People have curiously mixed emotions about sharks. Once again this summer moviegoers were thrilled by a shark-attack movie, Deep Blue Sea. Genetically engineered mako sharks that were both huge and smart unabashedly devoured their scientific creators, carrying on the Hollywood tradition begun in 1975 with the huge success of the movie Jaws.
Yet at the same time, the public is learning to appreciate sharks for their beauty. During the past 20 years, tourists have come from around the world to swim with the hammerhead sharks at the Espiritu Santo seamount in the Gulf of California. Not only are hammerheads magnificent to observe swimming gracefully in their schools, but these sharks also have keen senses and sophisticated behavioral repertoires. For example, female scalloped hammerhead sharks perform an elaborate display—resembling a springboard diver's reverse flip with a full twist—in order to induce subordinates to leave the center of harem-like schools, where the chance of a female mating with a male is highest.
The fact that the hammerhead can perceive very weak electromagnetic fields, an ability human beings lack, makes this species so abundant and a tourist attraction around the world. Like other sharks and rays, hammerheads migrate extensively, using their detection and navigational abilities to find prey buried out of sight and also to guide their movements back and forth from their seamount homes to neighboring feeding grounds. Ecotourism for sharks is currently very common in Florida, the Caribbean, Southern California, South Pacific islands and Australia, where divers are taken to sites to which oceanic and reef sharks are attracted by bait. Tourists are also taken in boats to islands inhabited by colonies of seals and sea lions off Australia and South Africa, where white sharks can be viewed from the security of a shark-proof cage.
But just as they are becoming tourist attractions, sharks may be vanishing. During my last two visits to the Gulf of California, I was stunned by how rare hammerhead sharks were in these waters. My research team managed to see only one small group of eight hammerhead sharks on a two-week cruise during summer 1998 to study pelagic fishes at a seamount near the tip of the Baja California peninsula. We spent much of our time searching for the hammerheads by making free and scuba dives.
During a similar cruise 18 years earlier, I discovered massive schools of the species at the same site. My colleague Don Nelson and I estimated the number of scalloped hammerhead sharks swimming around this underwater ridge less than one-half mile long by using a Lincoln Index capture-recapture analysis modified for our observations of sharks. One August morning we made breath-holding dives into the schools of sharks and quickly tagged 21 sharks with color-coded, plastic-streamer tags. The sharks often accelerated momentarily after tagging, but usually remained within their groups. That afternoon, nine tagged sharks were observed again with a group of 225, yielding an estimate of 525 sharks in the vicinity of the seamount.
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