Seals wearing special tags are playing a vital role in collecting temperature and salinity measurements from the polar oceans, delivering insights for weather forecasters, climate scientists and biologists, according to a new review.
Polar seas play a critical role in climate and weather systems, but they are poorly understood. Data collection is difficult and expensive, especially during the long, harsh winter, and sea ice prevents access by ships or floating buoys.
This is where seals can help – they are ideal for collecting data close to shore and in pack ice. Elephant seals, for example, spend 90 per cent of their lives at sea, travelling up to 4000km on months-long feeding trips. They dive about 60 times a day, reaching depths of up to 2000 metres – deeper than a military submarine.
Although the idea of using animals to collect data had been around for some time, it was a biologist, Professor Mike Fedak, and his colleagues at NERC's Sea Mammal Research Unit who persuaded the scientific community to take it seriously. 'We wanted to know more about the places the animals were visiting,' he explains. 'We'd ask the oceanographers "what's the ocean like here?" but they didn't know. NERC was using Autosub [a robot submarine] to explore under the ice, and we thought "we've got our own Autosub - the elephant seal!"'
The Sea Mammal Research Unit developed sophisticated tags that can measure temperature, salinity and pressure. They record a series of measurements at different depths while the seals are coming up from a dive, and transmit the stored profile up to a satellite when the animals surface to breathe. The tags are attached using fast-setting glue, and fall off naturally when the animals moult, up to 11 months later.
The first large-scale use of the tags was in 2003, when they were glued to the heads of 80 elephant seals at four colonies in the Southern Ocean. 'Apart from being the coolest animal on earth, elephant seals are very approachable,' explains Fedak. 'It's easy to fix the sensors on and it doesn't seem to bother them. They spend a lot of time at sea, so the tags don't get scraped off on ice or rocks, and they go to some interesting places where it's hard to get data any other way.'
Now the tags are being used by many countries including Norway, France, the US and Australia, and scientists are recruiting other species including Weddell seals, hooded seals and ring seals. 'We choose animals and program the tags to tailor them to where the oceanographers want to get data and to suit the behaviour of a particular species,' says Fedak. 'The beauty of it is that it's a partnership between oceanographers and biologists. It took a while for the oceanographers to accept it, but now they've really embraced it. It's so cost-effective compared to using ships.'
Marine animals – including seals, turtles and diving birds – have now provided over 1.4 million temperature profiles. Over 300,000 of these also measured salinity – crucial for a full understanding of ocean processes. Animals have provided 70 per cent of all the profiles in the World Ocean Data Base south of 60oS.
The profiles are made freely available every day via the World Meteorological Office, for immediate use by weather forecasters and ship operators. After further processing they are stored in the open-access World Ocean Database. Scientists around the world are now using the data in models to study the role that oceans play in climate change, including how ice sheets form and melt.
The data can also reveal important insights into the behavior of the animals themselves, including migration routes, feeding grounds and the distribution of different colonies. This can help with conservation policy, so animals benefit too.
Now seals are being recruited to the iStar project, to help find out why the ice sheets around the huge Pine Island glacier in Antarctica are breaking up, and what this means for sea levels. 'It's a really exciting project,' says Fedak. 'There's a whole bunch of geologists, oceanographers and biologists camped out on the glacier, and seals are playing a really important part - helping to get data from under the ice, and during the winter.'
The tags are being constantly improved. 'In a couple of months, all our tags will have fluorescence sensors to measure chlorophyll concentrations, showing where plankton are abundant,' adds Fedak. 'Next we're working on sensors for oxygen, and then carbon dioxide. The possibilities are really expanding.'
Dan Costa has used seals to gather data, as reported last year:
SANTA CRUZ - Researchers at UC Santa Cruz, who pioneered the use of satellite tags to monitor the migrations of elephant seals have compiled one of the largest datasets available for any marine mammal species, revealing their movements and diving behavior at sea in unprecedented detail.
A new study published Tuesday in the journal PLoS ONE focuses on the annual migrations of adult female elephant seals, with data from nearly 300 animals.
The results show elephant seals traveling throughout the entire northeast Pacific Ocean on foraging trips 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, professor of ecology and evolutionary biology and leader of the elephant seal research group at UCSC. "This represents the efforts of a large number of graduate students, postdoctoral researchers, and undergraduate volunteers who have all worked very hard to make this happen."
The researchers found that individual seals pursue a variety of different foraging strategies, but 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 and supporting a robust food web. This likely leads to a concentration of prey along the boundary, said Patrick Robinson, a researcher in Costa's lab and lead author of the paper.
"The highest density of seals is right over that area, so something interesting is definitely going on there," Robinson said.
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 elephant seals, 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 620 miles 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.
Smaller numbers of female elephant seals feed in coastal regions, pursuing bottom-dwelling prey along the continental shelf, or in other areas outside of the boundary zone such as around seamounts.
Among these is a large female that feeds near Vancouver Island and holds the record for deepest recorded dive by an elephant seal. The data analyzed in the PLoS ONE paper include one dive to 5,765 feet, and the same seal dived even deeper on a more recent foraging trip, reaching 5,788 feet, Robinson said.
Female northern elephant seals make two foraging trips every year. After the breeding season in February and March, they head out to sea for two months before returning to the rookery to molt.
Then they leave on a long post-molting migration that often lasts eight months, from June to January. 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," Robinson said.
In addition to tracking the foraging migrations, the researchers monitor the health of the seals and track birth rates over time. Tags are attached harmlessly onto the animals' fur and recovered when they return to the rookery. Before and after each migration, the researchers get weights and blood samples from the tagged seals, which always return to the same rookery.
The tags used today are far more sophisticated than the first ones deployed by UCSC researchers in the 1980s. Current devices, used on a subset of the seals in this study, can capture an animal's location, swim speed, and depth and duration of dives, as well as the temperature and salinity of the seawater and how that changes with depth.
Most of the animals in this study were tagged at the rookery on Año Nuevo Island, where UCSC researchers have been studying elephant seals for decades.
But the study also involved a collaboration with researchers in Mexico to tag elephant seals at Islas San Benito, which is nearly 700 miles southeast of Año Nuevo. "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."
These findings highlight the adaptability of elephant seals, suggesting that they may be able to withstand environmental perturbations such as climate change because the population is not dependent on a single foraging strategy.
This research is also providing valuable oceanographic data. While ocean surface temperatures can be measured by satellites, oceanographers have limited temperature data from deep waters.
Costa's group has organized the temperature data collected by the elephant seals into a format that oceanographers can use and uploaded it to the World Ocean Database, providing millions of ocean temperature data points not otherwise available.