ANCHORAGE, Alaska — An already-fragile population of killer whales that hunts Prince William Sound never recovered from the Exxon Valdez oil spill and is doomed to die off, biologists said last week.

Marine mammal biologist Craig Matkin of Homer, Alaska, has tracked the animals since the mid-1980s and said he never thought he’d see an entire population of whales _ even a small one _ disappear.

“To blame it all on the spill would not be fair, but that’s the final death blow,” Matkin said.

The plight of this group of killer whales contrasts with the full or slow, partial recovery of many other animal populations, including another group of whales, since the 1989 oil disaster.

Twenty years after the massive spill, as much as 16,000 gallons of oil lingers in Prince William Sound. Arguments linger over whether Exxon should pay more for cleanup work. And federal scientists and other researchers at an environmental conference in Anchorage last week said they’re still learning what the massive spill meant for local wildlife.

One of the most striking surprises to emerge from the annual Alaska Forum on the Environment was the tale of the so-called “AT1” population of killer whales.

Twenty years ago, the population numbered 22 whales. Today, only seven remain.

“These are the unexpected things. In killer whales, not recovering for this long length of time is something that we certainly didn’t foresee or predict,” said Jeep Rice, senior scientist with the National Marine Fisheries Service’s Auke Bay Laboratory in Juneau, Alaska.

Even before the spill, the AT1 whales were in deep trouble.

They eat harbor seals, which had been in decline for decades by the time of the oil disaster. The whales were also assailed by pollutants and pesticides that might have arrived in Alaska on weather systems from Southeast Asia and might hamper reproduction _ toxins were found in the whales’ blubber, Matkin said.

Then the tanker Exxon Valdez ran aground on Bligh Reef in Prince William Sound, and the estimated 11 million gallons it spilled killed thousands of birds and other wildlife.

The whales were known to be in the area. A Los Angeles Times photo shows four AT1 whales swimming near the leaking tanker.

Over the next year, more than a third of the whales died, and the population continued to fall.

The group is more than a pod. It’s the remains of a larger genetic population bound by family ties and social bonds, with its own distinct “language” of calls, said Eva Saulitis, a marine biologist who works with Matkin at the nonprofit North Gulf Oceanic Society.

The depleted population won’t mate with other groups, Saulitis said. The remaining whales may be too closely related to have calves, and the two remaining females are getting to old to reproduce.

She estimates this group of whales will be extinct within 25 years.

Their distinct dialect of calls, such as the long, loud blast a lone male sounds when he’s separated from the group, will die with them.

For other animals, scientists report mixed results.

If you ask Exxon, local animals are all on the road to recovery. The company insists the spill caused no long-term damage to the region. “The exposure to the remaining oil is very small and is decreasing,” said Paul Boehm, a chemical oceanographer and petroleum chemist consulting for the oil company in Alaska.

Boehm’s job includes gauging how much oil remains in the Sound. He says nearly all that’s left is weathered patches of non-toxic oil residue, and that the oil is in places or depths where animals like otters don’t dig and ducks and their food sources aren’t exposed.

The Exxon Valdez Oil Spill Trustees Council disagrees. The council is charged with using the nearly $1 billion Exxon agreed to pay for restoration efforts in a 1991 settlement, and it funds the continuing killer whale studies in the Sound.

Biologists have wrestled since the spill with deciding how much of the animal-population changes can be pinned on the oil spill, as opposed to other factors. Relatively little research was devoted to many of the animals before the spill.

But the killer whales were different.

Biologists began counting them in the Sound years before the oil spill. Sea World had proposed plucking killer whales for their aquariums, and that triggered substantial research on the whales. Biologists got a detailed record on the small groups that hunted in the region. They took photos of each whale. They could identify each by its markings or even nicks on its fins. Most of all, they knew exactly how many whales hung around Prince William Sound. (Sea World never got its whales.)

Two of the groups were already local celebrities _ swimming near shore or popping up next to anchored boats in the nooks and crannies of Prince William Sound.

The larger of the two was the AB pod, a group of “resident” whales, meaning they feed on fish.

The AB pod numbered 36 whales in 1989, having rebounded from shootings by longline fishermen who didn’t like the whales stealing black cod from their lines, Saulitis said.

Six days after the spill, Matkin saw the whales swimming through heavy sheens of oil.

Breathing oil vapor can be lethal, and seven of the whales went missing that first week. Another six disappeared over the following winter, Saulitis said.

Since 1989, the pod has been slow to recover, she said, probably because it lost five reproductive females, plus several juveniles who could have replenished the group.

The pod’s official count is now up to 29 whales, but that includes several who split off to join another group _ a move unprecedented among resident killer whales, Saulitis said.

“They’re just eking along.”

The other most commonly seen collection of killer whales in the Sound was the AT1 population.

It’s a transient group, meaning the whales hunt mammals and are more quiet, stealthy and secretive than their resident cousins, Saulitis said.

The population of 22 whales dropped to 13 over the first winter following the spill. It lost two reproductive females and two juveniles, Saulitis said.

Between 1990 and 1991, another two whales died.

The biologists waited for years to draw a connection between the initial decline in transient killer whales and the oil disaster to make sure the missing whales didn’t return or show up somewhere else.

“We were making darn sure that those animals were dead before we brought them into this whole picture,” Matkin said.

The biologist, with Saulitis, Rice and others, published a 2008 study that said the spill accelerated the AT1 whales’ path to extinction while the larger AB pod hadn’t recovered to pre-spill numbers.

Without studies of the bodies of dead whales to declare a concrete cause of death, evidence linking the spill to the initial decline in whales is circumstantial.

A murder mystery without the body, Matkin said.

But the sudden disappearance of 30 to 40 percent of the AB and AT1 whales 20 years ago was either caused by the spill, or it’s the biggest coincidence ever, he said.

“There’s no doubt they’re dead, and there’s no doubt they died at the time of the spill.”

(c) 2009, Anchorage Daily News. Distributed by McClatchy-Tribune Information Services.