There’s solid evidence that well-enforced marine protected areas (MPAs) that prohibit fishing benefit a wide variety of species within their borders. However, the impact outside their borders is a matter of debate, especially when it comes to highly sought tuna species. Experts differ about how much tuna can “spill over” from an area closed to industrial fishing and be caught in the sea beyond.

Recent research published in the journal Science indicates that large-scale MPAs do generate spillover. The study found that purse seiner fishing vessels catch 12-18% more tuna per unit of effort in the 100 nautical miles (185 kilometers) surrounding large MPAs than they would if the areas hadn’t been closed to fishing.

“There’s some arguments that protected areas are kind of ineffectual compared to other forms of fisheries management,” John Lynham, a marine economist at the University of Hawai‘i at Mānoa who co-authored the study, told Mongabay. “So, I think we’re contributing to some of that debate. And then really big picture, I think what we’ve found certainly provides evidence in favor of one day trying to have large MPAs on the high seas.”

The next few years could see an expansion of MPAs not just in national but also in international waters once a 2023 international agreement known as the High Seas Treaty takes effect. The effort is fueled largely by a separate 2022 agreement to scale up MPA coverage to 30% of the oceans’ surface area by 2030 — “30×30,” as it’s known.

Fully enacting such agreements may create pushback from the fishing industry. Yet research such as this could help convince industry players that large-scale closures are a win-win, Lynham said. 

New evidence of spillover?

Recent decades have seen a surge of MPAs established across the world, with some closed to fishing. The impact of such closures on the abundance of marine species and commercial fisheries outside MPA borders remains unclear. There’s limited direct data on abundance, or the size of a species’ population in a given area, so catch per unit of effort (CPUE) is often used as a proxy, albeit an imperfect one. For purse seiners, CPUE captures catch per set: how much catch purse seine nets pulled in for every time they were deployed.

A 2016 meta-analysis in Journal for Nature Conservation determined that most studies show spillover benefits. Another 2020 meta-analysis by some of the same authors published in the journal Fish and Fisheries also found evidence of spillover.

In 2022, Lynham co-authored a study in Science that found spillover benefits for bigeye and yellowfin tuna following the 2016 expansion of Papahānaumokuākea Marine National Monument, near Hawai‘i, the largest MPA in the world at more than twice the size of Texas.

The new study by Lynham and co-author Juan Carlos Villaseñor-Derbez, a marine scientist then at Stanford University, is broader in scope than the 2022 study. Its main analysis deals with spillover at six large MPAs that have contiguous no-fishing zones of more than 100,000 square kilometers (38,610 square miles).

The authors used publicly available catch data to compare CPUE during the ten years before and after the establishment of the no-take MPAs in two distinct zones: the “near” area 0-100 nautical miles (0-185 km) from an MPA’s border and the “far” area 100-200 nautical miles (185-370 km) away. They then used the difference between the gains in the “near” and “far” areas as an indication of spillover, operating under the cautious assumption that the gains in the “far” area came from other causes.

The authors found an average increase of 12% in CPUE, versus a counterfactual in which the MPA didn’t exist, when counting only the first decade after a closure. They estimated an 18% increase when also counting later observations for those MPAs that have existed longer than a decade.

The statistics include bigeye (Thunnus obesus), yellowfin (Thunnus albacares) and skipjack (Katsuwonus pelamis) tuna catch. The effect was greatest for bigeye tuna, perhaps because the species had been heavily exploited and the MPAs helped it recover, Lynham said.

In the new paper, Lynham and Villaseñor-Derbez, who is now at the University of Miami, cautioned that the most statistically robust evidence of spillover comes primarily from three MPAs in the Pacific Ocean for which there was strong CPUE data. All three MPAs once had “large amounts of fishing effort” and faced “stiff opposition” from fishing interests when they were established and are now considered well enforced, the study says. These three MPAs are at Revillagigedo in Mexico, the Galápagos in Ecuador, and the Phoenix Islands in Kiribati, the last of which was reopened to tuna fishing in 2023 after closing in 2015. There was also good CPUE data for a fourth MPA, the Chagos MPA in the Indian Ocean, which had less fishing previously and is not considered well-enforced, according to the study. Observations from Chagos also weighed heavily in the total result, but the spillover effect found there was relatively minimal. CPUE observations from two other MPAs were fewer and thus weighed less heavily.

