A new study focused on the annual migrations of elephant seals has just been published in the journal PLoS. More than 300 individuals were tracked, using satellite tags measuring a wide range of different data.
“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 individual seals used a wide range of different foraging strategies, though most targeted an oceanographic region where the cold waters of the north mixed with warmer subtropical waters. This mixing drives the growth of phytoplankton and supports a large food web.
“The highest density of seals is right over that area, so something interesting is definitely going on there,” said Patrick Robinson, a post doctoral researcher, and lead author of the study.
Previous studies done by the same team have shown this boundary zone to also be important for sharks, tuna, and albatrosses.
The boundary zone is detectable in satellite images by a surface feature caused from phytoplankton blooms that changes location every year by up to 1,000 km. But the seals target the same deep boundary zone every year, regardless of where the phytoplankton surface feature is.
There’s also a smaller number of female elephant seals that feed in coastal regions, or in other areas, such as around sea mounts.
One of those seals, a large female that feeds near Vancouver Island, holds the record for the deepest dive by an elephant seal, down to 5,788 feet.
The female elephant seals make two foraging trips every year. The first one, right after the breeding season in February or March, and the second one from around June to January. The amount of food they find during these trips determines their success breeding and the health of their offspring.
“If foraging is not good, the pups are smaller at weaning because the females produce less milk,” Robinson said.
The satellite tags tracked the foraging migrations, but also the health of the seals and their birth rates. The tags currently used can measure the location, the swimming speed, the depth and duration of dives, and the temperature and salinity of the water.
Most of the seals were tagged at the Año Nuevo Island Rookery, but some were also tagged at the Islas San Benito rookery much further south.
“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.”
Source: University of Californua Santa Cruz
Image Credits: Dan Costa, Robinson,et al.