Whale Confab in the Antarctic

by Bill Chameides | May 11th, 2011
posted by Erica Rowell (Editor)

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Duke scientists, studying whales in relation to their primary prey, krill, found hundreds of humpbacks in a remote area of the West Antarctic Peninsula. Why so many? (Ari Friedlaender, Duke University)

Want whales? Bring krill.

What a Marine Biologist Dreams About

Imagine spending four weeks on the West Antarctic Peninsula in Wilhelmina Bay in the company of more than 300 humpback whales. A dream for many marine biologists, but reality for Duke scientist and fellow Nicholas School faculty member Doug Nowacek and his colleagues who reported their findings in the open-access journal PLoS One.

How unusual was their whale sighting? Well, the density of humpbacks they saw — about five per square kilometer of ocean surface — was not only the largest ever recorded, it was roughly 10 times larger than previously recorded sightings.

So what explains a humpback congregation of this magnitude? One factor may be that whale populations are now on the rise as a result of the greater restraints on the whaling industry. Perhaps. But Nowacek et al’s data point to a more important reason: literally tons of food.

Climbing up the Southern Ocean Food Web: Phytoplankton to Krill to Humpbacks

The marine food web starts with mostly one-celled green organisms called phytoplankton. These little guys produce virtually all of the food that supports ocean life and about 50 percent of all the food on the planet. Next up are zooplankton, which include small (often single-celled) animals as well as krill (shrimp-like crustaceans) and jellyfish that drift with the ocean currents and feast on phytoplankton. In the Southern Ocean’s frigid waters, krill grow to about two inches in length and live to about five or so years.

There’s a huge variety in the size the of critters that eat krill ranging from fish, penguins, seals, and seabirds to the leviathans that Nowacek et al found lazing in Wilhelmina Bay.

The Story of Krill

Krill may be dinner for many of the Antarctic’s species, but their populations have been declining. A 2004 study found that Antarctic krill stocks have declined by as much as 80 percent since the 1970s (see here and here).

Why? Part of the trend may be due to our growing appetite — krill-based feeds are increasingly cropping up in fish farming and the growing dietary supplement market. It’s estimated that in 2010 the global fishing industry harvested some 150,000–180,000 metric tons of krill, a 40-percent increase over the size of 2009’s catch. But given the amount of krill in the ocean, these are unlikely to have been major factors in the decades-long decline.

A potentially more important factor is climate change. As Nowacek et al write: “The life history of Antarctic krill is intimately tied to sea ice.”

During summer months krill move out into the open Southern Ocean waters providing a smorgasbord for all, but come autumn, they hightail it to the relative safety of Antarctica’s coastal waters and bays where adults and juveniles spend the winter (overwinter) and feed on phytoplankton snug under the cover of sea ice.

But this winter habitat just happens to be located in one of the globe’s fastest warming areas where temperatures have increased by 2.5 degrees Celsius over the last 50 years — several times faster than the global mean rate. And warming temperatures mean less ice. Compared to 1979, the springtime retreat of sea ice in recent decades begins about a month earlier, and the onset of winter-ice formation happens about two month later.

“Diminished sea ice cover,” Nowacek et al write, “reduces habitat available for overwintering juvenile and adult krill and reduces the size of the food-rich marginal sea ice zone in summer. Over the past 50 years, significant decreases in both total sea ice cover and the timing of winter sea ice advance around the [West Antarctic Peninsula] WAP have accompanied dramatic reductions in the standing biomass of krill.” Less ice, less krill for humpback whales.

Whales Have Their Fill of Krill

So what’ve we got? Rebounding whale populations, diminishing sea-ice cover, and declining krill populations. How is all that playing out in the Antarctic ecosystem? Grappling with that question took Nowacek and his team to Wilhelmina Bay’s icy waters in May 2009 where they found the huge humpback confab.

But what became quickly apparent as they began gathering data on what was happening beneath the surface was that this was no random gathering of humpbacks for near-shore fun and games. No. These whales had flocked to the bay in pursuit of a meal, and more specifically to dine on krill.

Using a combination of acoustic-survey and netting techniques to plumb the waters, Nowacek’s team estimated that their 100-square kilometer study area contained about two million tons of krill. Holy krill, Batman — that’s about 10 times more krill in that little ocean patch than the entire fishing industry caught during the entire 2010 season.

And what brought all those krill to Wilhelmina Bay? Protection. In winter adult krill, traveling in larger swarms, move inshore into bays seeking sea ice to protect them from predators while juveniles gravitate to this below sea-ice habitat to feed.

But what made the whole thing such a bonanza for the humpbacks was the absence of sea ice. With occasional brash ice covering just 10 percent of the bay, the krill were easy pickings for the whales, who slept on the surface by day and feasted on krill at night.

Nowacek et al worry that the extended krill season in Wilhelmina Bay caused by the lack of ice may change migration patterns for whales and other predators putting more pressure on krill. I’m more worried about an entirely different predator. If whaling boats change their migration patterns to include an austral fall visit to bay it will be a very sad day for the humpbacks.

Further Reading

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