A traditional fisherman on a coral reef in Papua New Guinea. Image credit: Tane Sinclair-Taylor

Coral reefs are suffering worldwide. Consider the global coral bleaching that began in 2014 and is still ongoing, making it the longest bleaching event in recorded history. The damage has been catastrophic, affecting 38 percent of all reefs. And yet some reefs are faring better than others. Why?

The cover story of the current issue of Nature explores that question using a decidedly unorthodox approach for the study of biological entities like coral reefs: social science. Joshua Cinner and his colleagues borrowed the social science strategy of identifying “bright spots”—a tactic previously applied only to human systems such as businesses, organizations, families, villages—to tease out the differences between thriving and dying coral reefs.

Cinner, of the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Townsville, Australia, explained to mental_floss, “Bright spots are reefs that have more fish than expected based on their exposure to pressure like human population, poverty, and unfavorable environmental conditions. They’re not necessarily pristine reefs, but reefs that are doing better than they should given their circumstances. They’re essentially punching above their weight.”

The long-term goal is to identify how those bright spots are doing well and what contributes to “dark spots”—reefs doing more poorly than others facing similar pressures. Ideally, finding out the differences between these outliers and their peers may allow communities to apply bright-spot strategies to dark spots and improve local reefs.

“Most scientific studies look at averages or trends, and there’s a lot of research looking at how global fish stocks are declining,” said Cinner, who led a team of 38 co-authors from 10 countries for the study. “We focused on the outliers, the places that are bucking the trend.”

The research was inspired by a presentation Cinner heard at a conference about how malnutrition in a Vietnamese village was alleviated by pinpointing bright spots. Save the Children interviewed Vietnamese families whose children managed to stay well fed despite having no more food than other families. The organization learned that mothers of the healthier children were adding small shrimps they collected from the rice fields to their children’s food and feeding them smaller but more frequent meals. When Save the Children helped these mothers teach other families these strategies, child malnutrition rates dropped.

The innovative strategy piqued Cinner’s interest. He thought they might be able to apply a similar approach to coral reef conservation. “All the scientists I was working with really wanted to try something different, so the amount of collaboration I received was really incredible,” he said. “It’s by far the biggest attempt to integrate social and ecological data on coral reefs.”

Dozens of marine scientists handed over their data for the project, allowing Cinner’s team to analyze nearly 7000 surveys of 2500 reef sites in 46 states, countries, and territories—the largest global data set available for coral reefs. They used the amount of fish as a measure of health because the fish population is integral to the local ecosystem. When a massive bleaching event occurs, an intact reef fish community can help the reef recover, Cinner said.

“No matter how much fish are on a reef, it’s not going to climate-proof that reef or make it resistant to bleaching,” he explained. However, on the “continuous battlefield between coral and algae, carnivorous fish on a reef can keep the algae in check.”

Tane Sinclair-Taylor

Using a previous study’s estimate that an unfished reef in average conditions should have approximately 1000 kilograms of biomass (the total weight of all animals) per hectare (kg/ha), he compared this figure to the biomass at each coral reef. The average biomass of coral reefs was 762 kg/ha, but half of all coral reefs had less than 391 kg/ha, revealing a wide variation in reefs across the globe.

After taking into account geographical differences of the reefs (slope, depth, shape, and other characteristics), as well as the size, economics, and governance of nearby human populations, Cinner found the biggest influence on coral reef health was the size of the nearest city or port and how accessible the reef was to it. 

Next, they tried to identify the cause of this reduced biomass. Was it tourism, sedimentation from erosion, agricultural runoff, or fishing? When they compared the biomass of fished versus non-fished species, only the species targeted for catch had smaller populations, indicating that overfishing was the culprit. If sedimentation, tourism, or pollution were to blame, both fished and non-fished species would have suffered proportionally; yet only the fish headed for dinner plates were declining.

Then Cinner’s team identified the top 5 percent of coral reefs—15 locations—as bright spots. These reefs have more fish then would be expected and are concentrated mostly in the Pacific and the Indo-Pacific regions. Only one-third were in remote areas while the others were in populated areas, such as the Solomon Islands, Papua New Guinea, and Bird’s Head Peninsula in Indonesia.

They also identified 35 dark spots. Interestingly, they're located near countries representing the entire economic spectrum: lower-income countries (including Kenya and Madagascar), middle-income countries (such as Seychelles and Venezuela), and high-income countries (including the U.S. and Australia). Even protected, remote areas near pristine islands, such as the Northwest Hawaiian Islands, weren’t doing as well as they should be.

To learn what was driving the differences, the team interviewed local experts at all the bright spots and dark spots, as well as at 14 average spots.

Some differences, such as having deeper water at bright spots or more coral bleaching and cyclones at dark spots, cannot be changed. Others, such as a community’s dependence on a reef, should not be changed because it would affect people’s livelihoods, Cinner said.

But other conditions are more malleable. Dark spots had more technology-driven fishing operations that optimized catching and storing fish. For example, they had more access to freezers and more intensive fishing gear such as seine nets, which are huge, weighted horizontal nets that fishermen drag across large areas.

Meanwhile, bright spots tended to have better social and institutional conditions, "with higher levels of engagement from locals in resource management and decision-making,” Cinner said. “This involvement and ownership creates a situation where communities can develop locally appropriate, creative solutions for themselves, and the high dependence on reef resources may create the incentives to create those solutions.”

The next step is in-depth field study at bright and dark spots and, eventually, seeing if lessons learned from bright spots can brighten up those dark spots. 

“Our findings introduce a realm of possibilities for improving the sustainability of reef systems,” Cinner said.