Many species of billfish and tuna have seen a decline in their numbers over the past decades. However, many species are now looking brighter thanks to decades of fisheries management and conservation efforts. But some sharks that live in these fishes’ open water habitats are still in trouble, new research suggests.
These sharks, which include porbeagles and oceanic whitetips, are often caught accidentally in tuna or billfish fisheries. These species are often caught by accident in tuna and billfish fisheries. Their chances of extinction are increasing.Researchers report in the Nov. 11 Science.
The analysis assesses the extinction risk for 18 species of large-sized ocean fish over a period of nearly seven decades. It provides “a view of the open ocean that we have not had before,” says Colin Simpfendorfer, a marine biologist at James Cook University in Australia who was not involved in this research.
“Most of this information was available for individual species, but the synthesis for all of the species provides a much broader picture of what is happening in this important ecosystem,” he says.
In recent years, major global biodiversity assessments have documented declines in species and ecosystems across the globe, says Maria José Juan-Jordá, a fisheries ecologist at the Spanish Institute of Oceanography in Madrid. However, these patterns are difficult to understand in the oceans.
To fill this gap, Juan-Jordá and her colleagues looked to the International Union for Conservation of Nature’s Red List, which evaluates changes in a species’s extinction risk. The Red List Index measures the likelihood of extinction for a whole group of species. The team specifically targeted tunas, billfishes and sharks — large predatory fishes that have influential roles in their open ocean ecosystems.
Red List Index assessments take place every four to ten year. The Red List criteria were used by the researchers to create a method of continuously tracking extinction risk over time rather than only within the IUCN intervals.
Juan-Jordá and her colleagues did this by compiling data on species’ average age at reproductive maturity, changes in population biomass and abundance from fish stock assessments for seven tuna species, like the vulnerable bigeye and endangered southern bluefin; six billfish species, like black marlin and sailfish; and five shark species. This data was combined by the team to determine extinction risk trends of these 18 species between 1950 and 2019.
The team discovered that billfishes and tunas faced increased extinction risks in the second half of the 20th Century. This trend reversed for billfishes in 2010 and for tunas in 1990. These shifts are linked to known reductions of fishing deaths for these species.
Simpfendorfer states that the results were positive for billfishes and tunas. The researchers found that three of seven tunas and three out of six billfishes were considered endangered, near threatened, or vulnerable. “Now is not the time for complacency in managing these species,” Simpfendorfer says.
However, shark species are in trouble in the very waters where billfish and tuna are fished. Sharks are also often caught as bycatch.
“While we are increasingly sustainably managing the commercially important, valuable target species of tunas and billfishes,” says Juan-Jordá, “shark populations continue to decline, therefore, the risk of extinction has continued to increase.”
Some solutions going forward, says Juan-Jordá, include catch limits for some species and establishing sustainability goals within tuna and billfish fisheries beyond just the targeted species, addressing the issue of sharks that are incidentally caught. And it’s important to see if measures taken to reduce shark bycatch deaths are actually effective, she says.
“There is a clear need for significant improvement in shark-focused management, and organizations responsible for their management need to act quickly before it is too late,” Simpfendorfer says.
