The pinnipeds turned up shot on beaches in southern Puget Sound, on the Hood Canal and near Westport on the Washington coast.
About five to 10 seals and sea lions turn up shot in state waters during a typical year, said Brian Gorman, a spokesman for the National Oceanic and Atmospheric Administration.
Now, seven have been found since early December, most recently on Monday. Officials don't know who shot the animals.
"It's not crazy unusual, but it's enough for us to perk up our ears and be on the alert," he said.
Killing a seal or sea lion is a federal offense under the Marine Mammal Protection Act, punishable by a fine of $20,000, Gorman said.
The animal found in Hood Canal was a Steller sea lion, listed as threatened under the federal Endangered Species Act, in addition to being protected by the marine mammal law.
The Sea Shepherd Conservation Society, based in Friday Harbor, is offering a $10,000 reward for information leading to the arrest and conviction of the perpetrators.
The group is urging people with information to call NOAA's national enforcement at 800-853-1964 or call 911 to report a crime in progress.
"Prosecuting these cases is a challenge because without a witness it's difficult to make a connection between a shooter and a dead sea lion," Gorman said. "We really are dependent on the public to report anything they think is helpful."
Bullets or bullet fragments have been found or recovered from all seven sea lions, but biologists are working to determine whether the bullets killed them, Gorman said. He said stranded mammals have been found with old bullet wounds that aren't fatal.
The locations where the dead pinnipeds were found is not necessarily an indication of where they were killed, Gorman said. Many of the animals had been dead for two or three weeks, he said.
"People shouldn't infer anything from where they were ending up," he said. "Their stranded location is a function of tides and winds and not of where they died."
A harbor seal yearling found dead in Edmonds on Jan. 24 was suspected of having been shot, but this was later determined not to be the case, Gorman said.
Seals hear through holes in the sides of their heads that sometimes are mistaken for gunshot wounds, he said.
The Associated Press contributed to this story.
Bill Sheets: 425-339-3439; firstname.lastname@example.org.
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