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Study: Sharks Bounce Back When Coral Reefs Are Strictly Protected

With more nations considering creating shark sanctuaries, new research shows that strict fishing bans can help predator populations recover rapidly, as has been the case on a remote Australian reef.

Written by Matthew O. Berger Published on Read time Approx. 5 minutes
Grey Reef Sharks prowling Pacific waters.Reinhard Dirscherl/ullstein bild via Getty Images

This month, Samoa became the latest among a handful of Pacific island nations to designate its waters a shark sanctuary to protect the top predators that are crucial to healthy ocean ecosystems.

New regulations, announced on March 1 by Prime Minister Tuilaepa Sailele Malielegaoi, ban commercial fishing of sharks and rays within Samoa’s exclusive economic zone, as well as the possession or trade of sharks or shark products. Fishing gear often used to target sharks will also be banned.

But will such measures ultimately make a difference for local shark populations? New research offers a ray of hope — but only if the bans are strongly enforced.

Today, sharks face a multitude of threats. The Pew Charitable Trusts estimates that, globally, anywhere from 63 million to 273 million sharks are killed in commercial fisheries a year – and the effects of fishing on shark populations can be long-lasting. One recent study found that even shark populations thought to be healthy on long-protected island reefs in the remote Chagos Archipelago are to this day still smaller than in the years before industrial-scale exploitation.

However, the latest findings, published in the journal Biological Conservation, show sharks in a marine protected area (MPA) off Australia recovering faster than expected, less than a decade after enforcement of no-take fishing rules in the conservation zone were ramped up.

“It’s unlikely that we would have observed such dramatic recovery over a short period if enforcement had only been sporadic throughout this time,” said Conrad Speed, the lead author of the study and a research scientist at the Australian Institute of Marine Science.

Map of 17 shark sanctuaries around the world, which all have been established since 2009. (Courtesy of the Pew Charitable Trusts)

The Ashmore Reef National Nature Reserve was established in 1983. Its fishing ban was more far-reaching than the one that will be imposed in Samoa. The government banned all fishing around an uninhabited group of islands 320km (200 miles) off the northwestern Australian coast. But there was only occasional monitoring there, meaning fishing of sharks and other species continued to some degree, according to the study. That changed in 2008, when the government stationed a vessel at the reef 300 days a year.

In 2004, scientists had surveyed species at the sanctuary using baited remote underwater video surveys, in which fish are lured into view of cameras so researchers can collect samples of the marine life in certain areas.

In 2016, researchers conducted another survey. What they found was a greater proportion of both bigger, apex predator sharks and reef sharks among the species observed than during the sampling 12 years prior. Conversely, there were relatively fewer species lower on the food chain.

They also observed more than a fourfold increase in the abundance of gray reef sharks, an uptick that showed what they termed a “rapid recovery,” with reef shark abundance increasing 1.6 to 6.6 times faster than models would have predicted. That recovery shows that “shark communities can recover rapidly after exploitation in a well-managed no-take MPA,” according to the study, which was supported by Global FinPrint, an initiative of Microsoft cofounder Paul G. Allen’s foundation that works to fill information gaps about sharks and rays.

Speed said that the unexpectedly fast recovery could be due to sharks coming to the reef from elsewhere – but that this should be rare, given the distance to the next reef and the low numbers of sharks there. Another possibility is that population prediction models didn’t fully take into account how having fewer sharks around in the pre-enforcement period meant those that remained faced less competition and higher survival rates.

Whatever the explanation for the speed of the recovery, the bounce-back itself was made possible by the high level of enforcement of the fishing ban, according to the researchers. The study noted that shark abundance at Ashmore Reef in 2004 resembled that around Scott Reef, a nearby reef where shark fishing still takes place. In 2016, it resembled the levels observed at Rowley Shoals, a reef group in the Timor Sea where fishing has been banned and strictly enforced for more than 25 years.

To get recovery rates like those observed at Ashmore Reef, strict enforcement may be required, agreed Mark Bond, a postdoctoral researcher who studies sharks at Florida International University (he was not involved in the study). “Though we may see an improvement with less strict enforcement, I believe these conclusive results are a direct relation to the level of enforcement,” he said.

But that doesn’t mean anything less than strict enforcement is pointless. “Strict enforcement is always going to be the ideal,” said Jen Sawada of the Pew Charitable Trusts’ global shark conservation campaign. “But having a framework to enforce is better than nothing, even if not strictly enforced. Enforcement is something that can improve over time.”

Bond found the study’s results to be “somewhat surprising,” since he’d expected the recovery to take more time given sharks’ long lives, slow reproduction and limited number of offspring. But mostly, he said, it’s “exciting that MPAs, given strict enforcement, are an effective conservation strategy for reef-associated sharks.”

Ashmore Reef is just 583 square kilometers (225 square miles) and has the added benefits of being isolated, but Speed said the results suggest similar, small MPAs could see similar recoveries in shark populations “provided there is adequate enforcement.”

However, Samoa’s planned shark sanctuary won’t be as comprehensive as the MPA at Ashmore Reef, since other kinds of fishing will still be allowed. Bond cautioned that these kinds of more narrowly targeted shark sanctuaries may not provide the same protections for the prey on which sharks rely, which could slow down their recovery in comparison.

The research could give further momentum to conservationists to push for new and expanded marine protected areas that aim to safeguard sensitive habitats and help replenish species depleted by fishing and other activities. Done right, they say, those protections could lead to more fish spilling over to waters outside reserves, replenishing fisheries and boosting tourism and recreational fishing.

That movement has ramped up recently, as the 2020 deadline to meet a United Nations goal of setting aside 10 percent of the ocean in reserves by 2020 draws nearer. A study published last month found “remarkable progress in the last decade” toward that goal, but that just 3.6 percent of the ocean was in MPAs and only 2 percent in “strongly” or “fully protected” areas.

Bond, while noting that Samoa’s protections aren’t the same as those at Ashmore Reef, said he hopes to see a similarly strong shark recovery. “I hope to see equally robust data from Samoa in the coming years,” he said.

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