Brain lesion helps explain seal loss.

Harbor seals --one of Prince William Sound's most common marine mammals - apparently suffered heavily from the Exxon Valdez oil spill, says Kathryn J. Frost, a biologist with Alaska's Department of Fish and Game in Anchorage. Even before the spill, she notes, these seals had undergone a dramatic and unexplained population decline. But 1992 aerial surveys showed that while seal counts outside spill-affected areas had fallen 18 percent from 1988, numbers at oiled sites had dropped 34 percent. Frost also presented data indicating that oil may have doubled or tripled the mortality of pups born in affected regions in the year of the spill.

Since 1975, harbor seals in the Sound have plummeted from perhaps 13,000 animals to 2,500 in 1991. Because spill-assessment teams recovered only 18 harbor seal carcasses following the supertanker accident, many scientists initially questioned the impact of oil on the seals' current low population, Frost recalls. Indeed, she notes, until this spill, marine mammal experts had speculated whether seals were even at risk from oil. Anecdotal evidence suggested the animals would avoid visible oil and that petroleum probably wouldn't stick to any that swam through it.

"But harbor seals violated the conventional wisdom," Frost now reports. Not only did they swim regularly through contaminated waters, but they also hauled out to rest or bear young on spill-blackened beaches. Depending on the site, 50 to 100 percent of the seals in oiled regions of the Sound also acquired oil-stained pelts. Initially appearing as "black hats:' the oiling soon developed into dark capes. Many animals ended up covered in a viscous black coat that lasted until they molted in the fall. These oiled seals also exhibited behavioral anomalies that observers said gave the animals the appearance of being sick, lethargic -- even drugged.

But since dead seals sink, Frost says biologists couldn't rely on body counts to gauge spill-related mortality So her team obtained permission to kill and autopsy 28 of the federally protected animals over a two-year period. These included 12 seals collected before molting.

All bodies appeared normal when examined externally, she says. Assays of internal contamination, however, showed that concentrations of hydrocarbon breakdown products were seven to 12 times higher in the bile of seals from oiled regions than in seals from other areas in the Gulf of Alaska. Because body enzymes detoxify hydrocarbons, the exposed animals carried low concentrations in most tissues other than blubber, mammary tissue, and milk.

The real surprise, Frost says, and the "most significant finding was lesions in the brains" of seals exposed to oil. Veterinary pathologists observed a swelling of the myelin sheath that protects nerves- primarily in the thalamus, a brain relay center for nerve impulses. Resembling damage seen in humans who die from sniffing glue or hydrocarbon solvents, this abnormality could have caused a "confusion" of sensory information, Frost says. Disoriented animals would be "predisposed to drowning:' she notes, particularly if they lost the ability to distinguish up from down or to tell when to breathe. In justifying the sacrifice of animals for this study, she notes that beached carcasses decompose too rapidly for scientists to identify such evidence of oil exposure.

While not conclusive, she says, the brain data offer strong circumstantial evidence that oil claimed the lives -- probably by drowning -- of 300 to 350 harbor seals, or 30 percent of those who hauled out onto oiled beaches during the spill.