Queensland’s coastal shark numbers are continuing a 50-year decline, in sharp contradiction of suggestions of ‘exploding’ shark populations, according to an analysis of Queensland Shark Control Program data.
Griffith University and University of Queensland researchers analysed data from the program, which has used baited drumlines and nets since 1962 to minimise human-shark interactions, and now spans 1760km of the Queensland coastline.
Dr Chris Brown from Griffith’s Australian Rivers Institute said the results showed consistent and widespread declines of apex sharks – tiger white sharks and hammerheads – along Queensland’s coastline.
“We were surprised at how rapid these declines were, especially in the early years of the shark control program. We had to use specialist statistical methods to properly estimate the declines, because they occurred so quickly,” Dr Brown said.
“We were also surprised to find the declines were so consistent across different regions.
Beaches that had nets and drumlines installed in later years, like the late 1970s and 1980s, already had lower shark catches than at beaches where fishing started in the 1960s. This suggests the declines were widespread.”
Dr Brown said some species, like hammerhead sharks, were recognised internationally as being at risk of extinction.
“Sharks are an important part of Australia’s identity. They are also survivors that have been around for hundreds of millions of years, surviving through the extinction of dinosaurs,” he said.
“It would be a great tragedy if we lost these species because of preventable human causes.
“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.”
By analysing the Queensland Shark Control Program data, the research team reconstructed historical records of shark catches to explore changes in the number and sizes of sharks over the past half century.
UQ School of Biological Sciences researcher Dr George Roff said historical baselines of Queensland shark populations were largely unknown, despite a long history of shark exploitation by recreational and commercial fisheries.
“What we found is that large apex sharks such as hammerheads, tigers and white sharks, have declined by 74 to 92 per cent along Queensland’s coast,” Dr Roff said.
“And the chance of zero catch – catching no sharks at any given beach per year – has increased by as much as seven-fold.
“The average size of sharks has also declined – tiger sharks and hammerhead sharks are getting smaller.
“Large apex sharks are able to prey on larger animals such as turtles, dolphins and dugongs, and their widespread movement patterns along the coastline connects coral reefs, seagrass beds and coastal ecosystems.
“Such losses of apex sharks is likely to have changed the structure of coastal food webs over the past half century.”
The research is published in Communications Biology (DOI: 10.1038/s42003-018-0233-1).