Estimating historic sea otter prevalence from archaeological and contemporary California mussel size structure

Along the northeastern Pacific, the extirpation and subsequent recovery of sea otters generated profound changes in coastal social-ecological systems. Today, most conservation targets for sea otter recovery are formulated on pre-fur trade population estimates reflecting ecosystems devoid of humans. However, evidence suggests that for millennia prior to European contact, complex hunting and management protocols by Indigenous communities limited sea otters at sites of high human occupation in order to enhance local access to shellfish. To make inferences about relative sea otter prevalence in deep time, we compared the size structure of ancient California mussels (Mytilus californianus) from five archaeological sites on the Northwest Coast of North America to modern mussels at locations with and without sea otters. To estimate mussel shell length from archaeological umbo fragments, we established a morphometric regression between modern mussel umbo thickness and maximum shell length. We also quantified modern mussel size distributions from eight locations on the central coast of British Columbia, Canada, varying in sea otter occupation time. Comparisons of modern and ancient mussel size revealed that pre-fur trade mussel size distributions are more similar to modern mussel size distributions in the absence of sea otters, suggesting that sea otters prior to the maritime fur trade were maintained below carrying capacity as a result of human intervention. These findings provide broader insight into the conditions under which humans and sea otters persisted over millennia, and potential solutions for their coexistence in the future.