Assessing Recruitment of Atlantic Sturgeon: What Does “Recovery” Look Like?

In April 2012, the Atlantic sturgeon was listed as a federally endangered species and although many state and federal agencies are working to recover depressed populations throughout the range, standardized assessment methods for evaluating recovery are completely lacking. Recent studies have shown, however, that assessments of juvenile abundance may provide a good quantified measure of population status, and ultimately, population trend.  The objectives of this study were to assess 1) annual abundance and 2) growth of juvenile Atlantic sturgeon in the Altamaha River, Georgia. From May-August 2004-2011, we used anchored gill nets and trammel nets to sample juvenile Atlantic sturgeon throughout the Altamaha Estuary. At the end of each sampling season, we constructed length-frequency histograms to distinguish age-1 from age-2 and older juveniles. We then calculated the abundance of each age class using the Huggins closed-capture model in Program Mark. We also assessed annual growth of successive cohorts using a hierarchical linear modeling approach. Over the eight years of the study, age-1 population estimates varied from 433-6225 individuals while age-2 population estimates varied from 130-5200.  We also documented a strong positive trend in total juvenile abundance, increasing from a low of 1194 in 2004 to >5,000 in 2010 and 2011. Analysis of juvenile growth showed a significant decline in response to increasing abundance, providing evidence of density-dependence and resource limitation. Our findings suggest that the Altamaha River Atlantic sturgeon population is recovering and that juvenile abundance may be nearing carrying capacity.  We suggest that the method used is this study could be easily adapted to other Atlantic Coast rivers to provide a standardized and quantified approach for assessing recovery of Atlantic sturgeon populations throughout the range.