(CNN) Since September, 13 dead California sea lions have washed up in Puget Sound near Seattle, a conservation group says. Eight of the sea lions were shot, and the rest died from other wounds, most likely also caused by humans.

The Seal Sitters Marine Mammal Stranding Network, a volunteer organization that responds to calls for stranded animals, said all the sea lions died from acute trauma -- including one found last week that had its head sliced off.

It's not uncommon for sea lions to get shot -- most often by fishermen who believe the animals are reducing salmon populations and harming their livelihood. But the current spate of sea lion shootings is much higher than usual for the months of September through November, the Seal Sitters group says.

And the organization said this is just the beginning. High season for shooting sea lions is typically in the winter months, during fish runs.

Dyanna Lambourn of the Washington Department of Fish and Wildlife Marine Mammal Investigations unit (left) and Casey McLean of SR3 examine the entry wound from a bullet on a dead California sea lion in West Seattle.

"According to those who live and work along the Elliott Bay and Duwamish waterfront, shots are being heard even more frequently this year," said Robin Lindsey of Seal Sitters in an online post. "Animals searching for food to survive and fishermen searching out fish for consumption or livelihood are on an annual, never-ending collision course."

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