Age and Growth Estimates of the Blacktip Sawtail Catshark Galeus sauteri in Northeastern Waters of Taiwan

Abstract

The blacktip sawtail catshark Galeus sauteri is a small, demersal species that inhabits tropical and subtropical coastal waters of the western Pacific region. It is one of the most important small shark species in terms of catch biomass for the trawl fishery in Taiwanese waters, but its stock status is poorly known. In this study, age and growth of this species were described from 739 specimens (388 females and 351 males, 8.5- 49.8 cm in total length; TL) collected from Nov. 2007 to Oct. 2008 in waters off northeastern Taiwan by demersal or shrimp trawlers. Sex-specific relationships between the weight (TW) and TL significantly differed and could be expressed as: TW = 2.09 × 10-3TL3.10 (n = 1052, p < 0.05) for females and TW = 3.45 × 10-3TL2.94 (n = 884, p < 0.05) for males. Age was determined using band-pair reading of sectioned vertebrae. A single growth band pair (comprising translucent and opaque bands) is formed each year, and up to 14 and 12 band pairs were respectively observed for females and males. An edge analysis indicated that the translucent bands formed from July to Aug. Four growth functions, the traditional von Bertalanffy (VBGF), VBGF with a fixed size-atbirth (L0), Gompertz, and Robertson (logistic) were used to model the observed length-at-age data. The sexspecific growth equations significantly differed. The logistic function had the best fit for both sexes, and the growth parameters including the 95% confidence intervals with a bootstrap method were estimated as follows: asymptotic length L∞ = 48.30 (47.16-49.47) cm TL, and growth coefficient k = 0.374 (0.344-0.402) yr-1 for females, L∞ = 44.29 (42.91-45.88) cm TL and k = 0.392 (0.356-0.429) yr-1 for males. The ages at maturity were respectively estimated to be 9.14 and 7.57 yr for females and males, by substituting the mean sizes at maturity into the Robertson growth equation. Longevities of females and males were respectively estimated to be 20.9 and 12.4 yr.