Travel distances of juvenile fish key to better conservation

Travel distances of juvenile fish key to better conservation

As part of the largest, most comprehensive study of larval dispersal ever conducted, scientists were able to determine that most of the juvenile clownfish stayed relatively close to home, settling at mean distances of 10-15 kilometers from their natal reefs. Credit: Simon Thorrold, Woods Hole Oceanographic Institution

Marine reserves—sections of the ocean where fishing is prohibited—promote coral reef sustainability by preventing overfishing and increasing fish abundance and diversity. But to be effective, they need to be sized right, and in a way that accounts for how far juvenile fish travel away from their parents after spawning.

Scientists at the Woods Hole Oceanographic Institution (WHOI), along with researchers from Australia, France, and Saudi Arabia, have successfully measured the dispersal distances of two species across a 3,000 square mile section of the ocean—an area the size of Yellowstone National Park. The study, published in the May 8, 2017, issue of the journal Nature Ecology & Evolution, marks the largest, most comprehensive study of larval dispersal ever conducted and has important implications for the sizing and spacing of .

“How far fish will disperse in their lifetimes is critical when you start thinking about how marine reserves should be designed,” said Simon Thorrold, co-author of the study and a senior scientist at WHOI. “This is the first time we’ve been able to measure dispersal distances on spatial scales that are relevant to marine reserves, which means we can now provide data that informs management on optimal spacing and sizing.”

Size matters

Marine reserves come in many shapes and sizes. But if a reserve is too small, it can’t accommodate enough larvae to sustain populations. And if it’s too big, larvae will simply stay within the confines of the reserve without contributing to surrounding fisheries—a critical secondary role marine reserves need to play to improve fisheries management.

To get a read on fish dispersal in the past, scientists relied on population genetics approaches that lacked the power to measure dispersal over space and time scales relevant to protected areas of the ocean. More recently, ecologists have turned to computer-generated models of water currents to track particles through virtual oceans. According to Thorrold, this approach also has limitations since there was no way to verify the accuracy of the models. “The software can generate a lot of cool-looking graphs, but it was impossible to test the skill of those models in any real way.”

An empirical approach

To overcome these limitations, Thorrold and his colleagues took direct measurements of dispersal distances in the field. They collected DNA samples from thousands of adult and juvenile clownfish and butterflyfish throughout Kimbe Bay, Papua New Guinea, in 2009 and 2011. The entire sampling process occurred underwater, with the 30-person science team spending thousands of man-hours on SCUBA over several weeks in the field each year.

Travel distances of juvenile fish key to better conservation

How far fish will disperse in their lifetimes is critical when you start thinking about how marine reserves should be designed, according to WHOI biologist Simon Thorrold, co-author of the new study. The research team found that butterflyfish dispersed further than clownfish, averaging distances of 43-64 kilometers before settling into their new habitats. Credit: Simon Thorrold, Woods Hole Oceanographic Institution)

When the scientists returned to the lab, they used DNA parentage analysis, a sequencing technique that allowed them to match the juveniles up with their parents based on the DNA samples and spawning and settlement location data. From that, they were able to determine that most of the juvenile clownfish stayed relatively close to home, settling at mean distances of 10-15 kilometers from their natal reefs. The butterflyfish dispersed further, averaging distances of 43-64 kilometers before settling into their new habitats.

“Since we knew the respective locations of the adults and babies, we were able to come up with the exact linear distances that the larvae had dispersed. We’re no longer talking about estimates,” said Thorrold.

Benefits beyond design

In addition to helping inform the design of protected areas, the measurements can help to test the ability of reserves to perform key conservation functions. For example, one way a marine reserve network may improve fish population sustainability is through the so-called “rescue effect.” In theory, if fish in a reserve suffer catastrophic mortality, the reserve can be repopulated by larvae from other reserves within the network. Thorrold and colleagues were able to track larvae from one reserve to another in the study area, confirming that rescue effect is likely to occur in real-world reserve networks.

The dispersal measurements could also allow fisheries managers to monitor the effectiveness of existing reserves, helping answer the question of whether or not a particular reserve is contributing to fish populations beyond its boundaries. This, according to Thorrold, has been a big unknown.

“If you can trace larvae from one reserve to a place that’s fished, you can come up with a direct measure of how many fish the reserve is contributing to exploited populations beyond the reserve,” he said. “This helps when trying to convince fishermen that networks of marine reserves are a good management tool.”

Future work

According to Thorrold, as coral reef seascapes continue to face pressure from man-made stressors, marine reserves will continue to serve as an important conservation management tool. As such, it will become increasingly important to be able to provide direct measurements of larval dispersal, and find ways to apply the information to other regions of the ocean.

