Reef sharks threatened by overfishing
A study by Australian scientists has warned that coral reef shark populations on the Great Barrier Reef are in the midst of a catastrophic collapse.
The research by William Robbins and colleagues, based at James Cook University and the ARC Centre of Excellence for Coral Reef Studies, found that grey reef shark numbers had already declined to around 3% of unfished levels, and are currently declining so fast that they could collapse to one thousandth of their unfished levels within 20 years if current conditions continue.
Whitetip reef sharks fared little better: they are currently at 20% of unfished levels, and are headed towards 5% within two decades.
The study is the first of its kind to combine direct underwater counts of shark abundance with mathematical models that project future population trends based on information about reef sharks鈥� current survival, growth and reproductive rates.
鈥淥ur research indicates that current reef shark abundances and levels of fishing pressure are simply not sustainable. Reef sharks are effectively on a fast track to 鈥榚cological extinction鈥� 鈥� becoming so rare that they will no longer play their part in the ecology and food web of the reef,鈥� says Robbins, the study鈥檚 lead author.
鈥淚t also suggests that immediate and substantial reductions in fishing pressure will be needed to give threatened populations any chance of recovery,鈥� he added.
The researchers also compared shark abundances in reefs that had been zoned for different levels of fishing in the decades preceding the study. They found that some types of no-take zones had worked very effectively for reef sharks, but that others had not. In particular, reef shark abundances in 鈥減ink zones鈥�, which are strictly policed no-take zones that require special permits to enter, were as large as on oceanic reefs with virtually no shark fishing. In contrast, shark abundances in 鈥済reen zones鈥�, where illegal fishing is much harder to prevent, were similar to abundances in legally fishable areas.
鈥淩eef sharks mature late in life, and, like many whales and dolphins, produce very few offspring,鈥� notes Mizue Hisano, a co-author of the study. 鈥淭his makes it hard for them to bounce back from even low levels of fishing, such as poaching in green zones.
The study highlights the importance of ensuring high compliance with no-take regulations. 鈥淭he recent re-zoning of the Great Barrier Reef, with its increased emphasis on building support for no-take zones among reef users, is an important step to increase the effectiveness of no-take areas,鈥� says Dr Sean Connolly, another of the study鈥檚 authors.
鈥淗owever, these efforts need to be combined with realistic limits on shark fishing for the ecosystem as a whole. Because shark fishing is intensifying, the population collapses that we have identified are likely to accelerate if we do not take action now.鈥�
It was especially disturbing that a collapse in shark populations had occurred on Australia鈥檚 Great Barrier Reef, says Hisano. 鈥淭he Great Barrier Reef is widely regarded as one of the world鈥檚 best-managed reef ecosystems. This means the situation may well be even more serious on reefs elsewhere in the world.鈥�
The team鈥檚 report 鈥淥ngoing collapse of coral reef shark populations鈥� appears in this week鈥檚 issue of Current Biology.
Source: James Cook University
