Whales are
hunters par excellence
By Nutan
Shukla
ONE of natures gentle
giants, humpback whales have developed a remarkable
method of hunting. They use air bubbles, which they
release in thousands, to form a fishing net. Measuring 35
to 45 feet in length, these large creatures feed on
comparatively very small fish, such as capelin, and
krill, small shrimp-like crustaceans, which are found in
sub-polar and polar waters.
They feed in groups,
co-operating in order to maximise their intake of food
which they need in a large quantity. This is necessary to
fatten them up for the winter months, when they set out
on long journeys to and from their breeding grounds. Once
they are out on their annual excursion, that may last for
about two-third of the year, they do not feed, but at the
same time undergo rigours of courtship, mating and
calving. In such circumstances, one can understand
feeding is at premium for humpbacks during the brief
Arctic or Antarctic summer.
While feeding, humpbacks
adopt two methods to collect small fish and krill, one is
bubble-netting and the other is lunge-feeding. In
bubble-netting, a submerged humpback manoeuvres itself
below a large shoal of prey and begins to blow a circle
of bubbles which it produces by forcing out air through
the blowhole. As the bubbles rise, they form a hollow
cylinder in the water column, trapping the shoal inside
it. It is thought that fish are frightened by the
flashing reflections of the bubbles, which act as a
bubble barrier or net. Consequently, trapped fish
congregate into a tightening mass in the centre of the
column and rush towards the surface. The whale,
meanwhile, rises up through the centre of the column with
its mouth open. The fish which are running upwards are
already trapped by the water surface. The massive
creature lunges up, jaws wide open as it gulps a mouthful
of fish and sea water.
Whales can alter the
size of the bubbles according to the size of the prey,
just as fisherman chooses a net with a mesh of a suitable
size for the fish they want to catch. These animals have
pleats in the skin of throat, chest and belly to
accommodate their mighty mouthfuls of fish-laden-water.
When they close their mouth and expel the sea water, food
is held fast behind a fringe of baleen, or whalebone,
that hangs inside the top jaw and serves as a sieve
through which the water is pushed out.
The second method,
lunge-feeding, can be vertical or at an angle. Whales
dive below a school of small fish and shoot rapidly
upwards with mouth wide open. Suddenly the still water
bursts with an alarming roar and the heads of whales
surface together with mouths wide open and throats
distended with a churning soup of water and fish.
After they have mouth
filled, they slowly sink back into the water to surface
again.
It has been observed
that up to 40-45 whales might come together to feed, but
soon they break up into small groups, lunging in
line-abreast a behaviour known as echelon feeding
and exploiting patches of fish or plankton (tiny
animals and plants which drift in the sea.
They include algae, and
small animals such as the larvae of crustaceans, molluscs
and fish. Although some kinds of plankton can swim, they
are usually swept about by the ocean currents).
The lunge can also be at
an angle of horizontally across the seas surface.
Horizontal lunges might be performed while the whale is
swimming upside down, or on its side with one flipper for
a pennant sticking upright in the water.
The groups of whales are
usually transient, but several individuals have been seen
fishing together in this co-operative way for several
seasons in Glacier Bay, Alaska.
Another aberrant fisher
feeding off the Massachusetts coast has its own unique
technique. It rises out of the water with its mouth
closed and then sinks back down again with it partly
open, creating a vortex into which the fish are drawn,
much like the spout of water going out of a bathtub.
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