A Movable Feast
Humpback whales practice an unusual technique for netting fish off Alaskan shores
Roger Di Silvestro
The humpback whales that spend summers off southern Alaska gorging on coastal Seafoods use a fishing technique unique among whale species. They create nets of air bubbles that entrap prey, allowing the whales to capture food more easily.
After spending winter breeding and birthing off Hawaii, the humpbacks begin their 4,500-mile trip to Alaskan waters in March, with pregnant females usually the first to depart. By August, at least 400 whales are feeding off Alaska's south shores, where photographer Michio Hoshino took these pictures. Other humpbacks zero in on coastal areas south to California. Individual whales usually return to the same site each year, gathering in groups as large as 150.
The whales come in hungry from their four- to six-week-long journey. They have fasted through winter because warm southerly seas lack the krill, anchovies, sardines and other seafood the humpbacks need to survive. When they arrive in Alaska, the whales have lost about 25 percent of their weight, which can approach 40 tons.
The humpbacks ingest much of their prey simply by cruising along at the water's surface, mouths wide open, engulfing huge quantities of food and water. Each whale's upper jaw is lined with some 270 to 400 baleen plates that extend downward into the mouth. Made of keratin (the protein of hair, horn and fingernails), the plates are fringed along one edge, allowing water to pass through. By pressing tongue against roof of mouth, a whale can force out the water, retaining a mouthful of fish.
Sucking up massive meals at the water's surface is the humpback's most fundamental approach to feeding. Bubble netting is a more complex technique. One or more whales dive below a school of fish or a mass of krill, then begin to effervesce, emitting countless bubbles from their blowholes as they swim upward, spiraling around the school. This creates a circular wall of bubbles that seems to frighten the fish, because they tend to congregate in the middle of the bubble net, which may exceed 100 feet in diameter. Then the Whales swim up through the school with mouths open and-gulp, cetacean fast food.
Researchers also have reported bubble netting in North Atlantic humpbacks. Some whales back up bubble netting with bursts of sound whose purpose is unknown. Perhaps the sounds stun the fish or cause them to clump together, making them easier to catch, or perhaps the sounds are signals that permit the whales to coordinate their activities.
The whales seem to be very successful predators. Scientists who dissected humpbacks prior to the whaling ban estimated that 90 percent of individuals in feeding areas have full stomachs. One 45-foot female contained 600 herring; a 35-foot male held 1,200 pounds of cod.
The picnic begins to wind down in autumn, when the whales start the long voyage to warmer waters. By December, most of the Alaskan humpbacks have returned to Hawaiian waters. However, a few whales seem to winter off Alaska.
Humpbacks range throughout all oceans and adjoining seas. Until whaling nearly wiped them out, the whales numbered perhaps 100,000 in the Southern Hemisphere and 15,000 in the North Pacific. No reliable figures for today's world population are available, though some researchers believe this endangered species is recovering and that a few tens of thousands survive. -Roger Di Silvestro