Why are shark populations struggling to recover?

Thanks to years of effective fisheries management and conservation efforts, the future looks better for some tuna and billfish species, including southern bluefin tuna, black marlin and swordfish, after decades of declining populations.

However, some sharks that inhabit these species’ high-ocean habitats remain under threat, according to a new study.

(Photo: Photo: Alex Steyn/Unsplash)

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These sharks, especially oceanic whitetips and porbeams, are often accidentally stranded in tuna and billfish fisheries. What’s more, the chances of these species becoming extinct are increasing due to a lack of careful management, according to a study published Nov. 11 in the journal Science.

Over seven years, the study assessed the likelihood of extinction for 18 oceanic fish species. It offers “a vast view of the ocean that we’ve never had before,” says Colin Simpfendorfer, a marine scientist at Australia’s James Cook University who was not involved in the study.

Much of this data was previously only accessible by certain species. Still, according to the authors, the synthesis of all the data allows for a more complete picture of the state of this important ecosystem.

Recent large-scale assessments of global biodiversity show that species and ecosystems are declining globally, says Maria José Juan-Jordá, a fisheries ecologist at the Spanish Institute of Oceanography in Madrid.However, we know little about these ocean trend.

Also read: Midwest Shark Sightings: Are They More Common?

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(Photo: Photo: Oleksandr Sushko/Unsplash)

Juan-Jordá and her colleagues turned to the International Union for Conservation of Nature’s Red List, which assesses changes in a species’ extinction risk, to fill the gap. The Red List Index assesses the risk of extinction for an entire group of species. The team focused on tuna, billfish and sharks – large predatory fish with significant ecological impact in high seas environments.

evaluation of Red List Index Held every four to ten years. The Red List criteria are expanded in the new study to create a way to track extinction risk continuously over time, rather than just within IUCN intervals.

To do so, Juan-Jordá and her colleagues compiled stock assessment information for seven tuna species, including the vulnerable bigeye tuna and the endangered southern bluefin tuna, six billfish species, including black marlin and swordfish , and five species of sharks. They also looked at changes in population biomass and abundance. The team combined the data to determine trends in extinction risk for the 18 species between 1950 and 2019.

The team found that extinction risks for tuna and billfish increased throughout the second half of the 20th century, then declined in the 1990s and 2010s, respectively. These changes were associated with a concomitant reduction in fishing-related deaths for these species that are known to have occurred.

For billfish and tuna, the results are promising, according to Simpfendorfer. However, the researchers found that three of the seven tuna species and three of the six billfish species were still considered near threatened, vulnerable or endangered. According to Simpfendorfer, now is not the time to be complacent about managing these species.

Why do sharks struggle?

but Shark species are struggling In these areas where tuna and billfish are fished, sharks are often caught as bycatch.

According to Juan-Jordá, “shark numbers continue to decline; therefore, the risk of extinction continues to climb,” even as we manage more responsibly economically important and lucrative target species such as tuna and billfish.

According to Juan-Jordá, the problem of sharks being accidentally caught could be tackled by setting catch limits for specific species and defining sustainability goals in tuna and billfish fisheries, not just target species. According to her, it is crucial to determine whether actions to reduce shark bycatch deaths are actually helping.

Simpfendorfer said the organizations responsible for its management must act quickly before it is too late. “There is clearly a need for major improvements in shark-centric management,” he added.

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