Sharks are the stewards of coral reef systems. As apex predators, they pick off sick and weak fish, leaving the stronger ones to reproduce, which helps maintain the health and vitality of the marine ecosystem. But according to a new study published this week in Nature, sharks have disappeared from many coral reefs around the world, marking a widespread decline in global shark numbers.

The landmark study, which involved 121 scientists and 731 volunteers, and took seven years to complete, surveyed shark populations in coastal regions around the world using video cameras planted on reefs. The project was supported by a number of institutions and groups, including Global FinPrint, a program that assesses the health of shark populations with underwater census work

Previous surveys have tended to rely on divers to visually count sharks, but this can lead to inaccuracies since sharks tend to rove around, the paper suggests. By contrast, the new study planted baited remote underwater video stations (BRUVS), or “Chum Cams,” on 371 reefs in 58 countries and territories. The BRUVS were always deployed in the daytime, and kept on the reef for approximately an hour at a time.

When possible, the researchers tried to survey two different types of reef sites in each nation: one site that was protected and one site that was open to fishing.

“As a result of doing that, we didn’t randomly survey reefs around the world,” Demian Chapman, co-author of the study, associate professor in the department of biological sciences at Florida International University (FIU), and co-lead of the Global FinPrint project, told Mongabay. “If we had randomly surveyed reefs … we mostly would have gotten reefs that were open to fishing because there are more of them than there are protected areas, and we probably would have found that reef sharks were missing … a lot more.”

After collecting 15,165 hours of video, the research team found that sharks were absent from 20% of the surveyed reefs. In six locations, including the Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles and Qatar, only three sharks were observed in about 800 hours of footage, suggesting that sharks were functionally extinct in many parts, and could no longer fulfill their normal role in the marine ecosystem.

In 38 of the surveyed nations, including Fiji, Madagascar and Indonesia, there were only a moderate number of sharks, which is also cause for concern, Chapman said.

“If these nations don’t put in these conservation measures, they’re going to go in the direction of the places that are really bad,” he said.

In most cases, the absence of sharks was related to the “socio-economic conditions” of nearby human settlements, the paper suggests, including “the size and proximity of the nearest market, poor governance and the density of the human population.” For instance, poorly managed fisheries, especially ones that use longlines and gillnets, could easily wipe out a local shark population.

“It’s probably a good wake up call for certain regions to pay attention, and to look at specific recommendations that could help the ecosystem recover,” Michael Berumen, co-author of the study and professor of marine science at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, told Mongabay.

But it’s not all bad news. The research team also found that sharks were thriving on reefs in places like the Bahamas, continental Australia, the Solomon Islands, the Federated States of Micronesia, and French Polynesia. This success was usually owed to full bans on shark fishing, or well-managed, science-based fishery management, according to the study.

The abundance of sharks in certain places was a pleasant surprise to Chapman, who said he was bracing himself for a “bleaker picture.”

“There were quite a few good places that still remained … scattered in the Central Pacific Ocean,” he said. “We think of these places as reservoirs of hope.”

The study also highlighted that shark populations can easily be maintained with strong conservation practices, such as fishery management and the implementation of marine protected areas. This information can help strengthen global conservation efforts to maintain healthy shark populations, and even help reinstate sharks in places where they’ve been depleted.

“Sharks can actually spread from those places [with healthy populations] and seed the recovery in other places, once these other places put some conservation measures in place,” Chapman said.

“It’s almost surprising how much potential there is for recovery,” Berumen said. “Some of the [conservation] tools could have a huge impact, and they’re not hard [to implement]. It’s great to see that there’s cause for hope, and [there’s an] ability for recovery in a lot of these populations.”

Chapman and his colleagues will be conducting further research based on the data collected for this project, including a study to try and understand how the presence of sharks influences other reef fish populations. “We’re nowhere near complete analyzing this data,” he said.

Berumen says that the study will also inform conservation efforts in many parts of the world, including Saudi Arabia, where the government is developing new ecotourism economies.

Researchers preparing BRUVS for deployment in the ocean. Image by Global FinPrint

“We are working very closely with those specific developments, and generally with the governing authorities, in terms of helping everybody understand the best ways to do this with minimal impact, and to keep the reef system in as good a shape as we can,” Berumen said.

While the paper found that shark populations near coral reefs systems are quite degraded in some parts of the world, other parts have great conservation potential.

“Sharks need help and they need our attention,” Berumen added. “There are places where that’s more urgent than others, but I’m not sure that there’s any place that we can just … ignore the issue.”

Source: Mongabay