The numbers of major shark species off eastern Australia, especially great whites and hammerheads, have crashed in the past half century and show no signs of recovery despite being protected, new research finds.
Subscribe now for unlimited access.
$0/
(min cost $0)
or signup to continue reading
The study used data from Queensland's shark control program that runs along some 1800 kilometres from Cairns to the Gold Coast. As sharks are migratory animals, the declines are similar to those in waters off NSW.
The research, published Friday in Communications Biology, found catches fell 92 per cent for hammerheads and whites, 82 per cent for whalers and 74 per cent for tiger sharks. The declines were "either equal to or exceeding rates reported in coastal oceans elsewhere in the world", the paper said.
Overfishing seems to be the primary cause for the drop - including some 50,000 sharks taken during the Queensland program over the past 55 years - although apportioning blame remains difficult.
"We just don’t know what the contribution of recreational and commercial fishing is," said George Roff, marine ecologist at the University of Queensland and lead author of the paper. "Also, shark control programs in Queensland and NSW - how they are contributing to this decline?”
While ecological impacts from the loss of most of the sharks remain unclear, outcomes are not likely to be good.
"Apex sharks play a critical role in coastal ecosystems," Dr Roff said. "They sit at the top of the food web and are important for healthy ecosystems.”
Humans and sharks
The paper noted that so-called interactions between humans and shark are on the increase despite shark numbers being "severely depleted". Soaring numbers of people flocking to beaches appear to play a role although the chance of a shark bite remains tiny.
"It is easier to remove humans out of the human-shark interactions than it is to remove sharks," Dr Roff said.
The researchers praised NSW as a "leading example" in the use of drones and devices such as clever buoys to provide earlier warning of the presence of potentially dangerous sharks.
Even so, data for NSW from 1950-2010 also showed long-term declines in the number of larger sharks being caught, including tigers and sharks.
Lately, the state had replaced baited drumlines that killed a range of animals with so-called smart drumlines that allowed targeted sharks to be caught, tagged and released.
"We think you can have large sharks and preserve these important species - they’ve been around for millions of years and survived through the extinctions of the dinosaurs," said Chris Brown, a researcher at Griffith University and one of the paper's authors. "It would be really tragic if we lost them now because of preventable human causes.”
Shrinking shark sizes
The researchers ruled out climate change as playing a role since the sharpest declines occurred early in the program.
Similarly, baitfish numbers, while variable, have been more consistent, reducing the likelihood they are a factor, the researchers said. The popular view that sharks are coming closer to the coast, pursuing scarcer food sources “largely doesn’t seem to be true”, Dr Roff said.
Great white shark numbers continue to fall at eight of nine regions despite a ban on commercial and residential fishing of the animals since 1999, the paper found.
Another ominous sign is that the size of the caught animals has been falling. The average size of great hammerheads dropped by more than a fifth to 2.15 metres, while those of scalloped hammerheads dropped 16 per cent in the two decades since 1997, the paper found. Whaler and tiger shark sizes had also shrunk.
With reproductive maturity of sharks typically tied to their size, the relative absence of large sharks being caught was of a particular concern.
"Declines in the number of sharks reaching maturity can strongly influence population dynamics and inhibit recovery rates," the paper said.
