Where elephant seals go and what they do during their annual migrations have been tracked in unprecedented detail, showing that these seagoing mammals can adapt to environmental changes, which could help them adjust to a warming world, researchers say.
Nearly 300 female elephant seals were monitored with satellite tags during the two foraging trips they made each year. The results, detailed May 15 in the online journal PLoS ONE, show elephant seals traveling throughout the entire northeast Pacific Ocean in search of prey such as fish and squid.
"This work is unprecedented in terms of the number of animals tracked. For the first time we can truly say that we know what the elephant seal population is doing," said Daniel Costa, leader of the research group at the University of California, Santa Cruz.
After the breeding season in February and March, female elephant seals head out to sea for two months before returning to the rookery (a colony of breeding animals) to molt. Then they leave in June on a post-molting migration that often lasts eight months. The amount of food a female is able to find on these foraging trips directly affects her breeding success and, if she gives birth, her pup's growth rate and chances of survival.
"If foraging is not good, the pups are smaller at weaning because the females produce less milk," said Patrick Robinson, a postdoctoral researcher in Costa's lab and lead author of the journal article.
The researchers found that individual seals pursue a variety of different foraging strategies, and most of them target one oceanographic feature in particular: a boundary zone between two large rotating ocean currents, or gyres. Along this boundary, the cold nutrient-rich waters of the sub-polar gyre in the north mix with the warmer waters of the subtropical gyre, driving the growth of phytoplankton (microscopic marine plants) and supporting a robust food web. Presumably all that potential prey is what attracts the elephant seals.
"The highest density of seals is right over that area, so something interesting is definitely going on there," Robinson said in a statement.
Previous studies by Costa and other participants in the Tagging of Pacific Predators program have shown that this boundary zone is important for a wide range of marine predators, including sharks, tuna and albatrosses.
A surface feature associated with the boundary zone, caused by blooms of phytoplankton, is detectable in satellite images, but it moves seasonally as much as 600 miles (1,000 kilometers) to the south. The deep-diving elephant seals do not follow this surface feature but continue to target the deep boundary zone between the two gyres.
Variety of strategies
Smaller numbers of female elephant seals feed in coastal regions, pursuing bottom-dwelling prey along the continental shelf.
One large female that feeds near Vancouver Island holds the record for deepest recorded diveby an elephant seal: 5,788 feet (1,754 meters), or well over a mile.
The elephant seals were tagged at the rookery on Año Nuevo Island off the coast of Northern California and 690 miles (1,150 km) to the southeast, at Islas San Benito off the coast of Mexico. [Images: Tagging Elephant Seals]
"A lot of those animals travel much further to get to foraging areas in the north, so they might spend an extra week traveling, and we wanted to see how that affects them," Robinson said. "The animals from San Benito that do go up to feed at the boundary zone do fine, but we also found that many of them stayed closer to home, feeding along the continental shelf, and they were successful, too."
This suggests elephant seals may be able to withstand environmental perturbations such as climate change because the population is not dependent on a single foraging strategy.
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