International Research Team Pinpoints New Baja California Nursery for Northeastern Pacific White Sharks

Mar 30, 2017

Aquarium News
Conservation & Science
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The warm coastal lagoons of Baja California are nurseries for migratory gray whales. But those aren’t the only babies they nurture. Newly published research pinpoints the surrounding waters in Sebastian Vizcaino Bay as an important nursery for Eastern Pacific white sharks as well.

The findings, by scientists from the Ensenada Center for Scientific Research and Higher Education (CICESE) and U.S. colleagues including the Monterey Bay Aquarium and California State University, Long Beach, also document a high mortality rate—between 25 and 175 newborn white sharks animals each year—when newborn white sharks are caught accidentally in commercial gillnet fishing gear along the Baja coast. CICESE scientists and the Mexican government are addressing the threat by training local fishermen how to release white sharks from their nets unharmed.

Mexico protected the species in 2007, when it banned all commercial and sport fishing for white sharks in Mexican waters. It added another layer of protection in 2012, when it closed some Baja waters to commercial fishing during peak white shark pupping season.  In 2014 Mexico provided additional protection when it prohibited the landing any and all parts of white sharks.

Additional education and action is necessary, the study’s authors say, to reduce catch levels that otherwise “could have a substantial negative impact on the larger white shark population in the Northeastern Pacific.”

The study confirming that Sebastian Vizcaino Bay and its lagoons—located on the west coast of the Baja Peninsula, 500 miles south of San Diego—represent an important white shark nursery and that the population may be at risk from accidental catch of newborn pups was just published online in the journal Fisheries Research. The paper will appear in the April 2017 print edition.

“We’ve known for more than 30 years that the waters from Point Conception to San Diego are a nursery area for white sharks,” said Dr. Oscar Sosa-Nishizaki, a biological oceanographer with CICESE and a co-author of the paper. “Now we’ve confirmed that Sebastian Vizcaino Bay is a second nursery area for white sharks that travel between Southern California and Baja California.”

“It’s important to identify and protect these nurseries in order to support successful juvenile recruitment and survivorship,” he added. “That’s the key to ensuring long-term conservation of the white shark population in the Northeastern Pacific.”

Earlier studies led by the Monterey Bay Aquarium, Stanford University and other colleagues have confirmed that the Northeastern Pacific population is genetically distinct from white sharks elsewhere in the world.

Subsequent research estimated there were only 300 to 400 adult (breeding age or near-breeding) white sharks in the Northeastern Pacific population as of 2008. Other scientists have suggested that when both juvenile and adult white sharks are counted, the total number may be in the low thousands.

“We’re just beginning to understand the lives of white sharks in this part of the world,” said Dr. Salvador Jorgensen, a senior research scientist who leads the Monterey Bay Aquarium 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 study confirms that Sebastian Vizcaino Bay meets three criteria that define potential nursery areas for sharks: It is home to immature sharks in numbers greater than are found elsewhere; the observed increase in numbers of newborns and young-of-the-year sharks occurs on a seasonal basis; and the seasonal increase occurs consistently from year to year.

Data in the study support all three criteria, with the number of newborn sharks at its peak between May and September each year. Sebastian Vizcaino Bay is home to an abundance of other fish species that are known prey items for young white sharks, meaning there is sufficient food available to them at this important stage in their lives, according to the authors.

At the same time, the researchers identified significant threats to newborn sharks in Sebastian Vizcaino Bay. By working with local fishermen and examining the remains of landed sharks at fishing camps along the western coast of the Baja Peninsula, they documented 390 white shark mortalities between 1999 and 2013. Because white sharks were a protected species for most of the period, the mortalities are considered incidental, or unintended, catch.

Baja fishermen may accidentally snare white sharks when they use gillnets to catch bottom-dwelling species like California halibut and guitarfishes, according to Erick Oñate-Gonzalez, lead author of the paper. These commercially important fishes frequent the same waters as young white sharks.

“We and our colleagues in the Mexican government are working with fishermen to find ways for them to avoid accidentally catching young white sharks,” Sosa-Nishizaki said. “This includes changing the size mesh they use in their gillnets, or reducing the amount of time the nets are left unattended in the water.”

It may be necessary to better enforce existing fisheries regulations, and to establish specific conservation zones and gear regulations in waters identified as part of white shark birthing grounds, the authors say.

“The results suggest the need to strengthen surveillance in the implementation of [existing closure areas],” the paper concludes. “Improved education and enforcement of this management measure should contribute to the survival of young white sharks by decreasing the incidental catch.”

We have enjoyed a long and productive collaboration with our colleagues in Mexico,” said John O’Sullivan, director of collections for Monterey Bay Aquarium and a co-author of the study. “Continued collaboration between research teams and government agencies in the U.S. and Mexico is essential in order to ensure the long-term conservation of white sharks along the eastern Pacific.”

Citation: 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.

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