As a rule, when a female shark gives birth, she stops feeding and swims offshore, away from the nursery grounds. She will not feed until she has traveled a considerable distance from her young. It is possible that this instinct reduces cannibalism, because large sharks are known to dine heavily on smaller ones.
There was something atavistic about the scene at Port Salerno, something from our earliest hunter-gatherer ancestry, when people came together to watch a large something being dismembered for dinner. As Worden said, "People are afraid of them, yet they want to see them. If all we had were bluefish, there wouldn't be a fraction of the crowd."
Blacktip, sandbar, tiger, bull, dusky, hammerhead, silky—we know very little about any of the sharks. We certainly do not know how many years it takes for a population to replace itself. Lemon sharks grow about four inches a year and don't reach sexual maturity until they are 13 to 15 years old, according to Dr. Samuel H. Gruber of the University of Miami. That seems slow for a fish, but among sharks, who knows? Several blue sharks tagged off New Jersey by NMFS scientists showed little or no growth on recapture seven years later.
We joined Gruber's research team as it worked off the Marquesas, the tiny uninhabited islands west of Key West. That town's clutter faded into the distance as we sped over the meadows of sea grass and coral reefs, heading for a rendezvous with the research vessel Columbus Iselin, a 175-foot symbol of the effort underway here to understand shark biology.
After lunch we joined a team that uses ultrasonic tags to track juvenile lemon sharks in the shallow sea-grass flats between the islands. The water was too shallow for an outboard, so an air-boat was used. Through a deafening roar Erich Glavitza, a manufacturer of snowmobile engines whose hobby is studying sharks, drove us over the flats in pursuit of two-foot baby lemons. Chip Pike, a graduate student of Gruber's, was stationed on the bow, dip net in hand. We zigzagged in hot pursuit of a fleeing sharklet, running it to exhaustion so that Pike could swoop it up in a net. Then the shark was rushed to a large chain-link pen that had been assembled in the shallow water of the flats.
When we arrived, several baby lemon sharks were moving restlessly around the pen, their tails undulating. After eons of evolution they have become creatures of motion, of streamlining, with triangular fins that cut the water cleanly, allowing them to pursue prey with both grace and violence.
With long-handled nets, Pike and John Morrissey, another graduate student, stalked the small lemons, grabbing at the sharks as they sped between their legs. Soon silt stirred off the bottom, and the dark forms moved like bombers through cloud formations. At last the students caught one. It squirmed as they held it aloft. Small holes were punched in the fish's dorsal and pectoral fins for quick visual identification. Sutures closed up an incision in the muscle where a sonic tag had been implanted. A yellow plastic tag, marking the implant, protruded from its back.
They turned the little lemon loose, and it sped off across the flats. Now and then a tiny set of triangular dorsal fins appeared on the water's surface for a second, throwing a small wake before vanishing. Morrissey waded behind, dragging a hydrophone through the water, listening through the earphones for the occasional ping that indicated contact with the little shark. The scientists had named it Moxie.
Slogging through the vast tidal flats in pursuit of Moxie, Morrissey said, "We've learned that baby lemons go back to their home base and stay in about a 400-meter zone, about 50 meters wide, along shore. They patrol the same piece of bottom over and over again, and we haven't the faintest idea why. They aren't territorial, they don't defend their strip, their populations overlap. But they all have their own strips staked out."
Another team, headed by veterinarian Dr. Charlie Manire, was tending research longlines a mile away. In those deeper waters a six-foot nurse shark snatched and jerked against the lines when the team approached. They hauled it to the surface, put a loop over the fish's tail and another around its head, just in front of the dorsal fin and behind the pectorals, and with much difficulty they secured it to the gunwale of their 20-foot skiff. "Hold the tape on the nose," someone called out, above the splashing and grunting. Quickly, but with caution, they measured the distance from the nose to the first dorsal fin, the space between the fins, and the length of the tail, checking for small differences between this shark and those elsewhere that might indicate separate populations.