Predators threat to sawfish

The critically endangered sawfish found in the Kimberley must contend with two major predators as part of a life cycle that hangs in the balance.

Murdoch University scientists have taken a rare photograph of a freshwater crocodile preying on a juvenile freshwater sawfish on the back of evidence collected over several years showing sawfish are also fighting off saltwater crocodiles and bull sharks.

Lead researcher Associate Professor David Morgan said management techniques could help protect vulnerable newborn sawfish as they migrated 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 800mm total length with formidable weaponry, one would assume that rates of natural predation would be low,” Professor Morgan said.

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

Professor Morgan and fellow Murdoch researchers and Nyikina-Mangala Rangers examined scarring on 39 freshwater sawfish at the Fitzroy River.

Evidence of bite marks were on 23 individuals (or around 60 per cent). Based on the appearance of the bite mark, the predators responsible for the attacks were crocodiles (21 occasions), or bull sharks (three occasions).

One sawfish had marks attributed to both predators.

But all of these attacked sawfish appeared to be in a healthy condition, Professor Morgan said.

“These scars suggest that freshwater crocodiles attempt to capture and consume sawfish regularly, but are unsuccessful possibly because of the size, sensory capabilities and defences of their prey,” he said. “Understanding the frequency of successful attacks by these predators is difficult but attacks by larger estuar-ine crocodiles are likely to prove fatal.

“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 of 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.