Use of Side-Scan Sonar to Identify and Count Alligator Gar: Potential for Evaluating Population Density from Sonar Imagery

Declines in distribution and abundance of Alligator Gar Atractosteus spatula have spurred management and research efforts to conserve remaining populations.   However, lack of efficient sampling gears currently hampers data collection. We tested consumer-grade, side-scan sonar in experimental-pond and field evaluations to determine its utility to identify and count Alligator Gar.  In experimental ponds, Alligator Gar density based on sonar image interpretation was consistently underestimated (52% - 92% of true abundance) and other fishes were seldom falsely counted as Alligator Gar (7% of possible occasions).  In the field evaluation, side-scan counts (mean = 70 fish, range = 59-77) were remarkably close to a mark-recapture population estimate of 76 fish (95% CI = 43-149).  We also tested a pool of fisheries professionals (N = 8) to determine the usability of this technique with and without training (1 h).  Prior to training, fisheries professionals were consistent but biased (overestimation N = 5; underestimation N = 3).  However, training eliminated estimation bias from all but one individual.  We concluded that side-scan sonar has great potential for efficiently identifying and enumerating Alligator Gar with minimal effort and training.  The technology may also prove useful for identifying appropriate sampling sites and evaluating Alligator Gar habitat association.