During the 1970s an estimated 350,000 dolphins were killed each year as an unintended but inevitable by-catch of the tuna fishing in the eastern tropical Pacific Ocean (see the box). This occurred because between 1959 and 1963 the tuna-fishing fleet switched from pole and line fishing to a technique using vast 'purse seine' nets. Dolphins are caught because fishermen have known for a long time that tuna gather beneath schools of dolphins and very often seabirds congregate overhead - if you want to catch tuna, look for dolphins.
Purse-seine fisheries involve a fast moving boat and a helicopter that search for the telltale signs of seabirds and dolphins. On arriving near a school of dolphins the main boat launches four to six speedboats that operate like sheep dogs, chasing the dolphins so that they are kept together while the main boat encircles them with a one-mile long, 400-foot deep curtain of net, the top edge of which is held on the surface by floats (Figure ). The chase tends to last between 15 and 30 minutes, but occasionally extends to up to an hour. Once the circle is complete, the fishermen pull a rope that draws the bottom of the net together. This captures the tuna, but also encloses the dolphins. Without assistance, the dolphins can get caught up in the net and, unable to reach the surface, drown.
An illustration of how purse-seine fishing can allow some dolphins to escape while tuna are caught, as long as the dolphins can swim fast enough to escape the closing net.
Since the 1980s various changes in fishing procedures have reduced dolphin mortality. Before pulling the net in, the main boat turns and, because of the way it is constructed, the circular net forms a finger-like shoot opposite the large boat. The speedboats head for this part of the net and hold it open so that the dolphins can swim out. Given that dolphins experience this procedure fairly frequently, they appear to learn what to do. Their escape is helped by the fact that the net in this area is made of a very fine mesh so that they don't get caught, and divers swim inside the net actively pulling some of the stragglers out to the open ocean.
While the effort given to helping dolphins escape has cut deaths significantly, conservationists have been puzzled to note that dolphin numbers are nevertheless failing to recover. One possible clue to the problem came when researchers examining data collected by observers placed with the tuna fleet found that more lactating mothers were killed within the net than nursing calves. Elizabeth Edwards, a marine biologist working for the National Marine Fisheries Service in San Diego, USA, notes that "70-80% of lactating females killed were not killed with a calf, and the findings were consistent over several years". Edwards and her colleagues started looking to see whether calves could become separated from their mothers during fishing operations [2
]. This would not be recorded in mortality statistics, but could radically influence the recovery of the dolphin population. "Dolphins are very precocious animals," says Frank Fish of West Chester University, in West Chester PA. "We always marvel at wildebeest and other ungulates that start moving with the herd within 20 minutes of being born, but dolphins are moving constantly, so the neonate doesn't even have the luxury of 20 minutes."
Marine biologists studying dolphin behavior frequently see mothers and calves swimming closely together at speeds of up to 2.4 m/sec, with young calves making little if any obvious swimming motions. The assumption is that the calf is drafting in its mother's slipstream, just as cyclists save energy by packing together during a race. The principle is widespread in biology. "We are always amazed at how smart animals are in their ability to conserve energy - such as ducklings' use of the flow pattern in the water that the mother produces as she swims along, so that they can reduce the amount of energy they need to expend to maintain that speed," says Fish. But in the case of dolphins there has never been any hard science aimed at seeing if this is a lovely myth or a physical reality.
Enter mathematician and aerodynamicist Daniel Weihs, who works at the Technion, Israel Institute of Technology, Haifa. He was involved in research some 25 years ago that aimed to find the speed dolphins needed to reach before they started jumping [4
], and now he came back to the topic, looking for mathematical evidence of forces between mothers and calves (see the box for more of the rationale of the work).