Genetic structure and historic demography of endangered unarmored threespine stickleback at southern latitudes signals a need for new management directives

Habitat loss, flood control infrastructure, and drought have left most of southern California and northern Baja California’s native freshwater fish fauna on the brink of extinction, including the endangered unarmored threespine stickleback (Gasterosteus aculeatus williamsoni). This subspecies, an unusual morph lacking the typical lateral bony plates of the G. aculeatus complex, occurs at arid southern latitudes in the eastern Pacific Ocean and survives in only three inland locations. Managers have been stymied by a lack of molecular data to prioritize conservation efforts, including basic questions about the ancestry and genetic distinctiveness of unarmored populations. We sampled G. aculeatus from 36 localities and used microsatellites and whole genome data to place unarmored populations within the broader evolutionary context of G. aculeatus across southern California/northern Baja California. We identified three genetic groups, with none consisting solely of unarmored populations. Un..., ,