July 31, 2012
Whaling Has Caused Humpbacks To Remain In Antarctic Bays Longer Than Usual
Brett Smith for redOrbit.com - Your Universe Online
As humpback whales struggle to recover from 20th century whaling that severely depleted their numbers, scientists have found that the aquatic mammals reside in the bays along the Western Antarctic Peninsula into late austral autumn where they feast on a bounty of cold-water krill.
According to a new study by Duke University scientists, these habits contradict previous theories on humpback whale behavior, which stated that the whales should have left for breeding grounds during the times when they were being observed in high densities throughout the narrow channels and bays in western Antarctica.
“The old dogma is that by late autumn, the ice is heading in and the whales have headed out,” said lead author David Johnston, a research scientist at Duke´s Nicholas School of the Environment.
“But 70 percent of our surveying took place in waters with no ice, and we detected 371 groups of humpback whales over a 654-kilometer (406-mile) survey area, with density estimates of up to 1.75 whales per square kilometer (0.6 mi).”
He added, “if you were to walk to the bridge of a ship and look around, you´d spot two whales within 500 meters (1,640 feet) of the boat. That´s higher than anyone expected.”
Johnson´s team found the peak densities of whales in small, enclosed sections of Wilhelmina Bay, Andvord Bay and the Errera Channel. They discovered the lowest densities, 0.02 whales per square kilometer (0.6 mi) in the open water of the adjacent Gerlache Strait, coincided with previous studies conducted in the same area that led to theories of the whales´ early departure from the polar continent.
“Establishing the autumn density of humpback whales in the inshore regions of the Western Antarctic Peninsula is crucial for understanding the role they play in this rapidly changing ecosystem,” said co-author Ari Friedlaender. “Our results provide a new perspective on the magnitude of predator-prey relationships in the region as the Antarctic winter sets in.”
Humpback whales have been known to split their time between feeding grounds around Antarctica and breeding grounds from Central America to New Zealand to South Africa. The Duke researchers noted that previous studies have focused on the summer densities of the whales feeding in open waters, but the details and timing of their migration northward has largely remained a mystery.
In addition to the annual advance and retreat of the Antarctic sea ice, the abundance of krill plays a major role in determining the timing of the whales´ annual migration. According to the study, the behavior and abundance of krill swarms is directly linked to the cyclical patterns of sea ice.
“In fall, adult populations of krill appear to move inshore into deep coastal areas, where they overwinter in large aggregations, covered by advancing sea ice,” the authors wrote. “Juvenile and larval krill are thought to overwinter in close proximity to winter sea ice that may serve as both a refuge and food source during winter months”
The Duke researchers also speculated that global warming could have an effect of the future behavior of the humpbacks. They noted in their report that if the “Gerlache Strait region remains largely ice free in future winters, non-reproductive humpback whales could choose to remain close to large overwintering krill swarms and forego migration to lower latitudes.”