Nearly all of the dead sea lions were shot to death, with the most recent attack happening Monday, The (Portland) Oregonian reported.
The sea lions have been found near the mouth of the Columbia River and a few miles north and south.
Dalin D'Alessandro, a Portland State University research assistant with the Northern Oregon-Southern Washington Marine Mammal Stranding Network, said the killings likely come at the hands of fishermen. There is typically a slight increase in the number of dead sea lions around the opening of salmon season, she said, but so far this year there have been about four times what is usually seen in the same time period.
Jim Rice, Oregon Coast coordinator for the Marine Mammal Stranding Network, said he believes the numbers are the highest in recent memory.