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Abstract
The Atlantic Sturgeon (Acipenser oxyrinchus oxyrinchus) is a large, benthic, anadromous fish that occurs off the east coast of North America. Many populations were nearly extirpated by commercial harvest and the construction of dams on spawning rivers, which resulted in the species being listed as federally endangered in 2012. Recruitment studies indicate that the Atlantic Sturgeon population in the Altamaha River, Georgia is among the most robust within the South Atlantic Distinct Population Segment (DPS). Unfortunately, population estimates for the large, migratory adults are lacking because quantifying them is logistically challenging. The recent availability of cost effective remote sensing technology offers a new method for estimating the abundance of adult spawners during their upstream migrations. In this study, we used side scan sonar surveys to assess the run size of Atlantic Sturgeon throughout the entire navigable reach of potential spawning habitat (~550 river km) in the Altamaha River system, from September-November of 2017 and 2018. We used N-mixture models to estimate that there were 115 (95% CI: 104-129) and 82 (95% CI: 70-95) adult Atlantic Sturgeon in the 2017 and 2018 spawning runs. Depth and bedrock were significant covariates affecting abundance in the top models across both years. Overall detection probabilities were 0.42 (SE = 0.13) and 0.14 (SE = 0.13), respectively. This study is one of the only recent estimates of adult Atlantic Sturgeon abundance in the Altamaha River or the South Atlantic DPS and provides managers with important quantitative data about the spawning population and its preferred habitat characteristics. Compared to previous mark-recapture estimates of spawning run size, side-scan sonar was less invasive and required substantially less effort while indicating that the Altamaha River spawning population has remained relatively stable.