“The next thing we are working on is developing a coupled bio-physical model of the area that will allow us to take the results from this study and generalize them to other coral reef seascapes around the world,” he said. “Limited resources for ocean management, particularly in the developing world, means that we need to maximize the chances of successful conservation outcomes from these efforts. These types of scientific insights will be critical for ongoing efforts to promote resilience of coral reef ecosystems in the face of human exploitation and climate change.”

Explore further: DNA evidence shows that marine reserves help to sustain fisheries

Sharks Hanging Out With People

New research finds human-shark interactions can take place without long-term effects on the sharks.

A multimillion-dollar global industry exists in response to scuba divers’ interest in getting face time with the ocean predators. Options include cage diving with white sharks in South Africa and Guadeloupe Island; shark feeding in the Bahamas, Mexico, or Fiji; and diving with huge schools of hammerheads in Cocos Island and Galapagos.

But as most scuba divers know—and previous studies have shown—sharks more commonly swim away from people than toward them. Does that avoidance behavior persist after the divers leave? Do sharks steer clear of sites that are frequented by divers?

“Unfortunately, human impacts on shark populations are ubiquitous on our planet,” says lead author Darcy Bradley, a postdoctoral researcher at the University of California, Santa Barbara’s Bren School of Environmental Science & Management. “That makes it difficult to separate shark behavioral changes due to scuba diving from behavioral changes caused by other human activities like fishing.”

The researchers went to Palmyra, a remote atoll in the central Pacific Ocean, where shark populations are healthy, fishing is not allowed, and divers rarely enter the majority of its near pristine underwater world. However, Palmyra is home to a small scientific research station, where researchers dive in a handful of locations. This made the atoll an ideal site for studying whether and how shark abundance and behavior differ between locations where diving is more common and those where it is not.

As reported in the Marine Ecology Progress Series, the researchers used baited remote underwater video systems—cameras lowered to the ocean floor with a small amount of bait—to survey sharks and other predators from the surrounding reef.

Tourists see fewer wolves when hunters get close

“After reviewing 80 hours of underwater footage taken from video surveys conducted in 2015—14 years after Palmyra was established as a wildlife refuge and scientific diving activities began—we found that shark abundance and shark behavior were the same at sites with and without a long history of scuba diving,” says coauthor Jennifer Caselle, a research biologist at UC Santa Barbara’s Marine Science Institute.

“Our results suggest that humans can interact with reef sharks without long-term behavioral impacts,” Bradley says. “That’s good news. It means that well-regulated shark diving tourism doesn’t necessarily undermine shark conservation goals.”

Researchers from Florida International University also contributed to the work.

Source: UC Santa Barbara

Fiji shows off sharks – Hawaii should do the same

PACIFIC HARBOUR, Fiji — As the divers’ air bubbles sashayed to the surface 100 feet above, their eyes focused on the scene emerging in front of them.

A 10-foot female bull shark appeared, almost mirage-like, out of the blue beyond. Another entered, stage left. Then one on the right. And another.

Soon a dozen sharks were in view, some swimming within several feet of scuba spectators lined up to watch along a row of coral and rocky rubble.


Bull sharks are the main attraction at a dive site in Fiji’s first marine national park.

Courtesy: Alana Hong Eagle

A few certified dive masters orchestrated the show. Some hold dual roles as marine biologists and sheriffs of the sea, working to enforce the laws of Fiji’s first marine national park while educating a steady stream of tourists on the importance of shark conservation.

One swam over to a submerged trash can that the dive operator had placed for the show. He pulled out a tuna head and with a flip of his wrist let it float away, its lifeless eyes unblinking.

A 9-foot bull shark cut through a school of smaller fish, devouring it in a couple of bites. A few of the dozen divers who paid to see this world-renowned spectacle emitted muffled squeals of joy through their breathing regulators.

As Ben Saqata of Beqa Adventure Divers explained on the boat ride out to the lagoon for the dive, these apex predators are key to maintaining balanced ecosystems. They keep species down the food chain in check so those animals in turn do not dominate the food sources below them, and so on.

He said he’s seen this no-take zone become a spawning ground for other fish, and there’s been a spillover effect, pleasing local fishermen who ply the waters outside its boundaries.

Sharks are revered by many Fijians but face threats from fishermen who target certain species for their meat or fins. Protecting them has been the reserve’s primary mission, but the benefit has extended far beyond by generating millions of tourism dollars for the local economy.


