What can scientists studying white sharks learn from an expert on mountain lions? As it turns out, quite a lot.
Such a collaboration is on display in new research published in the Journal of Applied Ecology. Models that estimate survival rates for top predators on land, according to the study, can also work in the ocean. The research also revealed important safeguards that can help protect white sharks while they’re young and vulnerable.
At the heart of the effort was the work of lead author John Benson. Before taking his current role as a professor at the University of Nebraska, John was a post-doctoral researcher at the Monterey Bay Aquarium, working with senior research scientist Sal Jorgensen.
“We always learn things from adjacent fields,” says Sal, who specializes in white sharks, and who coauthored the paper along with six others. “John made his name studying mountain lions in Southern California.”
John’s past work also involved black bears in Louisiana, panthers in Florida, wolves and coyotes in Canada, and moose and their various predators in Alaska. After so much experience on land, John saw working with Sal at the aquarium as a chance to—as the saying goes—get his feet wet.
Monterey Bay Aquarium and its research colleagues have been tagging juvenile white sharks in southern California since 2002. Now they’ve gained new insights into white shark survival from those data tags. : Steve McNicholas
Estimating sharks’ survival rates
John realized there was an opportunity for a new approach for estimating juvenile shark survival, tapping data from electronic tags placed on the sharks to develop what are called “known-fate models.”
“They’re a very direct way of estimating survival and mortality,” he says.
The technique hinges on determining what happens to individuals—information that has been elusive historically, because of the difficulty of tracking animals in the ocean.
“It’s generally much easier to study animals on land,” John notes. “The animal tracking technology that helped us learn so many new things about terrestrial species decades ago, with radio telemetry and collars, has only recently become available in the marine environment.”
Researchers from the aquarium, California State University, Long Beach, and Mexico’s Ensenada Center for Scientific Research and Higher Education have been tagging and tracking juvenile white sharks with various devices since 2000. Sal and his colleagues have deployed devices called PATs, or pop-up archival tags, on both adult and juvenile white sharks.
A recovered white shark tag, full of data!
These tagged sharks had already taught us a lot about their travels, but the tags contained additional untapped information. Specifically, they let researchers to know each shark’s exact fate—whether it lives, died naturally, or ended up caught in fishing gear. John was able to use the data to develop a survival estimate for the population of juvenile sharks in Southern California and Baja California.
Over the course of 16 years, the team of scientists in California and Mexico tracked 37 young white sharks. From the known fate of all these tagged animals, John estimated that 63 percent of juveniles survive each year.
The paper showcases a powerful statistical tool, Sal says—one that’s useful not just for sharks. Thousands of pop-up tags have been deployed on marine species worldwide, but until now, nobody had used their data in this way.
“We were surprised to see that we were the first to apply this methodology on PAT tags,” he says. “This will pave the way for researchers to estimate annual survival, so vital to conservation for many other ocean species.”
Tagging a young white shark in November 2014 in Santa Monica Bay.The new paper analyzed data from 37 sharks tagged since 2002.
Young sharks and gillnets
Of the 37 young white sharks studied, one was eaten, though by which predator is unclear. Six died after run-ins with fishing gear, most often gillnets—a kind of mesh curtain designed to trap halibut and other bottom-dwelling fish in nearshore waters.
“We were able to see when sharks interacted with gillnets, when they were entangled but released, and when they were killed,” Sal says.
From these data, a few patterns emerged. One is that the bigger a shark grows, the less danger a gillnet poses—possibly because larger sharks snagged in gillnets are better able to fight their way free. The paper also found that more young white sharks died off the coast of Baja, compared with Southern California.
Thank you to @noaasanctuaries for the graphic!
That might stem partly from differences in the way fisheries are managed. White sharks are a protected species in both countries, and it is illegal to catch or sell white sharks. However, other related regulations differ between the U.S. and Mexico. In the U.S., gillnets are banned within three miles of the California coast; Mexico has instituted a three-month moratorium on shark fishing every summer—peak pupping season for white sharks. Both approaches help conserve sharks, and the study illustrates the added benefits of inshore gillnet measures for survival of juvenile white sharks.
Coauthor Oscar Sosa-Nishizaki, a professor at Mexico’s Ensenada Center for Scientific Research and Higher Education, agrees. Oscar has worked with the Monterey Bay Aquarium since 2002, contributing to research in places like Vizcaino Bay—a nursery area for young white sharks.
“It’s very important to work with the fishermen,” he says. “We want to know whenever they incidentally catch a white shark.”
Another best practice might be encouraging fishing crews to check their gillnets more than once each day, so a shark accidentally entangled in the net doesn’t stay there for long. This small step can cut a shark’s chance of death by gillnet in half.
“We have learned that if fishermen check their nets frequently, white sharks are quite hardy,” Sal says. “If released promptly, there’s a good chance they’ll survive.”
“This research suggests the importance of a collaborative approach to management in California and Mexico, and opportunities to innovate on best practices that can support fishermen, research and protections for white sharks,” he adds.