Dispatches from the Deep
Outfitted with high-tech tags, far-ranging and deep-diving elephant seals are collecting data critical to understanding the oceans, including changes tied to global warming
IT'S A COOL, SUNNY mid-winter morning at northern California's Año Nuevo State Reserve, an ideal day for watching a wildlife spectacle. And from the top of a dune that separates the reserve's brushy meadows from its rocky beach, the scene below looks like it came straight out of a nature documentary: A dozen female northern elephant seals, snorting, barking and flinging sand, are surrounded by at least as many fat, doe-eyed pups screeching for their mothers' attention. At the center of the hubbub, sporting the pendulous nose for which the species is named, sprawls the group's 2-ton, 15-foot-long dominant, or alpha, male. Sensing a rival has come too close, the snoozing giant suddenly rears up on his hindquarters, throws back his head and bellows a warning. Intimidated, the interloper flops down the beach on his belly, moving surprisingly fast for such a colossal creature.
Surveying the scene, biologist Jason Hassrick is concerned, however. Ordinarily at this time of year, the peak of the mating season, "you can hardly walk on the beach," says the University of California–Santa Cruz graduate student, "because it's wall-to-wall seals." For some reason, says Hassrick, most of the seals this season arrived late to breed and left the rookery early.
A quarter mile up the beach, Hassrick and his colleagues locate the next group of seals. It looks similar to the first, except that one female wears what appears to be a small orange hat on top of her head. The "hat" is actually a satellite tag that will allow scientists to track the animal as she forages in the Pacific Ocean over the following months. Attached to it is a pack-of-gum-sized electronic device that will simultaneously record the time and depth of each of her dives, as well as light levels and temperatures as the seal moves up and down the water column. According to Hassrick, who tagged the 10-year-old known as W1412 two days earlier, these data "may help us understand why the seals had such a truncated breeding season."
Data collected by W1412, along with 19 other seals tagged during the season, promises a wealth of additional insight into the animals' underwater world as well. While scientists for many years have been tagging marine creatures to learn about their movements and behavior, "now we're also using elephant seals and other animals as sensors to tell us about oceanographic conditions," says biologist Dan Costa, Hassrick's advisor and a professor of ecology and evolutionary biology at the University of California–Santa Cruz. Their work is part of a larger, multi-institutional program, Tagging of Pacific Pelagics (TOPP), in which dozens of scientists have tagged more than 2,000 animals of 23 species--all of them top, or apex, Pacific Ocean predators--ranging from seals and sea lions to seabirds, whales, tunas and sharks. "We're learning how these animals use the ocean--where they go and what they do--at the same time we're giving oceanographers data to improve their models," says Costa. Such data are important not only to understanding Earth's largest wildlife habitat, he adds, but also to predicting the course and detecting the consequences of global warming.
Traditionally, oceanographic data have been collected by sensors either towed from ships or attached to floating buoys or unmanned underwater vehicles. But scientists like Costa argue that elephant seals and other marine species are gathering more of this information far less expensively than machines. The animals also routinely visit parts of the ocean that sensors alone cannot reach, including the deepest waters and seas that are covered by ice. In the Southern Ocean, for example, Costa and a team of British, French, Australian and U.S. colleagues are using southern elephant seals to sample waters under the Antarctic ice pack--collecting profiles of more than 10,000 dives a year from an inaccessible area that's critical to understanding how the world's oceans will respond to climate change. In the Pacific, the planet's largest ocean, northern elephant seals and other TOPP animals are producing some million profiles a year.
Elephant seals make particularly good data collectors. After lumbering out of Año Nuevo and other West Coast rookeries, the animals travel thousands of miles on foraging trips throughout the eastern North Pacific. Among the deepest-diving of all marine mammals, they routinely plunge to depths of 2,000 feet, making more than 60 descents a day. And because the seals return faithfully to the same beaches twice a year--to breed in winter and molt in spring and summer--"they're really easy to tag, and we get more than 90 percent of our tags back," says Hassrick.
Costa attached the first time-depth recording device to an elephant seal 15 years ago. Based on what scientists knew about the species' physiology, "we assumed they were deep divers," he recalls, "but we were blown away when we found out just how deep they dive." (The deepest recorded dive so far, by a female, was to 4,800 feet, or nearly a mile.) Equally amazing was the discovery that seals come to the surface just 2 to 3 minutes for every 20 to 30 minutes they spend diving, meaning they spend about 90 percent of the time at sea underwater.
As tagging technology becomes more sophisticated, a more complete picture of elephant seal life is emerging. One of the more interesting discoveries is that males and females have evolved very different foraging strategies. While males leaving Año Nuevo head directly north to feed along continental margins between Oregon and Alaska's Aleutian Islands, females fan out into the Pacific, foraging for fish and squid in deep waters across a wide swath of ocean. Within this vast region, researchers have identified several "feeding hotspots," which Costa likens to "finding the grasslands and the water holes" that are important to terrestrial creatures. Attracting multiple species, including seals, sea turtles and seabirds, these critical feeding grounds are defined by specific combinations of temperature and current patterns that work together to concentrate prey.
