in nets easily. This makes sawfish highly susceptible to being caught in gill and trawling nets.
To help address this, we’re working with Australian fisheries to get a better understanding of bycatch and develop ways of reducing bycatch of sawfish in trawl and gillnet fisheries. This is part of the Australian Government’s National Environmental Science Program Marine and Coastal (northern) Hub[Link will open in a new window].
Cutting edge sawfish research
Richard was pleased to record so many sawfish. However, we need more data to better understand the size of the Archer River population.
The team took genetic samples which will help determine the size of the adult population through a method called close kin mark recapture.
Matching the sawfish DNA will reveal if any of the sawfish share a parent. If the adult population is small, the chances of the sawfish sharing a parent is higher than if the population is large.
We've succcessfully used close kin mark recapture (CKMR) to estimate population size in great white sharks[Link will open in a new window], grey nurse sharks[Link will open in a new window], speartooth sharks, river sharks, school sharks and southern bluefin tuna.[Link will open in a new window]
Determining how many breeding adults contribute to the population is a crucial piece of information to help conserve and manage sawfish populations.
In addition to CKMR, 13 sawfish were also tagged with satellite tags to understand long-term movements. Tagging will also reveal more about survival of sawfish captured as bycatch in commercial fishing operations.
The fish bone of collaboration
APN Ranger Coordinator Aaron Woolla said it was good to be involved in this research.
“We helped pull up the nets and held the sawfish from kicking, and checked the size and gender," Aaron said.
“The sawfish is a totem for some Archer River people here in Aurukun. It’s good to be out there catching those different type of fish. I have seen them a couple of times before. Our old people used to hunt for them in the freshwater time," Senior Ranger Horace Wikmunea said.