The abundance of very small mussels (Mytilus californianus) at sites in northern Santa Barbara County has been attributed to human overexploitation. However, other explanations are possible. Small valve size can be caused not only by overharvesting by humans, but also by environmental conditions adverse to mussel growth or survival; by intensive predation by sea otters, starfish, or other animals; or conceivably by intentional or unintentional size selection by aboriginal shellfish collectors.