University of Queensland and Griffith University researchers analysed shark control program data to measure changes in shark populations along the Queensland coastline in a 55 year period.

The shark control program has used drumlines and nets since 1962 to try to reduce the risk of shark attacks, and now spans 1760km of the Queensland coastline.

The scientists studied the number of hammerhead, white, tiger and whaler sharks caught in nets from 1962 to 2016.

The data showed the number of hammerheads and white sharks had each declined by 92%, whaler sharks by 82% and tiger sharks by 74%.

The researchers examined and eliminated the possibility that factors such as the location and materials of nets, reliability of historical figures or the possibility that sharks were learning to avoid drumlines had influenced the numbers.

Instead, they concluded that the most likely cause for the dramatic declines was commercial fishing.

“The key thing that our study shows is that large apex sharks on the Queensland coastline have been declining over the last 50 years,” the study’s lead author and post-doctoral research fellow at the University of Queensland George Roff said.

“We found drastic declines in the population of large apex sharks.”

The data also showed the sharks caught were getting smaller, which Roff said had implications for population dynamics and how well shark populations could recover.

“The bigger the shark is, that’s when it reaches a reproductive stage,” he said.

“If we’re catching more young sharks there’s fewer reproductive individuals out there and they breed less.”

The research team said factors such as climate change, which is affecting shark populations, could not explain the scale of the decline.

“We can be fairly certain the cause of the decline is fisheries related. There’s not very many other things it can be,” Roff said.

Dr Chris Brown, from Griffith University’s Australian Rivers Institute, said some species, such as hammerhead sharks, were recognised internationally as being at risk of extinction.

“It would be a great tragedy if we lost these species because of preventable human causes,” he said.

“Sharks play important roles in ecosystems as scavengers and predators, and they are indicators of healthy ecosystems. These declines are concerning because they suggest the health of coastal ecosystems is also declining.”

Shark attacks off Australian beaches have created an uneasy relationship between beachgoers and the apex predator.

But the researchers say their study demonstrates the need for more protections for sharks in Australian waters, as opposed to increased measures to control them.

“We also need more research into understanding the movements of these animals,” Roff said.

Reported by Lisa Cox

Graphics by Nick Evershed