UCSD scientists: Silky shark population structure can guide conservation

A new study shows how information about vulnerable silky sharks in the Eastern Pacific can impact conservation of similar species.

A new study led by researchers at UC San Diego’s Scripps Institution of Oceanography, The Nature Conservancy and the Inter-American Tropical Tuna Commission reveals the population structure of silky sharks (Carcharhinus falciformis) in the Eastern Pacific Ocean for the first time.

The work offers insights that could guide future conservation and management of this vulnerable species and others.

Silky sharks are the second-most commonly caught oceanic shark species worldwide (surpassed by blue sharks) and 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.

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

Marine ecologist Brendan Talwar, lead author and postdoctoral scholar in SIO’s Semmens Lab, teamed up with colleagues to conduct a comprehensive analysis of existing data on silky sharks, focusing on both large- and small-scale fisheries using an extensive dataset including population genetics, movement patterns collected through tagging, life history information and more, resulting in the most complete scientific resource on silky sharks to date.

The researchers propose a preliminary three-stock model for silky sharks in the region, identifying core populations in the northern, central and southern regions (offshore of Mexico, Central America and South America).

By defining these distinct stocks, the study paves the way for improved management and conservation of silky sharks across nearly 19.3 million square miles) of ocean.

“This study gives us one huge step forward in achieving more accurate population models for silky sharks,” Talwar said in a press release.

“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.”

In looking at bycatch data, the researchers also found that juvenile silk sharks account for 81 percent of the silky sharks caught by purse seines set on floating objects (such as fish aggregating devices) and 45 percent 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,” Talwar said.

The researchers’ collaborative approach offers a model for studying other vulnerable species in similar regions or elsewhere.