Sawfish v crocodiles and bull sharks

April 12, 2017

A freshwater crocodile preying on a freshwater sawfish in the Kimberley (Picture provided by David Woods, WA Department of Parks and Wildlife)

A freshwater crocodile preying on a freshwater sawfish in the Kimberley (Picture provided by David Woods, WA Department of Parks and Wildlife)

Murdoch scientists have shown the critically endangered sawfish found in the Kimberley region must contend with threats from two species of crocodiles and bull sharks.

A rare photograph has been taken of a freshwater crocodile preying on a juvenile freshwater sawfish, and Murdoch researchers have collected evidence over several years that show sawfish are also fighting off saltwater crocodiles and bull sharks.

Lead researcher, Associate Professor David Morgan from the Centre for Fish & Fisheries Research, said management techniques can help protect vulnerable newborn sawfish as they migrate up the Fitzroy River from their birthplaces in the estuaries.

The river acts as a nursery to the young freshwater sawfish where they spend the first four or five years of their lives before journeying downstream to the ocean to mature and breed.

“For a fish that is pupped at around 800 mm total length with formidable weaponry, one would assume that rates of natural predation would be low,” said Professor Morgan.

“But their upstream migrations are fraught with danger, and we suspect they don’t always survive.”

Professor Morgan and fellow Murdoch researchers, along with Nyikina-Mangala Rangers, examined scarring on 39 freshwater sawfish at the Fitzroy River, finding evidence of bite marks on 23 individuals (or around 60 per cent).

The upstream migrations of freshwater sawfish are fraught with danger (Picture provided by David Woods, WA Department of Parks and Wildlife)

The upstream migrations of freshwater sawfish are fraught with danger (Picture provided by David Woods, WA Department of Parks and Wildlife)

Based on the appearance of the bite mark, the predators responsible for the attacks were crocodiles (on 21 occasions), or bull sharks (three occasions) and one sawfish had marks attributed to both predators. Some sawfish had multiple bite marks, ranging from recent to healed scars. But all of these attacked individuals appeared to be in a healthy condition, said Professor Morgan.

“These scars suggest that freshwater crocodiles attempt to capture and consume sawfish regularly, but are unsuccessful possibly due to the size, sensory capabilities and defences of their prey,” he added.

“Understanding the frequency of successful attacks by these predators is difficult but attacks by larger estuarine crocodiles are likely to prove fatal, especially on newborn sawfish migrating upstream.

“The three natural predators, including the two protected species of crocodile, have the potential to seriously deplete the freshwater sawfish population, so it is important that management of the species ensures the upriver pools remain safe habitats.”

Professor Morgan suggests the removal or modification of human-made instream barriers, which attract crocodiles and bull sharks, will allow sawfish safe passage up the river.

Freshwater sawfish are listed internationally as critically endangered and it is the only sawfish from the five sawfish species with a juvenile freshwater phase. Their extinction risk has been linked to the susceptibility of their unique rostra to entanglement in fishing nets and through habitat loss. There is also evidence of hunters collecting sawfish rostra for fishing trophies.

Professor Morgan’s paper on the research has been published in the May issue of Ecology in the newly announced series of The Scientific Naturalist. It can be read here.

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