Human Interaction

Whale Wars Why are there whales in Antarctica?

posted: 05/15/12
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DCI

Every year, the Southern Ocean — the body of water encircling Antarctica — produces the greatest concentration of whale food on the planet.

Called "krill," these tiny, shrimp-like creatures once supported millions of baleen whales in Antarctic waters. That is, until the rise of industrial whaling in the early 1900's.

Advances in whaling technology, including faster ships and rocket-propelled harpoons, ushered in the greatest slaughter of marine mammals the world has ever seen. During the peak of Antarctic whaling, nearly 40,000 whales were killed each year.

By the late 20th century, what was once a stronghold for whales became a hiding place for frightened factions. Today, the Southern Ocean is a sanctuary and recovery zone for the world's remaining southern whale populations.

So why is whale food, or krill, so abundant in Antarctica? The answer begins with pack ice.

Pack Ice

The two major ice packs in the world are the Arctic ice pack and the Antarctic ice pack. The Antarctic ice pack — or Antarctic ice sheet — covers about 98 percent of the Antarctic continent. It represents the largest single mass of ice on earth, spreading over 4 million miles in the Antarctic summer and 19 million in the winter. It accounts for 90 percent of the world's ice and 70 percent of its freshwater. The amount of water that the ice pack alternately puts into or pulls out of the ocean and atmosphere has a major impact on global climate change.

In winter, krill feed on algae beneath the pack ice. Thus, the amount of pack ice in Antarctica has a direct effect on the total number of krill.

New studies show that winter temperatures in the Antarctic are climbing and the ice pack is beginning to deteriorate. It is forming later and retreating earlier, and this is having an impact on a number of species — particularly krill. The ice pack is often considered the engine that drives the Antarctic ecosystems, and changes in the ice sheet could have serious consequences for the region.

Fronts and Currents

The abundance of Antarctic krill isn't entirely dictated by pack ice. In summer, krill leave the pack ice to feast on huge blooms of phytoplankton in the Southern Ocean. Phytoplankton are microscopic plants that float freely in the ocean. They are the basis for the vast majority of oceanic food webs. The phytoplankton blooms in Antarctica are enormous. The reason? These tiny plants are crucially dependent on nutrients, and Antarctica has a globe-spanning "machine" that works day and night to bring upwellings of nutrient-rich waters to the surface.

The Antarctic Circumpolar Current (ACC) is a powerful, wind-driven current that wraps around the entire planet, connecting the Atlantic, Pacific and Indian Oceans. Driven by strong westerly winds that can create tumultuous and treacherous seas, the ACC is the largest wind-driven current on Earth. It is a massive flow of water — extending to depths of more than 2.5 miles with a width of more than 120 miles — that acts a barrier between the Southern Ocean and the northern oceans. Its temperature ranges from 1 to 5 degrees Celsius and its speed can reach up to 2.3 miles per hour. Sometimes referred to as the "great ocean conveyor," the ACC allows water, heat, salt and other properties to flow between the three great ocean basins.

Krill

The word "krill" is a Norwegian word for whale food, and that is an appropriate description of these tiny crustaceans. These small (2 inches on average), semi-transparent, shrimp-like creatures are the primary source of food for most of the world's large whales.

They belong to a group of animals called "zooplankton," meaning they are animals that float in the upper portions of the water column and are at the mercy of the ocean currents. They rise and fall with the currents, drifting around in enormous quantities. Krill swarms have been estimated to have 2 million tons of krill spread over more than 450 square kilometers. It has been said that the krill population of the world outweighs the human population (and it's not surprising, considering how many tons of krill some whales eat every single day!) Baleen whales have the most effective system for catching krill, using great baleen plates to filter out water and small plankton but trap the nutritious krill for swallowing.

Before they become whale food — or food for a host of other animals ranging from penguins to seals to birds — krill feed on microscopic phytoplankton, which are plentiful in Antarctic waters thanks largely to upwellings of water at the Antarctic convergence. But worrying new studies show that stocks of krill in Antarctica are declining, likely due to a decrease in the amount of sea ice formed during the winter months. Krill numbers may have dropped by as much as 80 percent since the 1970s. This dramatic decline will likely make it difficult for the great baleen whales to return to pre-whaling population numbers.

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