Computer model (above) showing four whales
insonify an annular bubble net with their 'feeding
calls'. The ray path shows the hollow-cored
cylindrical bubble net where the prey is
trapped.
| The
functioning of bubble nets is largely unknown and for
decades people have speculated as to how the bubble nets
produced by humpback whales trap fish. Most theories
have centred on the idea that fish are unwilling to swim
through the bubbly water and so become trapped in what
has become known as a ‘bubble net’.
Professor Tim Leighton at the University’s Institute
of Sound and Vibration Research (ISVR) believes this is
not consistent with the behaviour of fish in other
bubble fields: ‘Why fish are unable to swim through a
wall of bubbles is a mystery given the ability salmon
have to leap up waterfalls. This led me to think about
other ways in which the trapping happens.’
The mechanism that Professor Leighton suggests is a
‘Wall of Sound’ generated by the intense calls of the
humpback whales when they feed in this way. ‘These
sounds are very different to the humpback calls that can
be found on relaxation tapes,’ says Professor Leighton.
‘The trumpeting calls emitted by the whales when they
produce these nets sound scary to human ears, and are so
loud they resound throughout the hull of any nearby
ship. These sounds become trapped within the wall of the
bubble net. However, inside the cylinder, where the fish
congregate, it is almost silent,’ said Professor
Leighton.
‘The bubble net turns a survival trait in fish, into
one which gets them eaten. If they try to leave the net,
they encounter the terrifying ‘Wall of Sound’. Startled,
the fish form a tight school, and so make a compact
target when the whales rise up from beneath the trap
with their mouths open for feeding.’
After his initial calculations, Professor Leighton
contacted a former student, Dr Simon Richards, at the
defence company Qinetiq, and asked him to verify the
calculations with the computing facilities available to
him. Dr Richards agreed with the results and before
publishing Dr Paul White of ISVR performed another
cross-check.
Professor Leighton added ‘There are some research
problems I have spent years solving, but this mechanism
took me about 30 seconds. Obviously the whale has to use
the net intelligently and produce the sound in a
specific way to get it to work... but it is an elegant
proposition.’ | 
