I agree with most - it is an old lamprey scar.

Lampreys feed by attaching themselves to the fish and use those teeth and rasping tongue to open a wound and hold on to the side of the fish so they can feed off the various body fluids. Since it is essentially a surface injury would not expect to see scar tissue very deep into the meat.

The parasitic stage of Pacific and River lamprey is in the salt and they stop feeding once they reach freshwater (returning to spawn). Thus the healed condition of the wound. The oval nature of the scar could be due to growth of the fish after the lamprey was shed or a slight repositioning of the lamprey during its feeding efforts.

Due to a region wide population crash of lampreys such wounds are much rarer than a number of decades ago.

curt