Age and growth of Indo-Pacific sailfish, Istiophorus platypterus, from the Arabian Gulf

Abstract

Dorsal and anal fin spines were collected from 85 sailfish from the Gulf and used to estimate age from putative annular growth bands observed in thin transverse sections. A total of 84 (98.8%) of these were successfully read with an average percent error in precision of 4.8%. Nine year classes (0+ to 8) were estimated from spines examined. Spines lacking a pair of translucent and opaque rings were placed in the 0+ year class; however the somatic size of these individuals suggested they were between 6 and 10 months in age. Estimated age classes for females ( n = 50) ranged from 0+ to 8 years, while males ( n = 34) ranged from 0+ to 6 years. Age class 1, followed by 0+ and 3 were the most abundant for both sexes, comprising 75% of all samples. Females exhibited the maximum sizes for lower jaw-fork length (LJFL) and weight within 101 sampled sailfish. The LJFL for females ( n = 65) ranged in size from 129 to 199 cm and weight ranged from 11.5 to 47.0 kg. For males ( n = 36), LJFL ranged from 125 to 177 cm and weight ranged from 12.5 to 38.0 kg. The von Bertalanffy growth function fitted to the observed LJFL, weight and age data indicated a rapid growth rate during the first 2 years, after which length stabilized for males and females continued growing to a greater maximum mean length and weight. The relationships of measured morphological traits to LJFL derived from the non-linear equation Y = a LJFL b and their regression coefficients showed a negative allometric growth for weight, head, and pelvic fin length. Most other measurements were effectively isometric, with the exception of the anal fin in males, which showed positive allometric growth. A comparison of allometric growth between females and males using a modified t-test against linear regressions indicated no significant differences between LJFL and other morphometric characters ( P ranged 0.135–0.980).