Studies on the Northern Channel Islands have demonstrated that human harvesting of California mussels (Mytilus californianus) caused a reduction in mussel shell length over thousands of years. These studies however, have relied on measuring whole mussels from archaeological sites, which can be rare in archaeological assemblages, limiting chronological and geographic visibility. Using an allometric approach, three regression formulas were developed to determine total shell length from the commonly found hinge portion of a California mussel. Of these three regression formulas one, umbo-width, proved to be statistically reliable and practical for estimating the total shell length from archaeological mussel hinge fragments. Using this method, 2,262 California mussel hinges were measured to determine if a reduction in shell size through time could be identified. These results were compared to modern datasets for California mussel shell length on the Northern Channel Islands. The findings discussed herein provide useful baseline data for resource managers in the coming decades as rising sea surface temperatures and increased ocean acidification threaten this species.