National Geographic : 1992 Sep
young in her life, while a skipjack tuna that lives only ten years may produce two million eggs in a single 90-day spawning season. "It's easy to destroy a dolphin population by harvesting it," I told them, "while fish such as tuna can be remarkably resilient under heavy fishing pressure." I could have added that the evolutionary ancestors of dolphins once lived on land. Per haps only because they have developed a supe rior sensory system-the ability to detect AWORLD OF DANGER AND NOWHERE TO HIDE More than 40 species of dolphins and porpoises thrive, but several groups are dwindling, threatened by overfishing, accidental capture, pollution, and damage to their habitat (map). Closely related to whales, dolphins and por poises are found in nearly every sea. Generally dolphins have conical teeth, a defined beak, and a curved dorsal fin. Often confused with dolphins, the six species of porpoises usually have spade-shaped teeth, a rounded profile, and triangular dorsals. Virtually all dolphins and porpoises share a benign "smile." It was in the 1950s that the author determined the function behind the animals'grin:The lower jaw bone flares outward and serves as an ultra sensitive ear that enables them to hear sounds from fellow dolphins. Those sounds, emitted through the animals' rounded fore heads, called melons, can be heard more than a mile away by other dolphins. SBall Lipotes vexillifer PAINTINGS BYKARELHAVLICEK Described by a Western scien tist in1918, the baiji is threat ened by development and fishing practices along Chi Ina's Chang Jiang (Yangtze River), its only habitat. things with tiny echoes of their own sounds, or echolocation-have dolphins survived in the sea at all. The fishermen looked puzzled and doubt ful. In 1971 they had trapped almost a thousand juvenile striped dolphins in one out ing, perhaps killing most of an entire genera tion of this species in the Kuroshio. But it wasn't their way to worry about such things to be asked to take the dolphins into account. Maybe increased boat traffic was a factor in Hector's dolphin Cephalorhynchushectori These dolphins school near the shores of New Zealand, making them vulnerable to gill nets.