October 03, 2018 18:09:39
They’re enormous “alien-looking” dinosaurs — fish who can reach up to seven metres long. They’re also one of the most endangered species on the planet.
But one Top End river is teeming with sawfish, and it’s hoped a five-year survey of the elusive ray will shed more light on its behaviour and breeding.
The first survey by CSIRO researchers and Timber Creek rangers on the Victoria River in the Northern Territory captured 25 dwarf and largetooth sawfish, as well as providing the first recorded evidence of the critically endangered speartooth shark and northern river shark in that waterway.
Sawfish are extinct in 40 per cent of the countries they once populated but they still exist in healthy populations in tropical Australia, even if their numbers have declined massively since the 1960s — mainly due to commercial fishing, CSIRO researcher Dr Richard Pillans said.
“They have a big kind of electric hedge trimmer on the end of their body which makes them incredibly susceptible to being stuck in gill nets and trawl nets,” he said.
“So while the fishermen aren’t trying to catch them, as soon as [sawfish] come into contact with a net they become stuck.
“And because they’re also one of the largest fish we have — they grow to seven metres in length — getting an animal that size out of a net is quite tricky.”
‘You feel like you’re looking back four million years’
Sawfish, also known as carpenter sharks, are a type of ray, and are Australia’s largest freshwater fish, occurring both in fresh and salt water.
They have been recorded as reaching up to seven metres in length and weighing about a tonne.
“They’re an enormous dinosaur,” Dr Pillans said.
“The biggest I’ve seen is only five metres, and you feel like you’re looking back four million years when you see something that size, and so alien-looking with that big saw on its body.”
Because sawfish populations are so low, researchers have struggled to track their longevity, and how many young they have.
“They may have as few as six pups every two years, or they may have 12 pups every year, but we don’t know, and that’s some of the information we’re trying to find out with the surveys,” Dr Pillans said.
Because sawfish can live for about 50 years, they take ten to 15 years to reach sexual maturity, so it has been difficult to study them accurately in captivity, he said.
The animal is something of an underdog due to its susceptibility to getting caught in fisheries, its young eaten by crocodiles, and it’s easily affected by coastal developments and water extraction, he said.
“All the things going on in northern Australia are a threat to them… it’s nice to be able to try and find out more about a species and hopefully save them from going extinct,” Dr Pillans said.
“Being a ray, they’re not really doing anybody any harm, they’re just going about their business.”
Aboriginal rangers conducting endangered species surveys
The CSIRO has teamed up with the Northern Land Council and Timber Creek rangers to survey the Victoria River twice a year for five years, having conducted their first survey in August.
The Victoria River is an important sanctuary for sawfish, Dr Pillans said.
“We caught more in two weeks than I’ve caught in 20 years,” he said.
The long-term monitoring survey will look at how many sawfish and speartooth and northern river sharks live in the river system, how the population is faring, and how they use the waterways.
“The Victoria River is 560 kilometres long and sawfish have been recorded all the way up to the very upper reaches of the river,” Dr Pillans said.
“They get there in the wet season during massive floods when the river’s up 15 metres, and they swim hundreds of kilometres to get to isolated pools where they’re fairly protected from predators.”
The surveying project is being undertaken with the support of the traditional owners of the area, with the aim of training local rangers to undertake the surveys, handle the sawfish, measure them, take genetic samples, and then safely release them.
“Catching sawfish is a really hands-on procedure: you’re setting long gill nets in crocodile-infested rivers and the sawfish we’re catching are four to five metres long, so it’s not something you can do by yourself; you really need a team of trained experts,” Dr Pillans said.
Ranger Floyd Rogers said he was keen to get out on the water again.
“This seven days on the river was the greatest I have done and learned a lot,” he said.
Ranger coordinator Newton Hobbs said the rangers gained a lot of experience being on the river learning about endangered animals and the river system itself.
“We all [came] home with 10 fingers and toes so that’s a success,” he said.
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