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Biological oceanography

In the Heart of the Sea: Following the Song of Humpback Whales to Study Migration

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Van Opzeeland I, Van Parijs S, Kindermann L, Burkhardt E, Boebel O (2013) Calling in the Cold: Pervasive Acoustic Presence of Humpback Whales (Megaptera novaeangliae) in Antarctic Coastal Waters. PLoS ONE 8(9): e73007. doi:10.1371/journal.pone.0073007

Feature image: Baby humpback whale swimming underwater. Credit: Flickr/Christopher Michel

Feature image: Baby humpback whale swimming underwater. Credit: Flickr/Christopher Michel

Whales migrate? Whales migrate. 

Underwater acoustics can tell us a lot about marine mammals: their age, mating habits, stress levels, and also their migratory patterns. Just like birds, whales migrate to access abundant food and breeding grounds. Humpback whales migrate annually from low-latitude tropical breeding grounds to polar feeding grounds near the Antarctic. Some humpbacks, however, choose not to migrate and stay at the feeding grounds for the remainder of the year. In this study, the calls of humpback whales in Antarctica were studied to better understand their migratory patterns. (Click here to listen to a humpback whale song!)

Using Passive Acoustics To Understand Humpback Whale Calls

Passive acoustics is commonly used to detect and monitor marine mammal populations. In this study, recordings were made by PALAOA (PerenniAL Acoustic Observatory in the Antarctic Ocean) by deploying a hydrophone under a 100-meter thick sheet of ice. Two kinds of calls were the strongest: “moans” and “high calls”. Moans, having low frequency, were included in this study instead of high calls. This was because low frequency arch-shaped calls were better received for automated detection as opposed to high calls, which were too variable to be detected.

Another common sound made by humpback whales is a series of “clicks” while they are foraging. The closer they are to the prey, the faster the clicks.

Credit: Ocean Acoustics & Signals Lab, Woods Hole Oceanographic Institution

Can you tell if the whale is close to or far away from its prey?

Image 1: A Spectogram depicting the moans of a humpback whale. Whale moans are considered as social sounds! Credit: NOAA North East Fisheries Service

Image 1: A spectogram depicting the moans of a humpback whale. Whale moans are considered as social sounds! Credit: NOAA Northeast Fisheries Science Center

Spectrograms were used for this purpose because they help to visualize sound and demonstrate changes in frequency over time. Here’s a sound clip accompanying the spectrogram shown above! Warmer color means the sound is loud, cooler color means the sound is quiet. Notice how the spectrogram aligns with the sound.

Credit: NOAA Northeast Fisheries Science Center

Image 2: Graph showing presence of humpback whale moans with respect to percentage ice coverage. Red triangles indicate acoustic presence of humpback whales. Black bar below the acoustic presence indicators indicates PALAOA recording.

Image 2: Graph showing presence of humpback whale moans with respect to percentage ice coverage. Red triangles indicate acoustic presence of humpback whales. Black bar below the acoustic presence indicators indicates PALAOA recording.

The researchers conducting this study wanted to know whether or not humpback whales spend their winters in polar waters. They calculated percentage open water (as opposed to ice-covered) based on the distance at which the moans occurred.

Findings & Significance

The researchers picked up humpback whale moans for 9 months in 2008 and 11 months in 2009. More moans were present when ice cover increased to 70% and then further increased to 90%. This goes to show that humpback whales do indeed stay close to the ice shelves. The percentage ice cover also shows that whales spend more time in the feeding ground when there is more ice, and migrate when the ice diminishes.

Given the threats that humpback whales face, it is important to study their migratory patterns and try to understand how they use different habitats. The presence of humpback whales in Antarctic ice-covered habitats tells us that whales prefer places where there is an abundance of krill (their usual diet). Krill are abundant in places where they can remain under ice as opposed to open water, which strengthens this theory. The male-to-female ratio in the feeding ground during the austral winter (June to August) suggested that perhaps females who are smaller in size and sexually immature may not wish to make such a long journey to migrate, so they stay behind in the feeding ground to further their growth.

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