Australian researchers believe they have detected a dramatic decline in shark numbers over the last half century, findings that could challenge the use of culls as a way of responding to attacks.
The number of some shark species caught in nets off the State of Queensland have fallen between 74-92% in the last 55 years, according to a study published in Communications Biology on Thursday.
Researchers led by George Roff of the University of Queensland studied the catch from government-installed mesh nets and drum lines designed to prevent attacks on humans near the Great Barrier Reef.
An estimated 50,000 sharks have been caught by the programme.
The team found that in 1962, an average of 9.5 hammerheads were found per year, declining to 0.8 by 2016. Hammerheads are more likely to get caught in nets because of their shape.
Similar drops were seen in the numbers of whaler sharks.
Catch rates for tiger sharks, which are involved in many more attacks on humans, were stable over the first thirty years of the study period, but have since fallen.
Mr. Roff and his colleagues cautioned that the data was not standardised until 1992, and different baits and nets may have been used, making the data imperfect.
But in 1992, the report showed the catch of “hammerheads declined by 68%, whalers by 69%, tigers by 69% and white sharks by 42%”. The declines were largely due to the programme, as well as “depletion by recreational, and commercial fisheries”.
The falling numbers of apex predators like sharks could have a wide-ranging and as-yet understood impacts on marine ecosystems, the researchers warned.
“The extent and magnitude of decline in apex predators in the marine environment is less well understood” than the decline of those on land, the study said. The fall, coupled with data showing continued or even increased shark attacks in the region, could be an argument against culls as a means of prevention. “The extent to which targeting shark populations reduces interaction rates with humans in coastal ecosystems is contentious,” the researchers suggested.
There have been 27 shark attacks in Australia this year, according to data compiled by Sydney’s Taronga Zoo.
One attack, in Queensland’s Whitsunday Islands, in early November, was fatal.
Many experts point to the increased number of people going into the water as a reason for any increase in attacks.