A growing taste for shark steaks and shark-fin soup has for the first time in 400 million years put the scourge of the sea at the wrong end of the food chain. Commercial landings of this toothsome fish have doubled every year since 1986, and shark populations are plunging. It is hardly a case of good riddance. Sharks do for gentler fish what lions do for the wildebeest: they check populations by feeding on the weak. Also, sharks apparently do not get cancer and may therefore harbor clues to the nature of that disease.     Finally, there is the issue of motherhood. Sharks are viviparous. That is, they bear their young alive and swimming (not sealed in eggs) after gestation periods lasting from nine months to two years. Shark mothers generally give birth to litters of from eight to twelve pups and bear only one litter every other year. This is why sharks have one of the lowest fecundity rates in the ocean. The female cod, for example, spawns annually and lays a few million eggs at a time. If three quarters of the cod were to be fished this year, they could be back in full force in a few years. But if humans took that big of a bite out of the sharks, the population would not recover for 15 years.     So, late this summer, if all goes according to plan, the shark will join the bald eagle and the buffalo on the list of managed species. The federal government will cap the U.S. commercial catch at 5,800 metric tons, about half of the 1989 level, and limit sportsmen to two sharks per boat. Another provision discourages finning, the harvesting of shark fins alone, by limiting the weight of fins to 7 percent of that of all the carcasses.     Finning got under the skin of environmentalists, and it helped to mobilize support for the new regulations. Finning itself is a fairly recent innovation. Shark fins contain noodle-like cartilaginous tissues that Chinese chefs have traditionally used to thicken and flavor soup. Over the past few years rising demand in Hong Kong has made the fins as valuable as the rest of the fish. Long strands are prized, so unusually large fins can be worth considerably more to the fisherman than the average price of about $10 a pound.      But can U.S. quotas save shark species that wander the whole Atlantic? The blue shark, for example, migrates into the waters of something like 23 countries. John G. Casey, a biologist with the National Marine Fisheries Service Research Center in Narragansett, R.I., admits that international coordination will eventually be necessary. But he supports U.S. quotas as a first step in mobilizing other nations. Meanwhile the commercial fishermen are not waiting for the new rules to take effect.”There's a pre-quota rush on sharks,” Casey says, “and it's going on as we speak.”