Age and Growth of the Shortfin Mako Shark in the Southern Indian Ocean

Abstract

The Shortfin Mako Isurus oxyrinchus is one of the major shark bycatch species in the longline fishery for tuna Thunnus spp. in the Indian Ocean; however, its biological information is still little known. This study estimated the age and growth of Shortfin Makos in the southern Indian Ocean. In total, 178 specimens (72 females and 106 males) were opportunistically collected by the scientific observers onboard Taiwanese large-scale longline fishing vessels from February 2013 to September 2016 in the southern Indian Ocean. The relationship of gutted weight (GW; in kg) to curved fork length (CFL; in cm) for both sexes combined was estimated as GW = 0.00001 x CFL2.517 (n = 170, r(2) = 0.85). Growth band pairs (including translucent and opaque bands) were counted based on sectioned vertebral centra from the caudal peduncle region. The periodicity of band-pair deposition on vertebral centra was estimated to be 1 year, with opaque bands deposited around August based on edge analysis. On this basis, the maximum observed ages of females and males were 18 and 14 years, respectively. The von Bertalanffy growth function best fitted the observed sexes-combined length-at-age data, with growth parameter estimates as L-infinity = 267.6 cm CFL, k = 0.123/year, and t(0) = -2.487 years (n = 159). The growth model could be improved for future stock assessments by increasing the sample size, particularly on very small (neonates and small subadults) and very large female Shortfin Makos.