Data: Spawning run estimates and phenology for an extremely small population of Atlantic Sturgeon in the Marshyhope Creek-Nanticoke River system, Chesapeake Bay

Once thought to be extirpated from the Chesapeake Bay, fall spawning runs of Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus have been rediscovered in the Marshyhope Creek (MC)–Nanticoke River (NR) system of Maryland, United States. This population is grouped within the Chesapeake Bay Distinct Population Segment of Atlantic Sturgeon and is currently listed as endangered under the Endangered Species Act. Survey captures in this system are predominantly recaptures of 43 adults tagged since 2014, indicating a very small population in the two connected spawning tributaries. Data from mobile side-scan sonar surveys and detections of acoustically tagged adults on stationary telemetry receivers were analyzed in an integrated model to estimate spawning season abundance and examine run timing and system connectivity for this population. An array of acoustic receivers was deployed throughout the MC–NR system to monitor the movement of tagged fish during the spawning run period from mid-August to late October. Side-scan sonar surveys were conducted weekly in September in an area of high spawner aggregation to generate count data on spawning run abundance. In 2020 and 2021, 32 (95% credible interval [CRI] = 23–47) and 70 (95% CRI = 49–105) Atlantic Sturgeon, respectively, used the MC–NR system. The lower estimate for 2020 coincided with an earlier end to the spawning run related to cooler September temperatures in that year. In both years, high spawning run connectivity between MC and the upper NR was observed. Overall, run estimates support previous hypotheses that the MC–NR system supports a very small population and that both MC and the upper NR serve as important areas for spawning activity.