The state Department of Fish and Wildlife said Tuesday that the dead otter was found floating in Morro Bay in September and a necropsy determined it was killed with a pellet gun.
A numbered tag identified the animal as an otter that had previously been found tangled in a fishing line. It was rehabilitated and returned to the wild in July 2016.
The department says the killing was the latest in a rash of sea otter crimes along the stretch of coastline between San Francisco and Los Angeles.
A skinned otter was found in September. Three otters were shot to death in August 2016.