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 this week.

Marine mammal biologist Craig Matkin of Homer 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 linger 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 this week say they're still learning what the massive spill meant for local wildlife.

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