Dolphins and Porpoises
Whats the difference?
Dolphins generally have conical teeth, beak-like snouts, and sleek bodies. They swim fast in large groups, often accompanying boats and ships.
Porpoises usually have flat teeth, blunt noses, and rather stocky bodies. They normally swim slowly in small groups, almost never accompanying watercraft.
| 
|
Mention dolphin, and most people picture Flipper, the smiling bottle-nosed star of tube and screen. Actually, there are about three dozen species of true dolphins, varying in size from Hectors at just under 5 feet (1.5 m) to the killer whale at almost 32 feet (9.6 m). The majority measure 8-14 feet (2.4-4.2 m). Dolphins come in shades of black, gray, and white; some are spotted, some are striped. Most have bulging foreheads that house the melon, a pad of fat that probably focuses the sonar beams dolphins use to navigate (a process known as echolocation).
Several fishing practices have taken a toll on dolphin populations. Most lethal are drift nets and nets called purse seines, used to fish for yellowfin tuna. Because tuna often swim beneath dolphin schools, dolphins are actively sought. Fishermen then circle the tuna with mile-long (1.6-km) seine nets that trap fish and mammals alike. When this practice started in the 1950s, it killed more than a quarter of a million dolphins a year. Now boat captains are trained to employ maneuvers that help release the dolphins. The dolphin kill now totals around 20,000 a year.
The drift net, though, is the most indiscriminate killing device yet used at sea. Up to 40 miles (64 km) long, drift nets hang draped from floats, trapping virtually everything in their path. Fishermen haul the nets aboard, store the fish or squid, and discard everything else, including dolphins that have drowned while trapped. Introduced in the early 1980s, drift nets were banned by the United Nations in 1992.
For further information, please see the National Geographic publications listed below, perhaps available at your library. For a broader search of the topic, please see the NGS Publications Index. Items available in the National Geographic Store are linked there.
|