Acoustic observations reveal whale behavior in the Weddell Sea

Underwater recordings have revealed new details of the elusive behavior of whales in the Weddell Sea. Scientists at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) have learned more about the lifestyle and habits of humpback whales, as well as smaller animals called Antarctic minke whales.

Research on baleen whales in the Southern Ocean was once limited to studies conducted during the Antarctic summer, which is when biologists were present. But then, the AWI team permanently installed underwater microphones. 

The hydrophones have been successfully recording sounds in the Weddell Sea for the last nine years. The goal is to help improve protective measures for the whales and their main food source, the Antarctic krill.

The audio recordings have provided unique insights. Based on the observations, the researchers have determined there are most likely two humpback whale populations in the Weddell Sea. Both of these groups have been heard singing and communicating the most frequently in the autumn. The recordings also indicate that the humpback whales avoid areas with sea ice.

By contrast, the AWI team found that Antarctic minke whales prefer ice-covered regions. These animals were mostly heard making their distinctive quacking sounds during the winter months.

Antarctic minke whales are very mysterious. Marine biologists are still working to understand exactly where they live and breed and how many of the whales actually exist. 

A few years ago, experts discovered that Antarctic minke whales produce characteristic calls that sound similar to a dock quacking. These distinctive calls provide scientists with a reliable way to track the small whales, which measure up to eleven meters in length.

AWI biologist Diego Filun and his team are using the quacking sounds for the first-ever comprehensive, long-term observation of Antarctic minke whales in the Weddell Sea.

“We’ve been monitoring our underwater microphones for nine years. They were deployed at 21 points throughout the Weddell Sea and along the prime meridian, allowing us to record the whales’ acoustic activities in regions where research vessels rarely venture,” said Filun. 

“Thanks to the recordings, we now finally understand in what parts of the Weddell Sea the minke whales prefer to be at different times of year, and know that at least some of them stay there for the winter and don’t migrate to warmer waters.”

The recordings confirmed that in both the summer and winter, Antarctic minke whales stay in regions of the Weddell Sea that are covered with sea ice. However, the frequency of their calls changes with the season. They can be heard far more often in the autumn and winter months. 

“On aerial survey flights over the Weddell Sea in the summer, minke whales were primarily sighted near the sea-ice edge and less frequently in areas with thick sea ice. But our audio recordings showed just the opposite: the minke whales were rarely found in the marginal ice zone, and much more often under thick ice – most likely in an attempt to avoid their archenemies, killer whales,” said Filun.

Unlike the minke whales, humpback whales in the Weddell Sea do not seek shelter below the ice. In a second hydroacoustic study led by AWI biologist Elena Schall, the researchers found that humpback whales avoid ice-covered regions altogether. Instead, they seek out Antarctic krill along the northern edge of the ice.

“Our audio recordings from 2013 indicate that at least two humpback whale populations come to the Weddell Sea in summer to build up their fat reserves,” said Schall. “Whales from South Africa seem to go hunting at the eastern edge, near the prime meridian. But humpbacks from South America tend to stay in the northern coastal waters of the Antarctic Peninsula, and can be heard until later in the year than their counterparts to the east.”

The recordings suggest that the humpbacks move southward in the summer as the ice retreats, but only travel as far as needed to find sufficient food. 

“If we want to protect the unique biotic communities of the Weddell Sea in the long term, we need to know as precisely as possible how many baleen whales come to the Atlantic sector of the Southern Ocean in search of food, what regions they hunt in, whether they overwinter there, and how much krill is needed for the whales to find sufficient food,” said Dr. Ilse van Opzeeland. 

“In this regard, long-term acoustic observations are a vital tool, because they offer us a far more detailed picture of life below the water than the handful of scientific whale sightings alone.”

The AWI researchers hope their findings will be taken into account in future discussions on the establishment of a marine protected area in the Weddell Sea. In the meantime, they will  continue to analyze an abundance of underwater recordings.

The study is published in the journal Royal Society Open Science.

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By Chrissy Sexton, Earth.com Staff Writer