|Large, predatory open ocean fish, such as this Atlantic bluefin tuna (Thunnus thynnus), have benefited from successful fisheries management and conservation efforts in recent decades. ONANDIA, IIGO|
After decades of population decline, the future of several tuna and billfish species, including southern bluefin tuna, black marlins, and swordfish, is looking brighter, thanks to years of successful fisheries management and conservation actions. However, new research indicates that some sharks that live in these fishes' open water habitats are still in danger.
These sharks, which include oceanic whitetips and porbeagles, are frequently caught by chance in tuna and billfish fisheries. Researchers report in the November 11 issue of Science that a lack of dedicated management of these species has increased their chances of extinction.
The study looks at the extinction risk of 18 large ocean fish species over the course of nearly seven decades. According to Colin Simpfendorfer, a marine biologist at James Cook University in Australia who was not involved in this research, it provides "a view of the open ocean that we have not had before."
"Most of this information was available for individual species," he says, "but the synthesis for all of the species provides a much broader picture of what is happening in this important ecosystem."
According to Maria José Juan- Jordá, a fisheries ecologist at the Spanish Institute of Oceanography in Madrid, major global biodiversity assessments in recent years have documented declines in species and ecosystems around the world. However, these patterns in the oceans are poorly understood.
To fill this void, Juan-Jordá and her colleagues turned to the International Union for Conservation of Nature's Red List, which assesses changes in a species risk of extinction. The Red List Index assesses the extinction risk of an entire group of species. The team focused on tunas, billfishes, and sharks — large predatory fish that play important roles in their open ocean ecosystems.
Every four to ten years, the Red List Index is evaluated. The new study built on the Red List criteria to create a method of tracking extinction risk continuously over time, rather than just within IUCN intervals.
Juan- Jordá and her colleagues accomplished this by compiling data from fish stock assessments on species' average age at reproductive maturity, changes in population biomass, and abundance for seven tuna species, including the vulnerable bigeye and endangered southern bluefin; six billfish species, including black marlin and sailfish; and five shark species. From 1950 to 2019, the team combined the data to calculate extinction risk trends for these 18 species.
The team discovered that the risk of extinction for tunas and billfishes increased throughout the second half of the twentieth century, with the trend reversing for tunas in the 1990s and billfishes in the 2010s. These shifts are linked to known decreases in fishing mortality for these species that occurred at the same time.
According to Simpfendorfer, the results for tunas and billfishes are encouraging. However, three of the seven tunas and three of the six billfishes studied by the researchers are still classified as near threatened, vulnerable, or endangered. "Now is not the time to be complacent about managing these species," says Simpfendorfer.
However, shark species are struggling in the same waters where tuna and billfish are fished, and sharks are frequently caught as bycatch.
|Many open ocean sharks, such as the silky shark (Carcharhinus falciformis) (pictured), are declining, and are frequently caught by fishermen looking for other large fish. Forget, Fabio|
"While we are increasingly managing the commercially important, valuable target species of tunas and billfishes in a more sustainable manner," Juan-Jordá says, "shark populations continue to decline, and thus the risk of extinction has increased."
Future solutions, according to Juan- Jordá, include establishing catch limits for some species and establishing sustainability goals within tuna and billfish fisheries beyond just the targeted species, as well as addressing the issue of sharks caught incidentally. It's also critical to determine whether the measures put in place to reduce shark bycatch deaths are actually effective, she says.
"There is a clear need for significant improvement in shark-focused management," Simpfendorfer says, "and organizations responsible for their management must act quickly before it is too late."
M.J. Juan-Jordá and colleagues Seventy years of tunas, billfishes, and sharks as global ocean health indicators. Science, November 11, 2022, Vol. 378, p. 617. doi: 10.1126/science.abj0211.