Why did the great white shark cross the northern Pacific Ocean? It may sound like the beginning of some bad environmental joke, but it’s exactly what researchers have been asking themselves for more than a decade. Now, they believe they may have some answers.
Every year, a large group of sharks who normally call the California coast home migrate to the open ocean halfway between Baja California and Hawaii in a Colorado-sized region dubbed the White Shark Café. But why they would leave the nutrient-dense waters, filled with plenty of seals and sea lions to get fat on, for the seemingly barren 260-kilometer-wide (160-mile) oceanic wasteland was a mystery for marine scientists from the Monterey Bay Aquarium and Stanford University. So, the team tagged three dozen white sharks with satellite pop-up trackers and followed them out to sea during their April migration.
On their way to the café, the sharks were recorded making dives of up to 915 meters (3,000 feet) by using warm currents to help move down the water column. Once they reached their destination around late winter and early spring, the sharks began to “bounce dive” in V-shapes, diving to 100-200 meters (330-650 feet) at night. During the day, the sharks dove as deep as 450 meters (~1,450 feet) where the water is cold and low in oxygen. The results, yet to published in a peer-reviewed study, are surprising.
“It’s the largest migration of animals on Earth – a vertical migration that’s timed with the light cycle,” researcher Salvador Jorgensen told the San Francisco Chronicle. “During the day they go just below where there is light and at night they come up nearer the surface to warmer, more productive waters under the cover of darkness.”
Located 1,200 nautical miles east of Hawaii at the North Pacific Subtropical Gyre, one of the largest biomes on the planet, the region is influenced by California’s current (which carries cold, nutrient-rich waters south along the US west coast) and by equatorial upwelling from the south that brings productivity.
While satellite images show a nutrient-poor landscape, the sharks’ behavior tells a different story about what lurks underneath – presumably large groups of phytoplankton, fish, squid, and jellyfish that they are feeding on. Since more than 40 percent of the world’s oceans are open, the researchers say biological laboratories such as the café could be key to understanding climate change and environmental adaptations and responses.
“We now have a gold mine of data. We have doubled the current 20-year data set on white shark diving behaviors and environmental preferences in just three weeks,” said marine scientist Barbara Block.
[H/T: San Francisco Chronicle]