Though the strongest findings came from the three MPAs, all six MPAs showed some level of spillover when the raw analysis was run, and five of the six still did so after controlling for other factors that influence CPUE, such as fluctuations in tuna abundance due to factors like El Niño, a cyclical climate pattern.

“[T]he real strength of our contribution is that we are able to move beyond single case studies … and present a set of results where the same patterns are being observed at different locations and at different points in time,” the authors wrote.

The debate over spillover

Though Lynham and Villaseñor-Derbez concluded that their work bolsters the case that large MPAs bring spillover benefits, the idea that banning fishing in MPAs could help fisheries remains hotly contested. A different set of fisheries scientists, including Ray Hilborn at the University of Washington, have thrown cold water on the idea, starting before the study by Lynham and Villaseñor-Derbez was published in December.

Hilborn and his co-authors argued in a paper in the journal Theoretical Ecology that the 2022 study by Lynham and colleagues was methodologically flawed. Lynham strongly disputed their reasoning. He said the editors of Science found Hilborn’s critique, which he and a co-author submitted to the publication, lacking in merit and didn’t publish it.

Hilborn, for his part, told Mongabay that Science’s editors are biased in favor of attention-grabbing results and pro-MPA results, and have recently published a number of “really bad” fisheries papers, the “worst” of any journal, while declining to publish critiques of the work. He said the same critique passed muster with reviewers at Theoretical Ecology, who were MPA experts. (“Science does not focus on publishing papers that advocate for a particular stance or approach; rather, we strive to publish papers that represent major advances in our understanding,” Meagan Phelan, the publication’s communications director, told Mongabay.)

Hilborn, who receives some of his research funding from the fishing industry and has defended the practice in the past, also criticized Lynham and Villaseñor-Derbez’s December Science study. He told Mongabay the spillover effects it found were small. Moreover, he said, an important question remained outside the study’s scope: Even if there’s spillover, does it represent an overall increase in tuna populations or the catch available to fishing vessels? The establishment of new MPAs may simply shift the fishing to different locations outside the MPA boundaries, he pointed out. Lynham and Villaseñor-Derbez wrote in the study that their research didn’t “by itself” address this broader question. Lynham said signs of post-MPA abundance beyond MPA borders were important to document and the study presented evidence consistent with there being more wild fish in the ocean.

Alexander Caveen, a marine resource management expert at the University of Hull in the U.K. who has critically examined the scientific evidence for MPAs in past work, said he was surprised the study didn’t address the potentially confounding factor of fish aggregating devices (FADs). Purse seiners deploy these floating structures to attract tuna for easy fishing, and they could be doing so preferentially just outside of MPAs, potentially creating an illusion of greater CPUE relative to the “far” area. Hilborn said he also suspected FADs were a factor.

Lynham said he and Villaseñor-Derbez are currently studying this, but while the idea that purse seiners are catching tuna using FADs just outside of MPAs grabs people’s attention, it’s not currently happening on a large scale.

Lynham also said that while the spillover debate is important, its terms are, from a conservation perspective, frustratingly narrow. “No one is talking about opening Yosemite National Park to logging because it’s not generating spillover benefits to the commercial forestry sector!” he said via email.

Banner image: A yellowfin tuna (Thunnus albacares). Experts debate the impact of marine protected areas (MPAs) on tuna populations. Image by Ellen Cuylaerts / Ocean Image Bank. 

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Citations: 

Di Lorenzo, M., Claudet, J., & Guidetti, P. (2016). Spillover from marine protected areas to adjacent fisheries has an ecological and a fishery component. Journal for Nature Conservation, 32, 62-66. doi:10.1016/j.jnc.2016.04.004

Di Lorenzo, M., Guidetti, P., Di Franco, A., Calò, A., & Claudet, J. (2020). Assessing spillover from marine protected areas and its drivers: A meta‐analytical approach. Fish and Fisheries, 21(5), 906-915. doi:10.1111/faf.12469

Lynham, J., & Villaseñor-Derbez, J. C. (2024). Evidence of spillover benefits from large-scale marine protected areas to purse Seine fisheries. Science, 386(6727), 1276-1281. doi:10.1126/science.adn1146

Medoff, S., Lynham, J., & Raynor, J. (2022). Spillover benefits from the world’s largest fully protected MPA. Science, 378(6617), 313-316. doi:10.1126/science.abn0098

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