Ben Saqata of Beqa Adventure Divers is a marine biologist and deputized by the Fijian government to enforce the laws restricting fishing inside the Shark Reef Marine Reserve.

Courtesy: Alana Hong Eagle

Hawaii’s Conservation Efforts

Hawaii officials have been looking to places like Fiji for marine management ideas, given their similarities as remote island chains with economies driven largely by visitors who travel from afar to experience the natural resources.

Gov. David Ige announced in September his commitment for the state to “effectively manage” 30 percent of its nearshore fisheries by 2030. It’s unclear what that will entail, but it has at least set a course.

The plan does not have specific shark provisions and the governor, through his spokeswoman, declined to comment for this report.

Ige has expressed concern about Hawaii’s marine ecosystems in a broader sense at recent environmental conferences, including the International Union for Conservation of Nature’s World Conservation Congress in September, where he raised the issue of climate change and its disproportionate effects on island communities.


Gov. David Ige during a press conference at the IUCN meeting in Honolulu in September.

Cory Lum/Civil Beat

A new marine reserve to protect nearshore waters may not be in the state’s future. But officials are looking at the process Fiji went through to establish its park

The idea of incorporating local knowledge, the best available science and traditional practices, as Fiji, the Republic of Palau and other nations have done, is being used to manage smaller areas around Hawaii, said state Department of Land and Natural Resources Director Suzanne Case.

The north shore of Kauai and west side of the Big Island now have so-called community-based subsistence fishing areas. Others are in the works for Maui and Oahu. Each has its own management measures specific to the area, developed by those communities in conjunction with the state.

DLNR spokesman Dan Dennison said the department doesn’t have any internal expertise on sharks and deferred to University of Hawaii scientist Carl Meyer, who did not respond to requests for comment. The state Division of Aquatic Resources also did not respond to a request for comment.


Blacktip reef sharks are one of several species regularly encountered in the Shark Reef Marine Reserve,

Courtesy: Alana Hong Eagle

“I’m sure that Hawaii can learn from anyplace in the world that’s trying new things,” said William Aila, a fisherman, diver and former head of DLNR who now serves as deputy director of the state Department of Hawaiian Home Lands.

“If you take a look at the community-based fisheries management initiatives that are going on around the state, you have sort of this lab of fisheries management that’s coming from the ground up,” he said.

“I know for a fact that those folks are researching everything that’s happened in the past in Hawaii, everything that’s happened in other parts of the world, and that’s being included in their individual assessments of what they’d like to see happen,” Aila said.

‘Conservation Project Running A Dive Shop’

Beqa Adventure Divers, based on the south shore of Fiji’s main island, has been taking customers out to dive the reefs of Viti Levu since 2004 when the Shark Reef Marine Reserve was established.

The reserve, which became a national park in 2014, offers exhilarating sights of several shark species amid plentiful corals and a wide range of fish of all sizes and colors.

The company’s director, Mike Neumann, said its offering a lot more than good diving.

“We’re a conservation project running a dive shop,” he said. “It’s not the other way around.”


A gray shark cruises by during a December dive in the Shark Reef Marine Reserve in Fiji.

Nathan Eagle/Civil Beat

He acknowledges that not everyone supports feeding the sharks. Critics say it changes their natural behavior and can have other consequences, but Neumann pointed at studies showing little to no effect on the shark populations in the reserve. He noted there have been no shark bite incidents during any of the tours.

One study found the contents of their stomachs was less than 1 percent tuna, which is what the dive shop feeds them. Another found less diversity in the sharks going to the site; the bulls were outcompeting the tigers and other species.

“People who feed sharks are called fishermen — not a few dive operators,” Neumann said.

The bigger point, he said, has nothing to do with feeding or not feeding sharks, but instead the reserve’s value as “a proof of the concept that something like this can be done in conservation.”


Ben Saqata, center, a marine biologist with Beqa Adventure Divers, says there’s been a spillover effect from the reserve that helps boost fish populations beyond its boundaries.

Courtesy: Alana Hong Eagle

Beqa Adventure Divers partnered with Fiji’s government and neighboring villages to establish the reserve and now runs the park’s day-to-day operations.

Its employees include marine biologists who conduct shark research that’s been cited in international studies. All the workers are deputized fish wardens who have police powers to enforce laws banning fishing. That makes up for the government’s lack of resources for enforcement — a major issue in any marine protected area, including the massive Papahanaumokuakea Marine National Monument around the Northwestern Hawaiian Islands. 

Customers pay a marine park fee to dive in the reserve. The money goes to the villages in exchange for giving up their right to fish there.

