Â鶹ÒùÔº

Fish and other marine organisms, though deeply affected by human activities, don't respect human borders. The ranges of many commercially important species in fact straddle the borders of countries' exclusive economic zones (EEZs) and international waters, known as the high seas. This arrangement, which makes fisheries management difficult, is set to get even more complicated as climate change continues to heat up the ocean, a new study says.

The , published July 30 in the journal Science Advances, found that more than half of the world's straddling stocks will shift across the maritime borders between EEZs and the high seas by 2050. Most of these shifts will be into the high seas, where fisheries management is much more challenging and stocks are more likely to be overexploited.

"It's an important issue and an important paper that I think should make anyone concerned about fisheries or the seafood on their plate sit up and pay attention," Malin Pinsky, an associate professor of biology at the University of California, Santa Cruz, who wasn't involved with the study, told Mongabay.

"The shift towards the high seas that they document would have some really serious consequences, in part because fisheries management tends to be much less effective in the high seas," added Pinsky, an expert on climate-driven fisheries distribution shifts. "Climate change is sending a whole bunch of fisheries out into the lion's den because the high seas doesn't have a great reputation for sustainable fisheries management."

Among the most serious potential consequences is a loss of fisheries resources for many tropical countries that did little to create the climate crisis, Pinsky and other experts said.

Significant shifts

The new study deals with projections, but there's already empirical evidence that these types of shifts are taking place. A 2016 in the journal Frontiers in Marine Science found that marine species were generally shifting to higher latitudes and deeper waters, among other responses to climate change.

Projections of distribution shifts of straddling fish stocks also aren't entirely new. A 2021 study in the journal Nature Sustainability found that in the southwest Pacific Ocean, where tuna are an economic lifeline, three key populations are likely to shift toward the high seas by 2050 and away from the EEZs of a set of small island developing states (SIDS) including the Solomon Islands and Kiribati. (EEZs generally extend 200 nautical miles [370 kilometers or 230 miles] from a country's coastline.)

The Science Advances study is, however, the first to assess climate-driven distribution changes of straddling stocks across the world. The authors, from the Institute for the Oceans and Fisheries at the University of British Columbia (UBC) in Canada and other institutions, identified 347 commercially important straddling stocks worldwide, encompassing 67 species. (Many species have distinct subpopulations that generally don't interbreed.) Silky sharks (Carcharhinus falciformis), blue sharks (Prionace glauca), skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) are among the most common straddling stocks.

To determine which stocks would shift, they used a that was established in the late 2000s by a team that included William Cheung, a UBC professor and co-author of this study. The model accounts for factors such as water temperature, salinity and dissolved oxygen.

Using the model, the authors found that more than one-third of the 347 stocks would shift into the high seas by 2050, while at least one-fifth would shift into EEZs. A shift, measured against a 1951–2014 baseline period, wouldn't necessarily mean that all fish in a population had crossed into or out of the high seas. Some fish would still remain in the original domain and perhaps continue crossing borders. But it would mean a statistically significant change in the relative share of the overall mass of fish.

The authors ran the model under a low greenhouse gas emissions scenario roughly consistent with keeping warming under 2°Celsius (3.6° Fahrenheit) and also under a high-emissions scenario; the model found only slightly more shifting under the high-emissions scenario, perhaps because oceanic changes up until 2050 would be "very similar" either way, the study says.

Juliano Palacios-Abrantes, a postdoctoral researcher at the Institute for the Oceans and Fisheries at the University of British Columbia and lead author of the study, said its aim was to encourage fisheries managers to do more detailed analyses of their stocks.

"It could actually be that [a given] shift is even more pronounced in the region, or it could be that it's not," he told Mongabay, calling for localized and specialized research to supplement the study's findings.

Palacios-Abrantes and co-authors cautioned that data collection for many species has long been weak, making firm conclusions difficult. Pinsky echoed this, saying that while there's decent catch data for highly valuable fish like tropical tuna species, the paucity of good data for other species generally makes analysis difficult. Independent survey data are especially rare in many regions.

