July 21, 2011
If I asked you to name a marine predator, your first answer would probably be a shark. But this category is so much bigger—sea turtles, tuna, elephant seals, whales, even birds reign at the top of the ocean’s food webs. Many of these species are at risk from challenges such as overexploitation and climate change. And scientists hoping to protect these animals have often lacked good data on their movements; it’s hard to see where creatures go beneath the water’s surface.
In 2000, marine researchers began the Tagging of Pacific Predators project as part of the decade-long Census of Marine Life. They deployed 4,306 electronic tags, which yielded 1,791 tracks from individuals of 23 marine predator species in the northern Pacific (for a total of 265,386 days of data over 2000 to 2009). The results of their study were published earlier this month in Nature.
“It is like asking, ‘How do lions, zebras and cheetahs use Africa as a whole continent?’ only we have done it for a vast ocean,” the study’s lead author, Barbara Block of Stanford University, told Nature.
The species were concentrated along two main routes: One followed the California Current, which flows southward off the U.S. West Coast, and the other along the North Pacific transition zone, the boundary running east to west between the cold waters of the sub-Arctic and the warmer waters of the subtropics.
The researchers found that the exact location of a species represented a trade-off between having access to a greater amount of ocean productivity (meaning more food) and the temperatures that the predator–or its preferred prey–could withstand. As a result, two similar predator species can occupy two different ranges without overlapping (thus avoiding turf battles between, for example, white sharks and mako sharks).
Another factor that is likely to be an important driver of predator migration is upwelling, in which cool waters rich in nutrients are brought up to the surface of the ocean. Those nutrients help microscopic organisms grow and multiply and feed bigger critters up the food web. “Using satellite observations of temperature and chlorophyll concentrations [a marker for the amount of microscopic organisms in the water], we can now predict when and where individual species will be,” study co-author Daniel Costa of the University of California, Santa Cruz, told Nature.
Some predator species, such as yellowfin tuna, salmon sharks and elephant seals, can even be found returning to the same place every year, like wildebeests of the Serengeti.
Researchers hope that this data will help them manage these species in the future. Because no one can predict what might happen to the rest of the species in the food web if these top predators are lost—and who knows what tasty seafood may become a taste of the past.
Last up for Predator Week: What preys on humans?
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