There has been a striking decline in the number of large sharks caught off Queensland’s coast over the past 50 years, suggesting that populations have declined dramatically.
Our study, published in the journal Communications Biology, used historical data from the Queensland Shark Control Program.
Catch numbers of large apex sharks (hammerheads, tigers and white sharks) declined by 74-92%, and the chance of catching no sharks at any given beach per year has increased by as much as seven-fold.
Coinciding with ongoing declines in numbers of sharks in nets and drum lines, the probability of recording mature male and females has declined over the past two decades.
Our discovery is at odds with recent media reports of “booming” shark numbers reaching “plague” along our coastlines. The problem with those claims is that we previously had little idea of what the “natural” historical shark population would have been.
Why is the decline of sharks on the Queensland coastline a cause for concern? Large apex sharks have unique roles in coastal ecosystems, preying on weak and injured turtles, dolphins and dugongs, actively scavenging on dead whale carcasses, and connecting coral reefs, seagrass beds and coastal ecosystems.
Australia’s mixed view on sharks
As a nation, Australia has a long history with sharks. Some of the oldest stories in the world were written by the indigenous Yanyuwa people in the Northern Territory some 40,000 years ago, describing how the landscape of their coastal homeland was created by tiger sharks.
European settlers in the late 18th and early 19th centuries further described Australian coastlines as being “chock-full of sharks”, and upon visiting Sydney in 1895, the US author Mark Twain remarked:
The government pays a bounty of the shark; to get the bounty the fishermen bait the hook or the seine with agreeable mutton; the news spreads and the sharks come from all over the Pacific Ocean to get the free board. In time the shark culture will be one of the most successful things in the colony.
With the rise of Australian beach and surf culture, and the growing population density in coastal communities in the mid-20th century, increasing numbers of unprovoked fatal encounters with sharks occurred along the Queensland and New South Wales coastlines.
White sharks were extensively targeted and killed in “game fishing” tournaments, and harmless grey nurse sharks were hunted almost to extinction through recreational spearfishing in the 1950s and 1960s.
Yet despite this long history of shark exploitation, the historical baseline populations of sharks off Australia’s east coast were largely unknown.
Through mesh nets and baited drumlines, the Queensland Shark Control Program targets large sharks, with the aim of reducing local populations and minimise encounters between sharks and humans. Records of shark catches dating back as far as the 1960s provide a unique window into the past on Queensland beaches.
While we will never know exactly how many sharks roamed these waters more than half a century ago, the data points to radical changes in our coastal ecosystems since the 1960s.
The exact causes of declining shark numbers are difficult to pinpoint, largely because of a lack of detailed records from commercial or recreational fisheries before the 2000s. The Queensland government also acknowledges that the program itself has a direct impact on shark populations by selectively removing large, reproductively mature sharks from the population.
The data indicates that two hammerhead species – the scalloped and great hammerheads, both of which are listed as globally endangered – have declined by as much as 92% in Queensland over the past half century.
Similarly, the once-abundant white sharks have also shown no sign of recovery, despite a complete ban on commercial and recreational fishing in Queensland, implemented more than two decades ago.
The idea that shark populations are reaching “plague” proportions in recent years may represent a classic case of shifting baseline syndrome. Using shark numbers from recent history as a baseline may give a false perception that populations are “exploding”, whereas records from fifty years ago indicate that present day numbers are a fraction of what they once were.
Our results indicate that large shark species are becoming increasingly rare along Australia’s coastline. We should not be concerned about a “plague” of sharks, but rather the opposite: the fact that previously abundant apex shark species are increasingly at risk.
George Roff, Postdoctoral Research Fellow, The University of Queensland and Christopher Brown, Research Fellow, Australian Rivers Institute, Griffith UniversityThis article is republished from The Conversation under a Creative Commons license. Read the original article.