Complex sentence: In historic times, though, aboriginal hunting had depleted sea otter populations only in localized areas, and as the sea cow would have been easy prey for aboriginal hunters, accessible populations may have been exterminated with or without simultaneous otter hunting.
Split into simpler sentences: In historic times, though, aboriginal hunting had depleted sea otter populations only in localized areas. The sea cow would have been easy prey for aboriginal hunters, who would likely have exterminated accessible populations with or without simultaneous otter hunting.