Twenty-five years after the Exxon Valdez oil spill in Alaska's Prince William Sound, sea otter and salmon populations have rebounded, but other species, including shrimp, crab, herring and the pigeon guillemot, have not. Alyeska Pipeline Service now requires double-hulled tankers to carry North Slope oil, and they must be escorted by tugs. Alyeska conducts two major spill drills a year.

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