Fast Food for Humpback Whales!

Having taken on a new job with an educational charity in addition to continuing my PhD part-time I have found that time is seriously lacking!  Therefore I have decided to consolidate my blogging efforts into just one place….here.  Hopefully this is successful. This is the first post I wrote for my own blog, as this has now been removed I am re-publishing it here as it still amazes me (and many of the students who I speak to on Outreach days)   Happy reading!

Humpback whales are known to participate in an unusual hunting technique.  Although the habit has been visually identified from the air and also from the perspective of the whale via a “Critter Cam” (Crittercam Reveals Secrets of Marine World, National Geographic News, Sept. 25 2009) little was actually known about its mechanisms until recent years.

Bubble net viewed from above

This phenomenon is known as bubble net feeding and is unique to humpback whales.  Whales, either individually or in a group, employ bubbles in a hollow cylinder to trap fish (see figure below).  These cylinders can be up to 30m in diameter.  The whales produce the bubbles by emitting air through their blowholes.  The fish are consumed in one go as a whale rises from below with an open mouth, in an act called ‘lunge-feeding’.

Schematic of a humpback whale creating a bubble net.

When this was first seen it was understandably difficult to interpret.  After all, bubbles are prevalent in the upper ocean why would fish be so unwilling to travel through these bubbles?  Only recently, by exploring the acoustics, has the scientific understanding of the process been expanded.

At the same time as the bubbles are emitted the whales call out.  This sound has been described as “haunting” and “beautiful” by fishermen who have heard the noises from the surface.  It is believed that it could be an interaction between the bubbles and the sound which creates a boundary that the fish are not willing to cross.  Sound can be trapped within bubbles.  If this is occurring in the bubble nets the walls of the cylinder would concentrate the sound, leaving the inside of the cylinder almost silent.  It is believed that the fish are more likely to stay in what they perceive to be the “safe”, quiet centre of the cylinder than travel through the “wall of sound” created by the bubbles.

In addition to the bubbles trapping the sound they also create a difference in the acoustic quality of the water.  This is something which you can hear for yourself when making a cup of instant coffee (this is one for the adults to try or help out with).  When you make your next coffee fill the cup with hot water and tap the side of the cup with a teaspoon.  Add your coffee granules and, before stirring, tap the side of the cup again with a teaspoon.  Does it sound the same or different? It should, and always has done when I’ve been making coffee, sound different; the sound is deadened after the coffee has been added.

What has this got to do with humpback whales I hear you thinking!?  Well it is all to do with bubbles!  This experiment only works with instant coffee granules, due to the freeze drying process by which the granules are made.  During the process, where brewed coffee beans are dehydrated, air pockets get trapped in the coffee granules.  When the coffee is added to water the granules dissolve and the air in the pockets is released, forming bubbles in the water.  This deadening of the sound caused by the bubbles was utilised in a bid to protect migrating salmon, when bubble screens were erected to reduce noise from pile-driving activities in the San Francisco Bay Area.  Imagine all the sounds in the world around you suddenly being muffled, would you fell comfortable?

However the bubble nets work they are an incredible feeding technique, and a fantastic application of scientific principles…even if the whales have no idea how interesting they are!

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