Whales are hunters par excellence
By Nutan Shukla

ONE of nature’s 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 sea’s 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.

This feature was published on November 28, 1999