The world's biggest marine reserve seems to be doing its job

Fishing boats around Papahānaumokuākea Marine National Monument in Hawai’i are catching more tuna than they used to, suggesting local populations are growing again.

The number of fish caught just outside a recently expanded marine protected area in Hawai’i has risen, a sign that quadrupling the size of the reserve in 2016 may have bolstered fish populations in the region.

When President Barack Obama enlarged the Papahānaumokuākea Marine National Monument around Hawai’i to 1,510,000 square kilometers, marine conservationists around the world rejoiced.

Fishers may have felt differently, however, as fishing inside the area is not allowed. Yet by creating a space for dwindling tuna populations to recover, proponents argued, the reserve would benefit fisheries as well.

As populations inside the reserve boundaries steadily increased, they predicted, the fish would spill over into the surrounding areas, increasing the amount of tuna available to catch.

Proving that is tricky, however, as tuna can’t be counted directly, and their numbers may rise or fall for a variety of reasons other than the expansion of a reserve. But the new study, published in Science this week, strongly suggests the number of fish caught just outside the MPA is higher now than it used to be, based on data collected between early 2010 and late 2019.

Alan Friedlander, chief scientist for the National Geographic Society’s Pristine Seas project, calls the study a “very rigorous test of spillover from marine protected areas.”

“This is one of the few studies to show actual spillover benefits, which are often difficult to prove. That is great news as it suggests a robust approach we can use to evaluate and improve protected areas elsewhere.”

More hooks in the water

Importantly, says John Lynham, an environmental economist at the University of Hawai’i and one of the study’s authors, the increase in tuna catches near the reserve held up even when looking at the average numbers caught by particular fishers. This shows the effect is not due to more effective crews now fishing local waters, he explains. To account for effort, catch numbers were divided by the ever-increasing number of fishing hooks in the area.

“There are about 150 fishing vessels based out of Hawai’i,” says Lynham, “putting on the order of 40 to 50 million hooks in the water every year. To keep catch rates high, fishers keep adding more.”

Hawai’i-based boats account for about two-thirds of regional fishing. “There are also vessels coming in from Japan, China, and Taiwan, but we don’t have access to detailed data on what they are catching,” he says.

Based on the catches reported by fishing vessels, as well as marine biologists from the National Oceanic and Atmospheric Administration, Lynham and colleagues found the catch per hook increased over the 10 years of the study. For every 10,000 hooks in the water, the team calculated for the period between 2010 and 2019, fishers were catching on average six more yellowfin and five more bigeye tuna per year after the expansion than before.

“That last one, especially, was a surprise,” says Lynham, “because it is economically much more important, and there were fewer indications of an increase.”

Catch rates were highest in the area right around the edges of the reserve, the team found, tailing off with increasing distance. “Exactly what we’d expect a spillover effect to look like,” Lynham adds.

A handsome handful

Some experts are skeptical. According to fisheries biologist Ray Hilborn of the University of Washington, fishing in the area was limited before expansion. “Only a handful of boats fished there.”

He believes the expansion therefore had a limited impact, and that any recovery would take more time. In addition, he says, tuna populations across the Western Pacific increased around the time of the expansion of the reserve.

Lynham, on the other hand, says the recorded number of boats fishing in the area before the reserve was expanded was hardly insignificant. “There were 95 boats in 2013, 89 in 2014, and 94 in 2015.”

He also points out that the study specifically accounted for the possible effect of an ocean-wide increase in tuna populations, and that catches nearer the reserve were usually higher nevertheless.

Whether the increased catch is enough to compensate for the negative effect that excluding fishers from the park had on catch rates is harder to calculate, says Lynham, and would need further analysis. This is complicated by the fact that tuna numbers had been decreasing for decades before the reserve was created and likely would have continued to do so, making it harder to predict how many would have been caught if there hadn’t been a reserve. The numbers being caught before the expansion were probably too high to sustain.

In any case, the protected area appears to have a positive impact on the population, increasing catches where fishing is still allowed, which is good news, and had never been shown in this much detail before, Lynham says.

That should help to make the longline fisheries around the islands more sustainable, he adds, though he doesn’t think they currently are. “There are catch limits, but nobody is really respecting those. The U.S. exceeds their catch limit every year, while the other countries—Japan, China, Taiwan—somehow never seem to reach their limit—and never stop fishing.”

“There is a need for greater control of the catch numbers each country is hauling in before we can say we are really on a path towards sustainability,” he argues. That would also help scientific monitoring.

Sustainable spillover

Such efforts may be even more important around many other marine reserves, says Lynham, where data of the kind the team used for the study are often lacking. “We need to know where catches are coming from.”

That would also help find out whether smaller reserves, or those that do allow for limited fishing— Papahānaumokuākea being the biggest one in the world that doesn’t—are also effective.

Friedlander, who is based in Hawai’i, agrees with Lynham and warns that “we should not conclude from this study that all protected areas provide these benefits,” citing possible limitations such as small size, poor design, or inappropriate location.

“Some of these other reserves have been put in place with the goal of creating spillover effects,” says Lynham, “but they haven’t fully demonstrated them. Our study finally affirms there is some promise.”

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