Young freshwater sawfish navigate a gauntlet of sharks and crocodiles

Freshwater sawfish rank among the world's biggest fish, reaching over six metres (19 feet) in length. They also have that preposterously long, saw-like nose at their disposal. You'd think life would be easy for such a formidable-looking creature – but not only are these fish critically endangered, but new research also reveals that some young sawfish spend their early years dodging the world's deadliest predators.

A freshwater crocodile nabs a local freshwater sawfish, despite its size and intimidating nose. Image: David Woods/WA Department of Parks and Wildlife

The freshwater sawfish found in Western Australia (also known as the common sawfish or the largetooth sawfish) breed in near-shore estuaries, and their young leave the ocean and swim upriver to spend several years growing to maturity.

A team of researchers from Murdoch University in Perth took a close look at the sawfish of the Fitzroy River to see what dangers the youngsters faced during their upriver swim.

The estuaries are home to bull sharks and saltwater crocodiles, while smaller but no less hungry freshwater crocodiles dwell farther upriver. Evidence from the fish themselves, as well as from the stomach contents of bull sharks (and some astounding photographs) reveal that all three predators have a taste for baby sawfish.

The researchers got a close look at 39 of the sawfish swimming around the Fitzroy River – and they found the animals were covered in scars. More than half of them bore the marks of bites from predators past – many from crocs and a few from sharks ... while one poor fish showed signs of having been bitten by both predators.

These clues offer some insight into the relationship between the fish upriver and their reptilian neighbours. "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," says David Morgan, lead researcher of the new study, in a press release. Even newborn sawfish are nearly a metre (three feet) long – not the easiest of prey.

For young sawfish, upriver migrations are fraught with danger. Image: David Woods/WA Department of Parks and Wildlife

Of course, what these scars can't tell us is how many young sawfish don't survive once they've caught the eye of a hungry hunter. Needless to say, the trip upriver is a harrowing one.

"Their upstream migrations are fraught with danger, and we suspect they don't always survive," adds Morgan.

This predator-prey relationship isn't surprising, but the study does provide a much clearer picture of the situation. This, the scientists say, is important for future sustainability of sawfish populations, an important concern for the university's Centre for Fish and Fisheries Research.

Freshwater sawfish have experienced major population declines and local extinctions all across their range. The threats of habitat loss, fishing bycatch, and illegal catching and trading (many people covet their iconic rostrums as a novelty, or cook them in shark fin soup), have wreaked havoc on the species.

"[The sharks and crocodiles] have the potential to seriously deplete the freshwater sawfish population," says Morgan, "so it is important that the management of the species ensures the upriver pools remain safe habitats."

For example, some instream barriers constructed by humans have a tendency to attract the crocs and sharks. Morgan recommends removing such barriers as a step towards keeping the rivers safe for the young sawfish.

The situation is further complicated by the fact that both saltwater and freshwater crocodiles are also protected species in Australia.

Preserving this aquatic ecosystem means maintaining environments where predators and prey alike can get what they need to continue surviving, and for the sawfish, hopefully thriving again one day.