Neumann said their waters have proven more valuable as a protected reserve than a fishing ground. A 2011 study determined shark diving contributed $42.2 million to Fiji’s economy the previous year.


Viti Levu, the main island of Fiji, is receiving a steady stream of international visitors, many of whom are coming to dive with sharks.

Nathan Eagle/Civil Beat

“You can’t protect jaguars if you don’t protect the jungle,” Neumann said.

Shark conservation efforts have increased in recent years in Hawaii but there’s debate in the scientific community over whether Hawaii’s nearshore sharks need further protection.

Unlike in Fiji and other parts of the Pacific, sharks are not targeted in Hawaii for food and the state passed a ban on the trade of shark fins in 2010.

The real threat to sharks in Hawaii comes from their food sources being depleted, be it from commercial and recreational fishermen or habitat loss due to polluted runoff.

Counting The Sharks

Scientist Marc Nadon of the Joint Institute for Marine and Atmospheric Research at the University of Hawaii and his colleagues wrote a paper in 2012 estimating that the number of Pacific reef sharks had plummeted 90 percent in Hawaii.

The study found reef shark populations to be below 10 percent of the level they should be around populated islands, but could not determine the reasons for this depletion. That paper speculated that commercial and recreational fishing and an overall reduction in the amount of fish the sharks eat could explain why.

A follow-up to that study is expected within a few months, which could make its findings more defensible. The original study was criticized by some people because it relied on the observations of divers towed behind boats at a maximum 30 meters’ depth. The follow-up study uses cameras that extend down to 100 meters or more.

“We’re not saying there are no more reef sharks in the Main Hawaiian Islands, they’re still there. We’re talking about abundance,” Nadon said.

“Most people just snorkel around the main eight Hawaiian Islands and that’s their experience of what this is,” he said. “But if you go to remote areas of the Pacific, you almost don’t need the data. You just see it.”

Designating certain waters as protected marine areas would not be enough to stop the decline in reef shark stocks, Nadon said.

“The recent implementation of marine national monuments at most isolated U.S. Pacific islands may substantially increase the probability of persistence of reef shark populations, but effective enforcement and additional fishing regulations elsewhere would also be necessary to slow the decline of these species,” he said.


A 2011 study determined shark diving alone contributed $42.2 million to Fiji’s economy.

Nathan Eagle/Civil Beat

Scientist Kim Holland of the University of Hawaii’s Hawaii Institute of Marine Biology was among those skeptical of the findings that showed reef shark populations at such low levels in Hawaii.

Just because people do not encounter sharks as frequently in Hawaii as they do in places like Fiji does not mean they are not around, he said, noting poorer water visibility as one reason.

“There’s a lot of hidden shark biomass in Hawaiian waters,” Holland said.

He was careful to make the distinction between nearshore sharks, like blacktip reef sharks, and open-ocean sharks, like tiger sharks. The latter category faces significant threats, he said, because they are targeted for their fins and killed as bycatch.

If there has been a depletion in nearshore sharks, Holland said he would agree with Nadon it’s likely due to insufficient food, which would mean that addressing fishing could help.

“One of the real pressures on our reefs is gill nets are still allowed to be used in Hawaiian waters and not in the traditional sense — Polynesians only had so much capacity,” Holland said.

Restricting gill nets would help control fishing pressure on the reefs, he said, which would in turn help maintain healthy shark populations.

“It’s a hard nut to crack though because of the intersection between modern fisheries biology and advocating for traditional harvesting rights,” Holland said. “You get into that whole tension between modern fisheries management and traditional gathering rights.”


Sharks are plentiful in Fiji’s Shark Reef Marine Reserve.

Courtesy: Alana Hong Eagle

That’s a familiar battle in Fiji. Neumann, the shark dive director, said the problem is that the villages there have been fishing the same waters for generations but the fish population did not keep up with the human population.

“You can go anywhere down the coast and there is nothing there,” he said. “But how do you tell subsistence fishermen that they have to manage their resource?”

To Aila, the answer could be in the Hawaiian concept of reciprocity.

“It’s not only about the ‘right’ to fish,” he said. “It’s about, I have a responsibility to fish and in how I conduct myself.”

In order to continue fishing, Aila said, “we have to make sure we give back in terms of management or on a personal level a relationship — cleaning up marine debris or fishing lines.”

Neumann, who has spent time in Hawaii and is close with many in the science and conservation communities here, said he isn’t holding his breath for Hawaii to take strong actions to protect its waters even though the state’s economy and the public’s health depends on it.

“Unless the government gets some balls, it’s not going to happen,” he said. “The question for Hawaii is what kind of ocean do you want to show your tourists?”



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