International partnership confirms a new Baja nursery area for white sharks
It’s relatively easy to spot when and where a pregnant animal gives birth on land. But in the sea, it’s a whole different story.
Over the past few decades, researchers studying the elusive great white shark have pieced together a picture of their underwater lives: The adults seasonally travel between a remote region of the Pacific Ocean—dubbed the White Shark Café—and their feeding grounds in Central California and Mexico.
But where do females give birth, and where do the offspring grow up?
“We don’t know whether [the sharks] pup in-shore or off-shore,” explains the Aquarium’s Director of Collections John O’Sullivan. “We don’t even know whether they pup in American or Mexican waters.”
But in a paper recently published online in the journal Fisheries Research, scientists have revealed a new piece of the juvenile white shark puzzle. The study—an international collaboration between researchers at the Ensenada Center for Scientific Research and Higher Education (CICESE); Monterey Bay Aquarium; California State University, Long Beach; and other institutions—revealed that Bahia Sebastian Vizcaino, a warm lagoon on the coast of Baja California, is a nursery area for newborn white sharks.
Defining a nursery
For 30 years, John and his colleagues have known that the waters between Point Conception and San Diego were home to young white sharks. In John’s words, this location was “grandfathered in” as a nursery area—it was simply understood that southern California was the place to find young white sharks.
But in the early 2000s, satellite tags deployed on the juveniles by the Aquarium and its research partners revealed that young sharks didn’t stay in California. In fact, they didn’t even stay in the U.S. They were long-distance migrants, traveling regularly across the Mexican border and back again.
So where did these young sharks actually come from: California or Mexico?
Bahia Sebastian Vizcaino seemed a likely spot for a Mexican nursery. It was known to be home to juvenile—“young of the year”—white sharks, and it also supported fish species that could provide plentiful food. But in order to prove that the bay was a nursery, researchers had to demonstrate that the location met three criteria: It had to be home to relatively large numbers of juvenile sharks; the number of juveniles had to increase on a seasonal basis; and the seasonal increase had to be consistent from year to year.
Two nations, working together
To evaluate whether Bahia Sebastian Vizcaino met these criteria, John teamed up with CICESE biological oceanographer Oscar Sosa-Nishizaki and his doctoral student at the time, Erick C. Oñate-González. Oscar has led Aquarium-supported white shark research projects in Mexico since 2002. With help from numerous students, he has collected detailed data on white sharks in the region.
To gather data, Oscar and his students work closely with local fishermen to monitor when white sharks are caught. Although the Mexican government has taken several steps to protect white sharks since 2007, including a complete fishing ban since 2014, young of the year and juvenile sharks are still accidentally caught in commercial gillnet gear along the Baja coast. In return for data on each shark captured, Oscar works with the fishermen to assure that they know the regulations, and shares methods they can use to safely release live white sharks from their gear.
“They know that they should not be catching white sharks,” explains Oscar. “And if they don’t know, we explain the details to them. That helps them to avoid tickets, and so most of them are open to working with us.”
Oscar’s catch data helped John and their colleagues formally define Bahia Sebastian Vizcaino as a nursery area. They found that it does indeed support a large population of newborn and young-of-the-year sharks. In addition, they found that numbers of these young sharks increase reliably between May and September of each year.
Protecting newborn white sharks
In addition to formally defining Bahia Sebastian Vizcaino as a nursery area, the study found that between 25 and 175 newborn sharks are caught there each year. In concert with recent data showing that sharks in the Northeastern Pacific are an independent population from other white sharks worldwide, these findings have important conservation and management implications.
Additional education and action is necessary, the researchers say, to reduce catch levels that otherwise “could have a substantial negative impact on the larger white shark population in the Northeastern Pacific.”
Baja fishermen may accidentally snare white sharks when they use gillnets to catch bottom-dwelling species like California halibut and guitarfishes, according to Erick, lead author of the paper. These commercially important fishes frequent the same waters as young white sharks.
“It’s important to identify and protect these nurseries in order to support successful juvenile recruitment and survivorship,” Oscar says. “That’s the key to ensuring long-term conservation of the white shark population in the Northeastern Pacific.”
In 2007, the Mexican government published standards to regulate fishing for sharks and rays. These standards list the nursery areas where fishing is prohibited. Oscar is now corresponding with government officials to explore the possibility of adding Bahia Sebastian Vizcaino to that list. In addition, researchers will continue to track numbers of sharks in the area to ensure that the population isn’t declining.
These steps are crucial to maintaining the health of white shark populations in the Northeastern Pacific, but Oscar and John’s data are also part of a larger ecological story.
The key to healthy ecosystems
“We’re just beginning to understand the lives of white sharks in this part of the world,” says Dr. Salvador Jorgensen, a senior research scientist who leads the Aquarium’s white shark program.
“We know they play key roles in maintaining the health of ocean ecosystems. That’s why it’s critical to identify waters important to them at different life stages. Newborns and young sharks are especially vulnerable to fishing pressures. Pinpointing important nursery areas makes it much easier for fisheries managers to address localized threats that could imperil young sharks.”
The international research team hopes to address key questions the coming years: What makes Bahia Sebastian Vizcaino such a good place for newborn white sharks? How long do the newborns stay there? Where exactly do female sharks give birth, and how often do they pup?
“We have a common goal to resolve certain questions,” says John. “And we’re methodically working through these questions to better understand the species. Ultimately, this will allow us to provide better information for better management.”
But until all these questions are answered, the identification of Bahia Sebastian Vizcaino as a nursery area will allow scientists and managers to better protect the offspring of this rare and charismatic species.
Importance of Bahia Sebastian Vizcaino as a nursery area for white sharks (Carcharodon carcharias) in the Northeastern Pacific: A fishery dependent analysis; by Erick C. Oñate-González, Oscar Sosa-Nishizaki, Sharon Z. Herzka, Christopher G. Lowe, Kady Lyons, Omar Santana-Morales, Chugey Sepulveda, César Guerrero-Ávila, Emiliano García-Rodríguez, John B. O’Sullivan. Fisheries Research, Volume 188, April 2017, Pages 125–137.