Utility of Alligator Gar Age Estimates from Otoliths, Pectoral Fin Rays, and Scales

Abstract

AbstractWe evaluated annulus formation and readers’ ability to recognize known annuli from otoliths, pectoral fin rays, and scales of alligator gar Atractosteus spatula using known‐age (0 and 1 years) and chemically marked fish. Chemical marks were generally associated with annuli, and confirmed annulus formation occurred around May. In all bony structures, annuli appeared as narrow translucent lines that immediately preceded wider opaque bands; however, annuli in pectoral fin rays and scales were more variable in appearance than otoliths. In otoliths, the number of observed annuli corresponded with the number of years fish were at large for 96% of the fish. In contrast, periodicity of annuli in pectoral fin rays and scales only corresponded with the number of years fish were at large in 19% and 12% of the fish, respectively. Precision of age estimates was initially poor for all structures; percent agreement between two independent readers was only 49% for otoliths, 43% for pectoral fin rays, and 37% for scales. Agreement increased to 83% for otoliths, 65% for pectoral fin rays, and 60% for scales following a second independent age assessment. Final average coefficients of variation between readers were 2, 5, and 11% for otoliths, pectoral fin rays, and scales, respectively. Increased precision was the result of experience and an improved ability to identify annuli because of chemical reference marks. Although correct identification of annuli required substantial training, our study validates annulus formation in otoliths of alligator gar. In contrast, annulus formation was not validated for pectoral fin rays or scales from alligator gar older than age 6, but age estimates from pectoral fin rays of young alligator gar may have some utility.