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When President Barack Obama greatly expanded Papahanaumokuakea Marine National Monument in 2016, the goal was pretty basic: protect the marine life found within that vast stretch of the Pacific Ocean up to 200 miles around the Northwestern Hawaiian Islands.

But new research out of the University of Hawaii indicates there’s been another benefit to what was at the time the world’s largest marine protected area, which the commercial fishing industry staunchly opposed.

The monument, researchers found, has actually helped to revive the stocks of the migratory tuna that have swum through its borders over the past several years since fishing has been banned there.

Those tuna species – bigeye and yellowfin, both known as ahi in Hawaii – are the same tuna that the Honolulu-based commercial fishing fleets depend on, and which the islands’ residents and visitors consume regularly and in large quantities.

“This is supportive of using large protected areas to help restore tuna populations … and other species that we care about,” said John Lynham, a UH economics professor and co-author of a study published Thursday in the academic journal Science.

Fishing is almost entirely prohibited within Papahanaumokuakea, a nearly 600,000-square-mile protected area. But a so-called “spillover” effect from those restrictions has increased the catch rate for bigeye tuna — prized for fresh sashimi and poke — by about 10% in the waters just outside the monument’s boundaries, according to Sarah Medoff, a researcher at the UH who was the study’s lead author.

Meanwhile, the catch rate for yellowfin tuna has increased by around 50% in those same waters outside the boundary, according to Medoff.

In 2016, leaders of the , which represents a fleet of some 140 vessels that fish for bigeye, yellowfin, swordfish and other species, vocally opposed the monument’s expansion from 50 miles to 200 miles out from shore. The commercial fleet similarly objected when President George W. Bush first created the monument in 2006. The move, they said, would be too restrictive to the fleet’s fishing operations.

The Science study indicates that there’s at least some benefit to the fleet from the expanded boundary. It’s not yet clear whether the spillover benefit boosting tuna stocks actually makes the monument more of an economic asset to the fleet than a liability.

“Whether this is a net benefit, we just don’t know,” Lynham said, noting that wasn’t a focus of the study. However, he and Medoff hope their work will spark more discussion and investigation into whether the monument has actually helped Hawaii’s commercial longline fleet.

Neither HLA nor the Western Pacific Regional Fishery Management Council, known as Wespac, were available Wednesday to comment on the Science study.

shows Hawaii’s longline fleet will again catch far more than the 3,554 metric tons of bigeye tuna that it is initially allotted under international agreements. For the past several years or more, the fleet has hauled in thousands more tons through special agreements with U.S. territories in the Pacific. Guam, American Samoa and the Commonwealth of the Northern Mariana Islands have in the past provided some of their quota, which the territories don’t use anyway, in exchange for the funding of fisheries projects in their regions.

Medoff and Lynham said they completed their study with data collected aboard fishing vessels by NOAA. They analyzed the catch rates in two rings of ocean surrounding the monument — one bordering its expanded 200-mile boundary and another one farther out.

Prior to the monument’s 2016 expansion, the catch rate in both rings of ocean remained about the same, Lynham said. After the expansion took place, the catch rate in the ring next to the monument’s boundary started to expand, he explained, thus creating the “spillover” effect.

In recent months, Wespac leaders have strongly opposed the proposed expansion of another marine protected area, the Pacific Remote Islands Marine National Monument.

Under that proposal, led by a coalition of Indigenous researchers, educators and community leaders, the Pacific Remote Islands monument would well surpass the size of Papahanaumokuakea, which remains the nation’s largest marine protected area.

Lynham said he’s “reluctant to extrapolate” whether his study’s findings could apply to the other monument because there are too many caveats.

The Pacific Remote Islands monument is in a different region, and the tuna there would primarily be a different species — skipjack, primarily used in canned tuna. Plus, that monument is made up of five disconnected areas in the region, whereas Papahanaumokuakea encompasses one contiguous area.

Still, he and Medoff said, the study generally indicates that pelagic tuna benefit from marine protected areas if they’re large enough.

Exactly how large the area needs to be remains to be seen, they said, but it’s clear that Papahanaumokuakea, at 583,000 square miles, is big enough.

Civil Beat’s coverage of climate change is supported by the Environmental Funders Group of the Hawaii Community Foundation, Marisla Fund of the Hawaii Community Foundation and the Frost Family Foundation.