Coral reefs in remote locations may sometimes be worse off than those near human fishing communities, says a new study that looked at why some reefs are thriving as others die off.

An international team of 39 scientists looked at the amount of fish at 2,514 coral reefs worldwide and tried to identify which ones had unexpectedly more or less fish than others, given the environmental and socioeconomic conditions nearby, such as the size of local populations and whether the reefs were in a marine reserve.

"We focused on the outliers, the places that were bucking the trend. And we looked for places that for all intents and purposes should have been degraded, but weren't," said Josh Cinner, lead author of the report in an interview with Nature, the journal that published the new research. Cinner is a professor of social science at James Cook University in Australia and at the ARC Centre for Excellence for Coral Reef Studies.

Reef fish

An international team of 39 scientists looked at the amount of fish, or biomass, at 2,514 coral reefs worldwide and tried to identify which reefs had unexpectedly more or less fish than others, given the environmental and socioeconomic conditions nearby. (Tane Sinclair-Taylor)

Fifteen reefs that were doing better than expected were identified as "bright spots," while 35 around the world doing worse than expected were identified as "dark spots."

In general, the researchers found that the bright spots, such as Karkar Island in Papua New Guinea, the Solomon Islands, and parts of Indonesia, were places where the local community relied heavily on the fish that lived at the reef for food and had the power to manage it locally as a community.

They were also places where the fish had access to deeper waters where they could take refuge.

Blocking outsiders

Rashid Sumaila, professor and director of the fisheries economics research unit at the University of British Columbia's fisheries centre, provided economic data and analysis about coral reefs around the world for the study.

He said when the local community has the power to manage a reef they rely on, they typically create a system to keep outsiders from fishing there.

Coral reefs

Traditional fishermen take their boats to a coral reef in Papua New Guinea. In general, the researchers found that coral reefs did unusually well in places where the local community relied heavily on the fish that lived at the reef for food and had the power to manage it locally. (Tane Sinclair-Taylor)

There were also often traditional customs that protected the fish. For example, Cinner said, on Karkar Island, the community would sometimes temporarily close part of a reef for a period of time in preparation for a special feast.

On the other hand, some of the dark spots were in relatively remote places such as the northwestern Hawaiian Islands, where there is no local community to protect the fish.

"Then it's almost [a] free for all," Sumaila said.

In many such locations, fishermen used nets that could snag and damage reefs. They also had access to freezers, which gave an incentive to catch and store extra fish, depleting stocks.

Coral reef

Fish around coral reefs did better if they had access to deeper waters where they could take refuge. (Tane Sinclair-Taylor)

Another factor was that reefs with dark spots had recently suffered an environmental shock, such as from a cyclone or from a rise in water temperatures that can bleach reefs. Many such shocks are occurring more frequently as a result of human-caused climate change.

With files from Reuters