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For the first time, researchers have mapped the population structure of silky sharks (Carcharhinus falciformis) in the Eastern Pacific Ocean, offering new insights that could guide future conservation and management of this vulnerable species.

Led by UC San Diego's Scripps Institution of Oceanography, The Nature Conservancy and the Inter-American Tropical Tuna Commission (IATTC), the recent study brings together more than 30 years of silky shark research and data to reveal how their populations are structured across the Pacific Ocean.

Silky sharks are the second most commonly caught oceanic shark species worldwide, surpassed only by blue sharks. Found in tropical waters across the globe, silky sharks frequently interact with pelagic fisheries, where hundreds of thousands are caught as bycatch each year, primarily by small-scale coastal fisheries and offshore longline and purse-seine gears.

Previous observations indicate that silky sharks face a serious risk of overfishing, but efforts to assess and manage their populations have been limited by a lack of data, including gaps in understanding how their populations are structured across the region.

To address these knowledge gaps, Scripps Oceanography marine ecologist Brendan Talwar and colleagues teamed up to conduct a comprehensive review and analysis of existing data on silky sharks, focusing on both large- and small-scale fisheries.

Working with international partners, they compiled an extensive dataset that included population genetics, movement patterns collected through tagging, life history information, and fishery data from both offshore and nearshore sources, resulting in the most complete scientific resource on silky sharks to date.

"This study gives us one huge step forward in achieving more accurate population models for silky sharks," said Talwar, lead author of the study and a postdoctoral scholar in the Semmens Lab at Scripps Oceanography.

"For a wide-ranging species like the silky shark, there has been incredible research at the local and regional scales. But every once in a while, it's important to take a step back to try to make sense of it all. That big-picture view can lead to conclusions with real impact in terms of managing the species or the fishery."

, published in Reviews in Fish Biology and Fisheries, reveals a surprising amount of structure in a population that was previously thought to be well mixed across the Eastern Pacific Ocean.

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The authors propose a preliminary three-stock model for silky sharks in the region, identifying core populations in the north, central, and southern regions—each likely distinct from populations in the Central and Western Pacific Ocean. By defining these distinct stocks, the study paves the way for improved management and conservation of silky sharks across nearly 50 million square kilometers (19.3 million square miles) of ocean.

The new study also sheds light on the whereabouts of juvenile silky sharks in the Eastern Pacific, revealing potential evidence of pupping or nursery grounds not only along the edge of the continental shelf—where juveniles are often caught by small-scale fishing fleets—but also far offshore, where they are routinely caught by larger pelagic fleets.

In looking at bycatch data, the researchers found that juveniles account for 81% of the silky sharks caught by purse seines set on floating objects (such as fish aggregating devices), and 45% of those caught by longline fisheries.

"We think the young sharks are drawn to these floating objects because they provide structure, food, and perhaps a sense of safety, in an otherwise dangerous open-ocean environment," said Talwar.

The authors noted that their collaborative approach offers a model for studying other vulnerable species in similar regions, or elsewhere.

"This project is a testament to what can be accomplished when the right people come together, piecing together all the available information on a vulnerable species to discover what we can learn," said study co-author Lyall Bellquist, an independent fisheries scientist and Scripps Oceanography alumnus.

"We demonstrated that multi-institutional support from key organizations and agencies can enable cost-effective, impactful science that will advance conservation of a highly vulnerable species."

Building on the success of this effort, the IATTC Ecosystem and Bycatch Working Group issued a formal recommendation to fund similar studies focused on three species of critically endangered hammerhead sharks in 2024.