NOW, in early springtime, is when female northern right whales and their newborns migrate northward from calving grounds off Florida and Georgia to around Cape Cod, Mass., taking about a month for the journey. It has been a good season for baby making among the northern rights. Scientists have sighted 20 calves, a record after years of falling counts. Only 320 or so of the behemoths now ply the North Atlantic, and a high rate of reproduction is seen as critical to the comeback of these big mammals, once hunted to near extinction and now the most endangered of the great whales.
But despite more than a half century of protection, as well as sustained Federal and private conservation efforts, the 55-foot, black and gray whales are failing to rally and their population remains dangerously low, baffling scientists and alarming environmentalists.
Six whales have died so far this year, including three calves, the highest number of deaths on record for so short a period. Part of the problem is that the lumbering giants swim through one of the nation's busiest sea lanes for commercial shipping and naval maneuvers, at times getting hit. Other whales get entangled in fishing gear.
But scientists say the roots of the problem go beyond such incidents and are increasingly a grim mystery, prompting a redoubling of protective efforts and detective work.
"We don't know what's going on," Dr. Scott D. Kraus, chief scientist at the New England Aquarium in Boston, the main research group working to save the animal, said in an interview. "It gets nerve-racking."
Environmental groups, including Greenpeace and the International Wildlife Coalition, recently contended that Navy war games off Georgia and Florida with five-inch guns and 500-pound bombs were probably responsible for many of the recent deaths.
But scientists, while happy to question Navy practices, often say that what causes the deaths is extremely murky. Right whales dying for any reason, natural or unnatural, float on the surface of the sea, their carcasses vulnerable to damage from passing ships and military maneuvers and often making cause and effect very difficult to disentangle.
"I fear our faddishness," said Dr. Charles Mayo, a senior scientist at the Center for Coastal Studies, a private group in Provincetown, Mass., on Cape Cod, that studies whales. "We're all so desperate to find out what's going on. We have to be careful not to overlook the less conspicuous things. Changes in the coastal ecosystem concern me a great deal -- no blood, no carcasses, just silent gasps.
"Something is happening. And given the status of the northern right as severely endangered, everything is guilty until proven innocent."
The past and current dangers confronting Eubalaena glacialis, the North Atlantic right whale, stem to a significant degree from its anatomy. The animal, to put it bluntly, is fat, with blubber making up about 40 percent of its body weight, more than virtually any other whale. It is a slow swimmer, seemingly unable to hit speeds over five knots.
The thick layer of blubber keeps the right whale afloat when it dies; most other whales quickly sink. And because of its inherent buoyancy, the whale also tends to rest, feed, court and mate at or near the surface.
For centuries, such attributes made the mammal the "right" whale to hunt and kill -- thus its common name. The attraction was mainly its oil, rendered from fat and used as lamp fuel and lubricants and eventually as an ingredient in soaps and paints. The baleen, or whalebone, was strong and flexible and used to make such things as whips and corset stays. Adding to its allure, the right whale tended to dwell near coasts, making it easy prey. It was the first of the large whales to be commercially hunted.
Starting a millennium or so ago, the species was pursued on a large scale, at first by Basque whalers around the Bay of Biscay. As that population of whales withered and knowledge of the world's seas grew, hunting shifted to the western North Atlantic and then the Pacific.
A single whale could yield as much as 90 barrels of oil and 1,200 pounds of baleen. One kill could pay for an entire voyage. Everything else was pure profit.
By 1935, the species had declined to such low numbers that the League of Nations, fearing the whale would become extinct, was able to talk most nations into giving up the hunt.
Since then, the animal's status has remained shaky because of an enigmatic mix of human and natural factors. The main suspects are ship collisions, entanglement in fishing gear and habitat decline in feeding areas. The toothless mammals are skimmers of the sea, feeding mainly on dense swarms of copepods, which are tiny crustaceans the size of a matchhead.
Federal and private efforts to save the whale increased in the 1970's and 1980's and generally focused on trying to achieve a better understanding the animal's habitat and habits. Dr. Kraus of the New England Aquarium pioneered surveys at sea and learned to tell individuals apart, creating a catalogue of photographs. Particular right whales bear distinctive calluses of hardened skin, as well as characteristic scars and coloration.