Antarctic Seals Survive Arrival of Massive Icebergs

Two Weddell seals strike a pose on ice. CREDIT: Jay Rotella, Montana State University View full size image

Updated on Wednesday, Sept. 26, at 12:42 p.m. ET

After an enormous iceberg called B-15 broke off from Antarctica's Ross Ice Shelf, it released chunks of ice — themselves massive icebergs — that wreaked havoc on emperor penguins that breed to the west.

But this doesn't appear to have been the case for a population of Weddell seals whose breeding sites were also in the icebergs' path, new research has found.

The calving of the 4,250-square-mile (11,000 square kilometers) B-15 iceberg gave researchers a unique opportunity to examine how living things respond to an extreme event in their environment — in this case, the arrival of the chunks of B-15.

Previous research has examined the impact of extreme events on emperor penguins. At one of the penguin breeding sites, collisions between the icebergs and the ice on which the penguins were nesting killed some and forced others to move to less-ideal breeding sites. At another site, an iceberg blocked the penguins' route to the open water where they feed, a research team led by Gerald Kooyman of the Scripps Institution of Oceanography reported in 2007. [See Photos of the Weddell Seals]

Weddell seals vs. ice chunks

In the latest study, another team looked at how the icebergs affected the seals living in Erebus Bay on the Ross Sea. Scientists have been monitoring the seal population there since 1969, and they honed in on the period from 2001 through 2005, when the remnants of B-15, which calved in 2000, passed through.

"Here, what we found is a pretty mild effect on the seals," said Thierry Chambert, a doctoral candidate at Montana State University and the lead researcher on the project. "They survived pretty well during the five years of iceberg presence, but their reproduction was lower than usual."

Chambert and colleagues focused on female seals, since they are more reliably spotted on the ice than males.

They found that in four of the five years in which the icebergs were present, the number of females that showed up at the colony was lower than usual. In addition, the probability that an individual female would be alive the following year did not change, but reproduction rates dropped.

In years without icebergs, there is a nearly 70-percent chance a female will give birth to a pup. During the iceberg years, this dropped by about 20 percentage points, down to a 50-percent chance.

How seals respond

Researchers don't know why the icebergs had the effect on the seals that they did, but they have some hypotheses.

The icebergs may have made the seals' breeding sites more difficult to access by altering the terrain of the sea ice. Other work has shown that the icebergs had a negative effect on the amount of tiny, floating plants, called phytoplankton, in the sea. This effect could have rippled up the food chain to the seals, which feed on various fish species and other Antarctic foods. [Album: Stunning Photos of Antarctic Ice]

The seals had a few advantages over the penguins. Unlike the emperor penguins, which must traverse the ice to feed, the female seals generally stay put once they haul out at the breeding site. The seals may also have benefitted from being farther from B-15 when it broke off the ice shelf, Chambert said. 

Chambert and colleagues believe that the drop in reproduction is indicative of seals' strategy for dealing with difficult times. Seals can live up to 30 years, and they breed annually beginning at age 6, meaning if they miss a year or two, they will have a chance again later.

"This is a long-lived species. Because of that, you expect a strategy that will favor survival over reproduction," he said.

By putting off reproduction, the seals can spend more energy on survival. This gives them the chance to breed again when conditions are better.

The researchers note the icebergs were only around for a short period. More frequent iceberg calving, such as those predicted as a result of global warming, could have had a more negative effect on the seal population, the researchers write.

The research was published today (Sept. 25) in the journal Proceedings of the Royal Society B.

Editor's Note: This story was updated to correct an error in the size of the B-15 iceberg.

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