Validation of Annual Growth Zone Formation in Gray Triggerfish Dorsal Spines, Fin Rays, and Vertebrae

The goal of this study was to validate annual growth zone formation in the gray triggerfish dorsal spines, fin rays and vertebrae. Adult gray triggerfish (n= 4) were chemically marked by injecting with 50 mg of oxytetracycline (OTC) per kg body mass and reared in a 2,300-L aquaculture tank. Fish were sacrificed after 262 days and their first dorsal spines, fin rays and vertebrae were extracted and sectioned. One translucent zone formed distal to the OTC mark in all hardpart types during winter. Additional fin rays and vertebrae with corresponding dorsal spines were sampled from fish (n =27 and 59, respectively) to compare translucent zone counts between hardparts. There was a significant difference between translucent zone counts between fin ray and dorsal spine sections (t_1,25 = -3.15, P = 0.004), with fin ray counts being on average 1 zone higher than dorsal spines. Translucent zone counts in vertebrae were similar to those counted in dorsal spines (t_1,57 = 1.90, P = 0.062). The percentage of dorsal spines with translucent margins increased in winter months. The combined results of this study validate annual translucent zone formation in gray triggerfish hardparts, with dorsal spines being the most reliable ageing structure.