Side-Scan Sonar

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. Abundance was positively associated with river depth in both years and availability of bedrock in 2018. Detection probabilities were 0.42 (SE = 0.13) and 0.14 (SE = 0.13), respectively. This study provides the first estimate of ATS spawner abundance in the Altamaha River since 2007 and the first since the species was listed as threatened, and my results suggest that the population has remained stable over the last decade. Our results also provide managers with important quantitative data about the spawning habitat locations and characteristics. Compared to previous mark-recapture estimates of spawning run size, side-scan sonar was less invasive and required substantially less sampling effort.