Age, growth and maturity of frigate tuna (Auxis thazard Lacepède, 1800) in the Southeastern Brazilian coast

Abstract

Age, growth and maturity of frigate tuna (Auxis thazard) were studied for the first time in Southeast Brazil. A total of 650 fish (265–494 mm Fork Length) were sampled from beach-seine landings between March 2018 and February 2019. Sectioned fin spines from 548 specimens were processed and age was determined by counting and measuring of increments. Growth parameters were fitted to length-at-age data using the von Bertalanffy growth function and were compared to the literature using the growth performance index (Phi). Macro and microscopic characterization of gonads and physiological indexes were used for the analysis of the reproductive cycle. The length distribution did not differ by sex. Assigned ages were between 0+ and 4 years, with annual increments formed during winter. Coefficient of variation between readings was 11%. Growth differed by sex, and the von Bertalanffy growth parameters were L∞ = 471 mm; k = 0.47 year−1; and t0 = –1.46 year−1 for females; L∞ = 498 mm; k = 0.35 year−1; and t0 = –2.01 year−1 for males. Phi was 5.0 for both sexes. Estimated growth rate is among the lowest recorded for the species, possibly related to the method used for age and growth assessment. Gonadosomatic index peaked in December, associated with the upwelling of the South Atlantic Central Waters, at a temperature of approximately 16 °C. Spawning activity in colder waters has never been reported for this species and may be influenced by a richer habitat caused by the intrusion of more nutritive rich waters during spring-summer, which allows larval development. Length-at-first-maturity (L50) was 345.4 mm for females and 329.8 mm for males.