First record of exotic alligator gar, Atractosteus spatula (Actinopterygii: Lepisosteiformes: Lepisosteidae), from Ganga River system, India: A possible threat to indigenous riverine fish diversity

A new record of an exotic alligator gar, Atractosteus spatula (Lacepède, 1803), from an open wetland of the Ganga River was presented in this paper and discussed along with the environmental parameters. Entry of the exotic fish into the natural system was probably a result of uncontrolled ornamental fish trading. Considering threats of this predatory fish to become invasive and disturb riverine fish diversity, possible ways to avoid such risk have been discussed.

Effects of Training on Side-Scan Sonar Use as a Fish Survey Tool: a Case Study in Alligator Gar

Consumer-grade side-scan sonar has become a versatile fisheries management tool. First applied to assess habitat, its use has expanded to surveying fishes in recent years. However, an important consideration is the skill and experience of users, which can impact both the accuracy and comparability of surveys. To this end, we characterized the ability of a small sample of novice users (N = 8) to identify alligator gar Atractosteus spatula in imagery, as well as the effect of a two-hour training exercise on user performance. Prior to training, mean accuracy (expressed as the difference between observed and expected counts) among participants ranged from -2.6 to 1.3 fish and precision ranged from ± 1.2 to 2.4 fish, with the majority of participants underestimating the number of alligator gar present in the imagery. False positives (i.e., identifying alligator gar in imagery when none were present) were common among participants. Post-training mean accuracy ranged from -3.1 to 0 among participants and precision ranged from ± 1.6 to 3.2 fish. The frequency of false positives was significantly reduced following training, and participants reported significant increases in confidence associated with image interpretation. The relatively high accuracy and precision we observed prior to training indicated that side-scan sonar can be easily incorporated into large-scale fishery monitoring efforts for alligator gar. However, our results also suggested that a rather minimal investment in training can further improve consistency and reduce uncertainty among novice users.