Beyond migrants and residents: complex partial migrations in differentially migrating shortnose sturgeon

BACKGROUND: Partial migration refers to populations that contain both resident and migrant individuals. Partially migratory populations exhibit broad behavioral plasticity, with individual variation stemming from a combination of environmental influences, inheritable genetics, and sexual dimorphism. The United States federally endangered shortnose sturgeon (Acipenser brevirostrum) have been observed to form partially migratory populations in the Gulf of Maine, with sex-biased migration behavior and complex differential timing. However, migrations in southern populations are poorly understood, as are the influences behind migratory life history determination for this species. Therefore, the aim of this study was to investigate the spatiotemporal characteristics and influences of partial migration behaviors in several shortnose sturgeon populations in the southern extent of its range. METHODS: Long-term (2011–2024) acoustic telemetry data from shortnose sturgeon across the southern United States (i.e., Georgia, South Carolina, North Carolina) were used to identify behaviorally distinct migratory contingents using an analysis of similarities with several migration metrics and residency indices. Population- and sex-level influences on migratory life history determination were then assessed using generalized linear and multinomial models. RESULTS: Multiple behavioral contingents were present within both the migratory and resident groups of southern shortnose sturgeon spawning populations. Individuals classified as resident did not move to other rivers, and differential behaviors between resident contingents were observed where some fish traveled into the lower river and connected estuarine/coastal environments of their spawning river system while others remained upriver. Migrants were detected in at least one river system beyond their putative spawning river, and differential behaviors between migrant contingents related to residency patterns, connectivity, and travel distances during coastal inter-river migration. Furthermore, females had a greater probability of being migratory than males, and larger populations displayed more migratory life histories than smaller populations. CONCLUSIONS: Shortnose sturgeon populations in the southern United States exhibited extensive behavioral plasticity, with sex- and population-level influences on migratory life history determination. Behavioral diversity is critical for stability and persistence of partially migrating populations, and our study demonstrates such behavioral complexity for shortnose sturgeon. Species management must account for the rull range of migratory capabilities observed in our study.