Validation of annual growth zone formation in gray triggerfish Balistes capriscus dorsal spines, vertebrae, and otoliths

Abstract

Uncertainty in age estimates from dorsal spines has been a persistent issue in stock assessments of gray triggerfish, Balistes capriscus. This study sought to validate the annual deposition of growth zones on dorsal spines, vertebrae, and otoliths of gray triggerfish through chemical marking. Fish (n = 101) were collected from offshore habitats and held in an aquaculture facility. 74 adult fish were chemically marked with a 50 mg/kg body weight injection of calcein, and reared for an average of 527 days post-marking. At intervals, fish were sacrificed and first dorsal spines, vertebrae, and otoliths were extracted and sectioned. Annuli, were enumerated for spines (n = 96), vertebrae (n = 94), and otoliths (n = 48) and ranged from 0 to 11 annuli for spines and vertebrae, and 1–12 annuli for otoliths. Age bias plots showed strong agreement between spine and vertebra annuli counts for all observed ages, while counts from spines and vertebrae appeared to underage beginning at age 5 when compared to otolith annuli counts. Tests of symmetry indicated that the annuli counts between paired age structures were not biased (p > 0.05). Analysis of growth zones observed distal to calcein marks in all of the age structures confirmed that these zones were deposited annually, and the expected number of these zones, or annuli, were observed in 91% of spine, 90% of vertebrae, and 100% of otolith sections. Marginal increment analysis of ageing structures indicated that annuli form during summer months. Percentages of annuli deposited on the margins peaked in June for spines (58%) and otoliths (29%), and August for vertebrae (30%). Results from this study validate the annual deposition of growth zones but further consideration needs to be taken when ageing older than age-4.