- Conventional wisdom holds that marine protected areas don’t offer much in the way of protections to highly migratory species of marine life, given that those species are unaware of the imaginary borders humans draw on maps to delineate such areas.
- New research finds that, to the contrary, large MPAs can confer benefits on migratory marine species — but only when they are carefully designed, strictly enforced, and integrated with sustainable fisheries management.
- The study, published last month in the journal Marine Policy, explores whether or not there are any benefits of “targeted spatial protection” measures, including large-scale fisheries closures and marine protected areas (MPAs), for highly migratory species like billfishes (such as swordfish and marlins), pelagic sharks (such as blue, great white, mako, silky, and thresher sharks), and tuna — and highlights ways that spatial protection for migratory pelagic species can be improved.
Conventional wisdom holds that marine protected areas don’t offer much in the way of protections to highly migratory species of marine life, given that those species are unaware of the imaginary borders humans draw on maps to delineate such areas. New research finds that, to the contrary, large MPAs can confer benefits on migratory marine species — but only when they are carefully designed, strictly enforced, and integrated with sustainable fisheries management.
The study, published last month in the journal Marine Policy, explores whether or not there are any benefits of “targeted spatial protection” measures, including large-scale fisheries closures and marine protected areas (MPAs), for highly migratory species like billfishes (such as swordfish and marlins), pelagic sharks (such as blue, great white, mako, silky, and thresher sharks), and tuna — and highlights ways that spatial protection for migratory pelagic species can be improved.
“It is a fairly common belief that protection fixed in space cannot benefit highly migratory species,” Dr. Kristina Boerder of Canada’s Dalhousie University, lead author of the study, said in a statement. “However, if spatial closures such as marine protected areas or fisheries closures are strategically placed and complemented by adapted fisheries management, even those who wander thousands of miles can benefit.”
All of the species included in the study “are highly mobile, undertake long-distance horizontal movements through the pelagic environment, are currently exploited by commercial fisheries, and are collectively managed” through the 18 Regional Fisheries Management Organizations (RFMOs) that exist around the globe, according to the study. “Ecological and economic benefits from spatial protection have been demonstrated for many reef and demersal species, but remain debated and understudied for highly migratory fishes, such as tunas, billfishes, and pelagic sharks,” Boerder and co-authors write in the study.
Many large pelagic species are of high commercial value, but their long life spans and the higher ages at which they reach maturity makes them particularly vulnerable to overexploitation by fishing fleets. A number of populations of large tuna species, for instance, have declined by 10 to 25 percent of their original spawning biomass, according to the study. Some species, such as Pacific bluefin tuna, have populations facing “extreme depletion” of more than 95 percent.
In response to these declines and the conservation targets adopted under multilateral international agreements like the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and the Convention on Migratory Species (CMS), the amount of ocean area placed under some form of spatial protection has increased dramatically over the last two decades.
“According to our analysis, unilaterally established MPAs and RFMO fishery closures targeting large pelagic fishes now cover nearly 15% of global ocean surface, in approximately equal proportions,” Boerder and colleagues write. At the start of 2019, about 7.6 percent of Earth’s ocean surface area was covered by MPAs. Seasonal and permanent fisheries closures covered another 7.4 percent. “This means that collectively more than 50 million [square kilometers] of ocean area are under some spatial management that could potentially benefit large pelagic fish stocks.”
Boerder and team consulted peer-reviewed and grey literature in order to determine the suitability of highly migratory species for these types of spatial protections. “Multiple modeling studies have suggested that highly mobile fish stocks within a system that includes closed areas appear more resilient to collapse, and fisheries yields are higher over time, when contrasted with a scenario that lacks spatial protection,” the researchers found. “These benefits are predicted to be especially pronounced where fishing mortality is difficult to control, [Illegal, Unreported and Unregulated (IUU)] fishing prevails, or fisheries are mismanaged,” they write.
While closed areas and MPAs can serve as buffers against overexploitation, the authors of the study determined that all species are not equally suited for spatial protection, “mainly due to variation in their distribution and behavior, including philopatric behavior and site fidelity, migration along fixed pathways, and aggregation for spawning.”
Philopatry, the tendency of members of a species to repeatedly return to specific areas throughout their life cycles, is one of the most important characteristics in determining which species will benefit from spatial protection, the researchers found. In other words, species whose movement and aggregation habits are predictable in space and time benefit the most from spatial protections. For instance, Atlantic bluefin tuna (Thunnus thynnus) return to the same spawning and feeding grounds every year using well-known migratory routes. “For predictable
cases like these, ‘targeted’ closures such as a closure in the Gulf of Mexico for Atlantic bluefin tuna, may be effective in protecting vulnerable life stages in defined areas such as spawning sites,” the researchers write.
But that same strategy would not work as well for species like skipjack tuna (Katsuwonus pelamis) that do not routinely migrate to particular spawning or feeding grounds. “A modeling study of the Chagos MPA in the Western Indian Ocean demonstrated the importance of design and scale of spatial protection for such ‘unpredictable’ fish species,” according to the study. “This MPA was found to have little effect on skipjack tuna stocks due to strong seasonal variations of habitat conditions that drive stocks into and out of the MPA. In contrast, a much larger hypothetical fisheries closure covering large parts of favorable habitat for skipjack tuna was predicted to stabilize spawning stock biomass (SSB) and yield higher catches over a 20-year [timeframe] especially compared to a contrasting scenario without any closure.”
The researchers say their findings bolster the call for protecting at least 30 percent of Earth’s oceans by 2030 through a network of MPAs and other effective conservation measures made by members of the International Union for Conservation of Nature (IUCN) in 2016.
“This study will help us to better design large marine protected areas and understand under what conditions they will benefit large highly migratory species like sharks, turtles, and whales,” Angelo O’Connor Villagomez, senior officer with the Pew Bertarelli Ocean Legacy Project, said in a statement.
Referencing a recent report issued by the U.N.’s Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, Villagomez added: “These species benefit most when we integrate well-designed and enforced marine protected areas with sustainable fisheries management. The recent UN Biodiversity report estimating that 1 million species are threatened with extinction strengthens the case for protecting 30 percent of every ocean habitat by 2030.”
• Boerder, K., Schiller, L., & Worm, B. (2019). Not all who wander are lost: Improving spatial protection for large pelagic fishes. Marine Policy 105, 80-90. doi:10.1016/j.marpol.2019.04.013