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Tunas, billfishes, and sharks are sentinels of global ocean health

While we are familiar with the ways in which human activities are currently threatening the abundance and diversity of wild species on land, information on biodiversity changes in the oceans is not as easy to obtain. It’s estimated that intense fishing across the planet’s oceans has led to roughly half of all commercially harvested fish and invertebrate stocks becoming overfished during the 20th century, and the effects of these fisheries on ocean biodiversity are poorly researched and understood. 

In a new study, scientists from Spain, the U.S. and Canada, have used the risk categories and criteria of the International Union for Conservation of Nature’s ‘Red List of Threatened Species’ to help develop an understanding of changing biodiversity among 52 populations of oceanic tuna, billfish and sharks since1950. The International Union for Conservation of Nature (IUCN) labels a species as critically endangered (CR), endangered (EN), or vulnerable (V) on the basis of how much its population has declined over the past three generations or 10 years, whichever period is longer. Other categories include near threatened (NT) and of least concern (LC).

Dr. Maria José Juan-Jordá from the Basque Research and Technology Alliance, and colleagues, built upon this classification system and developed a continuous Red List Index (RLI) of yearly changes in extinction risk for populations of seven tuna species, six billfish species, and five shark species over the past 70 years. This helped them to understand the health of oceanic biodiversity as well as the overall impacts of fishing mortality and conservation efforts on these populations of predatory fish.

The researchers found that, between 1950 and 2008, the global RLI trajectory of these fish populations grew worse by around 27 percent. This reflected an increasing extinction risk due to fishing pressure for these iconic oceanic predatory species. However, as calls for better monitoring and effective fisheries management strategies were heeded, mortality due to fishing declined and populations of tuna and billfish gradually recovered. For tunas, the RLI started to improve in the 1990s and end of the 2000s whereas the RLI for billfishes deteriorated until the early 2000s, improving only during the past decade. 

In particular, since 2008, populations of southern bluefin tuna have improved from critical to vulnerable, while yellowfin tuna, swordfish, blue marlin, striped marlin, and black marlin have moved to the categories near threatened or of least concern. These findings suggest that effective ocean fisheries management has shifted the biodiversity loss curve for tunas and billfishes. These target species are now being more successfully managed to ensure maximum sustainable catches.

However, the same cannot be said for oceanic shark species. The extinction risk for sharks, which remain largely undermanaged, continues to rise. Sharks are often captured as bycatch in other fisheries, and their numbers and biodiversity continue to decline, leading to increasing risk of biodiversity loss. The researchers report that there seems to be high resistance to any measure that might meaningfully curb fishing mortality for sharks, and emphasize that unless effective management actions are adopted to reduce shark mortality for each fishery and shark species – including international trade regulation – their risk trajectories will continue worsening in the future. 

The authors state: “Our study connects annual changes in fishing mortality and extinction risk globally over the past 70 years for oceanic tunas, billfishes, and sharks and reveals how effective management for highly valuable commercial species of tunas and billfishes has reversed the biodiversity loss curve while the extinction risk of undermanaged sharks continues to increase.”

In a related Perspective, Matthew Burgess and Sarah Becker write: “The conservation statuses of threatened target species can be improved by managing the fishing industry, which can benefit the industry economically in the long run while allowing the threatened species to recover. However, the protection of high-vulnerability bycatch and non-target species is expected to be more difficult because they will require fisheries to invest in better fishing gear and targeting practices, or reduce fishing efforts, without directly benefiting from these changes.”

The research is published in the journal Science.

By Alison Bosman, Staff Writer

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