Indeed, how completely seals and other TOPP predators rely on such underwater "weather" is among the program's most significant findings--and cause for concern as humans continue to alter global climate. Already, Costa and other researchers have found--to their surprise--that even deep-diving elephant seals can be adversely affected by weather anomalies at the sea's surface. During El Niño years, when eastern Pacific temperatures rise, the scientists discovered that female seals spend more time searching for prey and gain less weight than they do other years. As a result, their pups the following breeding season weigh less at weaning, and fewer offspring survive.
Fortunately, elephant seals are resilient creatures. Hunted nearly to extinction for their blubber by the end of the nineteenth century, "elephant seals have made one of the most remarkable recoveries of any species ever," says biologist Burney Le Boeuf, a retired University of California–Santa Cruz professor who has studied the animals for more than 30 years. In an upcoming paper, he and former student Richard Condit, now a biologist for the Smithsonian Tropical Research Institute, report that, thanks to protection by the Mexican and U.S. governments, the number of females breeding at Año Nuevo grew from 12 individuals in 1961 to nearly 3,000 today. The species' total population has increased to at least 150,000. "There's no question that elephant seals, like all animals, will be affected by global warming," says Costa, "but my hope is that they'll be able to deal with it given they can forage throughout the North Pacific."
Hassrick, who in the short term worries more about the impact of overfishing on prey, is less optimistic. He notes that individual seals, while they vary widely in foraging strategies, end up sticking to the same routes year after year. "Their route fidelity is amazing," says Hassrick, "but that could become a problem if ocean conditions change too quickly in the future."
Senior Editor Laura Tangley visited Año Nuevo State Reserve with University of California–Santa Cruz researchers during the 2006–2007 breeding season.
The Mating Game
Elephant seals may spend very little of their lives on land, but what they do there--mainly fight, mate and rear their young--is fascinating, not only to scientists, but to visitors who flock to the handful of rookeries that are open to the public. Males arrive on the beaches first, where they engage in fierce battles for dominance. Though bulls rarely die in these fights, they can be badly injured, and many sport scars as evidence of previous clashes. Once winners have established control over their territories, females haul out. Pregnant from the previous year's breeding season, they give birth and nurse their pups for about 4 weeks--fasting the entire time and losing up to 40 percent of their body weight.
After the pups are weaned, it's time to mate again before heading back to sea. Yet because elephant seals have what's known as a highly polygynous mating system, very few males actually do get to breed. In one season, a dominant bull may sire 50 pups--and more than 500 in a lifetime--while the majority will never produce a single offspring.
But not for lack of trying. Lurking at the fringes of seal territories, legions of subordinate males continually try to sneak in while the alpha bull is busy or sleeping. When females leave the beach at the end of the season, these males "line up along the water line, basically harassing the females back to sea," says Jason Hassrick, a University of California–Santa Cruz biologist. To decrease the distance she must travel, a female often waits for high tide to make her escape. "And sometimes," says Hassrick, "you'll get a really good alpha male who will escort a female all the way back into the water."
To check out the action first-hand, you can visit the elephant seal rookery at Año Nuevo State Reserve, which is 55 miles south of San Francisco. Winter is the best time to go, but some seals--at least juveniles--can be seen every month. The reserve is open to the public year-round except for the first two weeks in December. For more information, go to www.parks.ca.gov.
NWF Priority: Fighting Global Warming
Combating global warming is a top priority for NWF, which is, among other activities, backing national legislation to reduce greenhouse gases, publishing reports on warming's impact on wildlife and collaborating with its state affiliates on grassroots efforts. See www.nwf.org/globalwarming.
As the leader of last season's elephant seal tagging effort at Año Nuevo State Reserve, biologist Jason Hassrick had his work cut out for him. He and his team attached satellite tags--complete with miniature electronic devices to record time, depth, temperature and light level--to a total of 20 seals during the animals' winter breeding season. Then they had to return to the beaches to locate and collect all the expensive gadgets when the seals molted several months later. The data collected by the tags are helping scientists gain a better understanding of the Pacific--the world's largest ocean--including how it may be changing as a consequence of global warming.
But Hassrick, a Ph.D. student in ecology and evolutionary biology at the University of California–Santa Cruz, also found time to pursue his own research. "For my dissertation, I'm focusing on how diving behavior develops in northern elephant seals," he says. Specifically, Hassrick wants to learn whether the diving performance of females improves as they get older. Because older females gain more weight during pregnancy and give birth to larger pups--meaning they invest more energy than younger moms--" my hypothesis is that they become more efficient foragers as they age," he says.
From what Hassrick has found so far, it looks like the hypothesis is correct. Taking blood samples from tagged females of known ages, he has discovered that older seals store more oxygen in their blood, "which tells me they have a greater aerobic diving capacity, so they can stay underwater longer," he says. Among the deepest diving of all marine mammals, both male and female elephant seals routinely plunge to depths of 2,000 feet while foraging for fish, squid and other deep-water prey. --Laura Tangley