Seals under siege: a heated warning.

Seals Under Siege: A Heated Warning

Seals have not found the past century a healthy one. Besides facing human predators, who have all but eliminated several regional populations, masses of the finfooted creatures have succumbed to viral plagues. Most recently, in the summer of 1988, nearly 18,000 dead harbor seals washed up on European shores, victims of a distemper virus (SN: 9/3/88, p.149).

Scientists have speculated that the distemper-resistant harp seal acted as a sort of marine Typhoid Mary in the 1988 outbreak, moving south into the harbor seal's range and passing along the virus. Some blame chemical pollutants known to disarm seal immune systems.

Two biologists now suggest, however, that investigators have overlooked an underlying factor that could explain five of the century's most conspicuous seal die-offs, including the 1988 deaths. Unusually warm weather and crowded seal herds combined to trigger the outbreaks, they propose.

David M. Lavigne of the University of Guelph, Ontario, and Oswald J. Schmitz of the University of British Columbia in Vancouver examined records of regional air temperatures preceding five major seal die-offs. "In each case, the mass mortalities began following three months when mean air temperatures were 1[degrees]C to 3[degrees]C higher than the preceding 10-year average," they write in the recently released June MARINE POLLUTION BULLETIN.

The researchers suggest that as temperatures rise, seals begin hauling out of the water and gathering in unusually dense herds on shore. If a contagious virus is present it may spread rapidly, like measles racing through a crowded classroom. Lavigne and Schmitz cite a 1973 study showing that a 2[degrees]C rise in air temperature coincided with harbor seals hauling out by the hundreds. "What is important is that the local [seal] densities on shore were much higher than densities preceding the [outbreaks]," Lavigne says.

"The act of hauling out does indeed make seals more vulnerable," says John Harwood, head of the Sea Mammal Research Unit in Cambridge, England. "But the problem with this hypothesis is that we really don't know why they haul out. It's unreasonable to think that seals haul out just because the air temperature has gotten warmer." Although Harwood agrees that higher temperatures might in some cases contribute to seal plagues, "its very hard to believe that [temperature] is the sole factor," he says.

Lavigne and Schmitz acknowledge that the hauling out behavior remains poorly understood. Nonetheless, they say, "four of the six documented mass mortalities in seal populations have occurred in the past 12 years, a period which includes some of the warmest years in the 20th century." Moreover, they suggest warmer weather may have done in other marine creatures. Recent die-offs of dolphins and whales also followed unseasonably warm temperatures, they point out.

Predictions of global temperatures rising by as much as 3[degrees]C within the next century (SN: 6/23/90, p.391) carry "profound implications for the future [of pinniped populations]," Lavigne and Schmitz write. "Our data show that a 1[degrees]C to 3[degrees]C increase in average temperature can trigger very significant ecological events," Lavigne told SCIENCE NEWS. Noting a recent rash of die-offs among seabirds, fish, coral reefs and sea turtles, he adds," If the record of the past 12 years is anything to go by, we probably have much more to worry about than seal deaths."