Humpback Whales Present in Antarctic Waters Over Winter a Surprise to Scientists

Humpback whale researchers listening to underwater recordings got a surprise upon hearing data collected in Antarctic waters over winter. It had been assumed that all humpbacks migrate to equatorial waters during the winter, but acoustic data from underwater recording stations along the ice shelf revealed the sounds of humpback whales even during the austral winter months.

The find came by way of a classic case of being in the right place at the right time, when researcher Ilse Van Opzeeland, a marine biologist and expert on large whales, walked into her office at Antartica's Neumayer Station III one morning in 2008 and switched on the monitor connected to a live-stream of underwater noise.

"I was totally surprised, because the textbook-opinion until that day was that humpback whales migrate to Antarctic waters only in the austral summer months," said Van Opzeelandat, who is employed by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research. "And even then, standing [beliefs] were that they would only be feeding on krill in the ice-free regions around 60 degrees south latitude."

However Van Opzeelandat observed that the whale calls were being picked up by monitors in an area 70 degrees south, which was much farther south than any known humpback feeding grounds.

"With this in mind, hearing the animals on a winter morning near our observatory was a double surprise," she said.

After spending the next several years collecting data and listening to more whale recordings, Van Opzeelandat concluded that there is a population of humpback whales that indeed spends their winter in Antarctic waters, particularly in the Weddell Sea.

"Today, we know that, in 2008, the humpback whales were present near the observatory with the exception of the months May, September and October. In the following year, they were absent only in September. Therefore, it is highly likely that humpback whales spent the entire winter in the eastern Weddell Sea during both years," Van Opzeelandat said, adding that audio interference by sea ice is a likely explanation of why the humback calls were not heard in some months.

"Near the observatory, open water areas in the sea-ice, also known as polynias, regularly form during winter. Such polynias form due to offshore winds which press the sea-ice off the continent out to sea. We suspect that humpback whales use these ice-free areas. When polynias close or change position, the whales may move with them and leave the recording radius of 100 kilometers, which our underwater microphones are monitoring. However, we do not yet have proof for this behavior," Van Opzeeland said.

While they have no visual evidence of the whales, the researchers suspect that the ones they are hearing over winter are juveniles who skip the more than 7,000-kilometer long journey towards equatorial waters off the African coast. Pregnant humpback whales need to consume huge quantities of food as they will lose 65 percent of their body weight when gestating and nursing a calf, so the journey to resource-rich waters is a necessity for a pregnant whale. But younger whales may save the time and energy expended by remaining in the Antarctic over winter.

"With this in mind, it appears energetically advantageous, from viewpoint of the young whale cows, to remain in Antarctic waters during winter," Van Opzeeland explained. "Furthermore, the coastal region of the eastern Weddell Sea likely provides krill concentrations substantial enough for the animals to find sufficient food, even in the colder season, to acquire sufficient fat-reserves for reproduction and the long trip in the following year."

Van Opzeeland and her colleagues' research is published in the journal PLOS One.