Marine reserves provide essential ecological benefits, new research confirms. However, protected areas near large population centers are increasingly under pressure.

When scientists compared the health of marine reserves close to heavily populated areas with those far from civilization, they found a significant difference.

"Fish stocks were extremely depleted on reefs that were accessible to large human populations," Josh Cinner, a researcher at James Cook University's ARC Center of Excellence for Coral Reef Studies, said in a news release. "Compared to marine reserves far from these human pressures, reserves near high human pressure had only a quarter of the fish and were a hundred times less likely to have top predators such as sharks."

Scientists also compared marine reserves to surrounding habitats. The largest differences in ecological heath were found at the borders of marine reserves exposed to human pressures.

"A really novel and exciting part of our study found that the greatest difference in fish biomass between marine reserves and places open to fishing was in locations with medium to high human pressure," Cinner said. "This means that, for most fisheries species, marine reserves have the biggest bang where human pressures are medium to high."

Fishing and human development is prohibited inside marine reserves.

Reefs inside marine reserves near large human populations hosted five times more fish than nearby reefs open to commercial and recreational fishing.

During their surveys of marine reserves around the globe, scientists rarely observed top predators inside reserves exposed to human pressures.

"You'd have to do about 200 dives to see a top predator in reserves with the highest human pressure. But where human pressure was low, you'd be likely to see predators more than half the time," said Aaron MacNeil, a scientist from Dalhousie University.

Researchers shared their analysis of global marine reserve health in the journal PNAS.

Marine reserves and a total prohibition on fishing aren't a viable conservation strategy in places where access to fish stocks are vital to local populations. But researchers found other types of conservation strategies, including restricting the types of fishing gear used near vital habitat, also provide essential ecological benefits.

"These restrictions certainly had better outcomes than doing nothing, but not as good as marine reserves," ARC researcher Michele Barnes said. "They were a sort of compromise."

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