Dan Costa, who talked to us here in Cambria about his research last year, has published his study about Elephant Seals as Oceanographers. He told about one female seal who was tagged when she was six or seven years old as part of the first tagging in 1995. Through some kind of serendipitous chance, the graduate students read a tag incorrectly and tagged her again in 2006. Seals that old weren't supposed to be part of the study. She was at least 17 at that time. They were surprised to find that she followed nearly the exact same route, far out into the Pacific, past the tip of the Aleutians.
In 2010, the last time they looked for her, she had another pup and was still going strong. This photo is a representative seal, not her.
His team has also tagged other species, which have produced additional information. I look forward to publication of those results.
ScienceDaily (May 15, 2012) —
Researchers at the University of California, 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 May 15 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 UC Santa Cruz. "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. Presumably, this leads to a concentration of prey along
the boundary, said Patrick Robinson, a postdoctoral 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 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.
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 1,747 meters (5,765 feet,
well over a mile), and the same seal dove even deeper on a more recent
foraging trip, reaching 1,754 meters (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 1,150 kilometers (690 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.