Some dolphins, however, prefer squid. The squid-eaters are pilot whales, false killer whales, melon-headed whales and Risso's dolphins, but other dolphins sometimes take squid as well.
Some dolphins have part-time preferences. Rough-toothed dolphins eat mollusks, and tucuxi eat shrimp. One river dolphin, the boto, eats "armoured" varieties of freshwater bottom fish, crushing them with its flat, molarlike teeth.
Orcas, the largest dolphins, eat almost all of the above and individually consume more than any other dolphin. One mature captive male devoured up to 160 kms of fish a day, but the average is 79 kg for males, 63 kg for females and 16 kg for babies when they stop nursing. In captivity, orcas eat dead fish, mostly herring, although some marine parks give them salmon or other fish species. In the wild, orcas catch dozens of species of fish and squid. Many orcas specialize in taking marine mammals - seals, sea lions, other dolphins, porpoises and sometimes even large whales.
Scientists determine the diet of dolphins and porpoises mainly by examining the stomachs of animals that strand on beaches and die or are found floating dead at sea or washed ashore. Occasionally, they find and analyze feces. Only rarely does a scientist actually see, much less photograph, a dolphin in the act of catching a fish. Feeding usually occurs underwater, obscured from view. Even at the surface, it happens quickly. However, when researchers in British Columbia suspect that orcas are feeding on salmon, they scoop nets into the water to recover the shimmering fish scales which often fall off and float at the surface; each species of salmon has different scales. Pilot whales often follow squid, which is good evidence of a food preference. The movements of other dolphins also sometimes correspond with those of squid or fish.
Probably all species of dolphins and porpoises use their sonar to "see" fish prey. But when orcas hunt seals or other marine mammals that have better hearing than fish, they keep quiet, watch their prey and sneak up on it. Of course, river dolphins, most of which have limited vision in muddy rivers, have to rely on sonar. When the prey is located, it is quickly snatched or chased and trapped against the river bottom or an underwater rock wall. In the open sea, schools of fish are sometimes corralled by a large herd of dolphins and forced to the water's surface, which acts as a wall. Researchers and fishermen often see fish jumping furiously out of the water in a desperate attempt to escape.
Dolphins and porpoises use their teeth to grasp prey. an orca may bite a sea lion into pieces, and the boto crushes its armoured food. But dolphins and porpoises do not chew as they eat - fish, squid and pieces of larger prey are swallowed whole.
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