"It's hard to get ready for the future when you don't know what's happening now," Pinsky said.

Still, Pinsky said, the model used by the authors is a "great way to start as a broad brush."

Pinsky said empirical work on how much shifting is already taking place should be possible for some stocks, though he warned that catch and location data are sometimes closely guarded, creating impediments to such research.

Study co-author Rashid Sumaila, a UBC professor and co-winner of the 2023 Tyler Prize for the Environment, which has been likened to a Nobel Prize, has long called for a ban on high seas fishing. He said he believes the study's findings show why such a ban is necessary.

"This projected redistribution strengthens the case for a ban: The high seas could serve as a biodiversity refuge for migrating species, prevent inequitable capture of displaced resources by wealthy distant-water fleets, and avert a carbon-intensive escalation of long-distance fishing," Sumaila told Mongabay in an email.

Compounding injustices?

Tropical countries, many of which are low- or middle-income, have done relatively little to contribute to climate change but are especially vulnerable to its impacts. The study suggests that when it comes to fisheries, there may be an added layer of injustice: Straddling stocks are projected to generally move away from tropical EEZs and into international waters. In those cases when commercial stocks do shift into EEZs, it will primarily be in temperate waters, not the tropics, the study says.

Without governance mechanisms to protect their interests, tropical countries could find it harder to catch fish or gain revenue from their fishing rights.

"Coastal nations in the tropics don't really have the resources to go to the high seas," Palacios-Abrantes said.

Palacios-Abrantes cited Pacific SIDS as a "very concerning" example, given that many rely on fisheries revenues. Eight of them have formed a bloc that collectively sells access to tuna fisheries within their EEZs and shares revenues—a successful arrangement that's often been suggested as a model for other coastal nations. These are the Federated States of Micronesia, Kiribati, the Marshall Islands, Nauru, Palau, Papua New Guinea, the Solomon Islands and Tuvalu; Tokelau, a New Zealand territory, also participates.

In the central Indo-Pacific region, which encompasses these SIDS, 58% of straddling stocks are projected to move into the high seas, while no stocks are projected to move into EEZs, the study says.

"These shifting stocks include important highly migratory species such as skipjack, yellowfin, and bluefin tuna (Thunnus obesus), as well as non–highly migratory straddling species such as Japanese flying squid (Todarodes pacificus) and chub mackerel (Scomber japonicus)," the authors write.

Johann Bell, a senior director at the Conservation International Green Climate Fund Agency and lead author of the 2021 Nature Sustainability study on the redistribution of tropical tuna in the region, said the study underscores the need for stronger international management. There are two multilateral bodies—regional fisheries management organizations (RFMOs), as they're called—that manage tuna and tuna-like stocks in the Pacific high seas: the Western and Central Pacific Fisheries Commission (WCPFC) and the Inter-American Tropical Tuna Commission (IATTC), which covers the eastern Pacific.

Bell told Mongabay in an email that the Science Advances study supports the idea that the two RFMOs "need to collaborate more closely to fulfill their conservation and management responsibilities under international law by commissioning further scientific research to develop climate-informed stock assessments and harvest strategies for shared tuna resources."

Bell pointed to two related studies that he's co-authored: a 2022 in Frontiers in Marine Science that analyzes the role of the WCPFC and the IATTC in dealing with tuna stock redistribution and a new, broader published this month in Fish and Fisheries that lays out the types of research that RFMOs should undertake to understand the effects of climate change on fish stocks.

"The results of this research will inform adaptations needed to enable Pacific Island tuna-dependent economies to retain the benefits they now receive from tuna, regardless of the redistribution of the fish," Bell said.

Ryan Orgera, global director of Accountability.Fish, a U.S.-based advocacy group, said that "RFMO governance is largely stuck in the 20th century" and change is "regrettably slow," but that it is important to adapt to shifting stocks.

"Most RFMOs, especially WCPFC, understand this and are trying to operationalize these concepts into their processes," Orgera told Mongabay in an email regarding the Science Advances study.

"[C]limate change can be an opening to retool RFMOs to manage and bridge the shifting sands we're standing on," he added.