Validation of age and growth of the Picasso triggerfish (Balistidae: Rhinecanthus aculeatus) from Okinawa Island, Japan, using sectioned vertebrae and dorsal spines

Abstract

To understand age and growth of the Picasso triggerfish, Rhinecanthus aculeatus, monthly sampling was conducted from February 2010 to October 2011 on the subtropical Okinawa Island, Japan, using trammel net and speargun. Fish were measured by weight (W) and standard length (SL) and age was determined by examining thin-sections of the first dorsal spine and the second abdominal vertebra. A total of 352 fish were caught ranging from 20.9 mm to 209.5 mm (SL). Marginal increment analysis indicated that translucent rings were deposited annually during the colder season from October to January. The relationship of observed SL and age was described by the original form of the von Bertalanffy growth equation. Male and female growth models were significantly different. Overall growth was rapid during the first 2.5 years for both sexes, thereafter growth curves diverged. Males consequently reached larger sizes and greater ages. Maximum age for males and females was 13.5 and 9.5 years, respectively. Regression models of somatic growth (SL) as a function of vertebral and spine radii were significantly correlated (r = 0.98 and r = 0.96, respectively). According to the weight–length relationship, growth in R. aculeatus was negative allometric. Small recruits (SL < 40 mm) appeared in high numbers in shallow coastal areas starting from August. This is the first record of age and growth for R. aculeatus. Our data suggest that the Picasso triggerfish is a moderately long-lived balistid and that the observed sex-specific growth pattern is driven by the fish’s polygynous mating system and different reproduction strategies of males and females.