Baleen whales eat swarms of tiny crustaceans called zooplankton, and they engage in all kinds of tricky tactics to get the biggest mouthful of food for the smallest amount of effort.

There are plenty of videos of baleen whale feeding events posted on YouTube, but the video above is one of my favorites from the BBC.

The Challenge

Whales are marine mammals, which means they breathe air. When they feed on underwater creatures, they have to do it while holding their breath, and that takes a tremendous amount of energy. Imagine swimming around underwater in a pool while trying to capture and swallow a swimming fish – it would not be easy!

All Together Now!

Luckily for baleen whales, zooplankton also need to stay together in large swarms because it makes it easy for them to find mates and helps them avoid being eaten by fish. Swarming behavior allows whales be very efficient eaters by swallowing many zooplankton in one giant gulp, like you see in the video above.

On the Move

Some whales also take advantage of migration patterns of zooplankton to make feeding more efficient. Many krill species, migrate to the surface at night to feed on phytoplankton, and then migrate deep near the seafloor during the day to avoid predators that hunt by vision, like fish. This behavior is known as diel-vertical migration (see figure below). Some baleen whales like Sei Whales wait until night when the krill are aggregated at the surface and do all their feeding at night, to avoid having to spend energy diving deep for their food.

Krill Tracking
Every time the glider profiles the water column it sends out a pulse of sound to measure zooplankton, like krill (see inset for a picture of the critters). The glider collects many profiles each day, so we can see the upward movement of zooplankton at night and downward movement during the day (labeled on the figure). This pattern, known as diel-vertical migration, is a way for zooplankton to avoid the well-lit surface waters during the day, when visual predators like fish could see and eat them.

We can measure the diel-vertical migration process using active acoustic echosounders, which we have mounted on our gliders. Acoustic echosounders work much like the echolocation used by dolphins – a pulse of sound is sent out, the sound reflects off of a surface (such as a layer of zooplankton) and then returns to the source of the sound. The difference between the sound sent out and the sound received tells you something about how many zooplankton are out there, and what they are doing. An example of the echosounder data showing diel-vertical migration from one of our gliders is given in the figure above.

In order to know how much zooplankton is out there by the difference in sound, the echosounder has to first be calibrated in a big tank. This is done by hanging a small sphere of known composition underneath the echosounder and recording how much sound reflects off of the sphere. In the photograph below, graduate students Gennavieve Ruckdeschel and Tara Howatt are suspending the glider in Dalhousie University’s  Aquatron tower tank, which is 30 m deep, to perform this calibration.

Graduate students Gennavieve Ruckdeschel and Tara Howatt submerge the glider into a 30 m deep seawater tank at Dalhousie to prepare the echosounder sensor for measuring zooplankton at sea.
Graduate students Gennavieve Ruckdeschel and Tara Howatt submerge the glider into a 30 m deep seawater tank at Dalhousie to prepare the echosounder sensor for measuring zooplankton at sea.

This post was written by guest blogger Kim Davies, Liber Ero Postdoctoral Fellow, Department of Oceanography, Dalhousie University.