Efficacy of Low‐Cost, Side‐Scan Sonar for Surveying Alligator Gars

Abstract

AbstractSide‐scan sonar (SSS) provides an efficient, nonintrusive sampling tool for many fisheries applications. Professional‐grade systems have proven useful for species‐specific detection of large, distinctive fishes; however, these systems are often cost prohibitive for widespread use. Lower‐cost, consumer‐grade systems are becoming increasingly popular for fisheries applications and have been used extensively to map aquatic habitat. However, these systems often produce lower‐resolution imagery that may limit species‐specific applications. We used experimental ponds stocked with known fish assemblages and a field‐based mark–recapture study to determine the efficacy of a consumer‐grade SSS system in surveying Alligator Gars Atractosteus spatula. Three ponds were stocked with known abundances of Alligator Gars and other sympatric fishes. A fourth pond remained unstocked to assess false positives; a fifth was stocked only with other species to quantify misidentification. Replicate sonar images of each pond were then recorded using a Humminbird 998c system. For the field‐based application, we used gill nets to mark and recapture Alligator Gars and estimate population size within a closed backwater of the Guadalupe River, Texas. Following final recapture, we recorded replicate imagery of the backwater. Imagery from both evaluations was interpreted by three independent readers of varying experience without knowledge of image origin. The results from the ponds indicated a low rate of false detection (7%) and misidentification (4%). The number of Alligator Gars identified was positively related to density; however, the probability of detection varied among readers (range, 0.27–0.71) and was positively related to experience. In contrast, all readers estimated Alligator Gar abundance in the backwater within 10% of the population estimated from the mark–recapture data. Our results suggest that low‐cost SSS can provide an effective, noninvasive alternative to traditional sampling techniques for Alligator Gars.