Estimates of life history parameters of the sharpspine skate, Okamejei acutispina, in the northeastern waters of Taiwan

The sharpspine skate Okamejei acutispina is one of the most abundant bycatch species of trawl fishing in the waters of northeastern Taiwan. However, its life history parameters, which are essential for stock assessment and management, are still poorly understood. The objective of this study was to estimate important parameters for this species, including age, growth, age at maturity and mortality. Age estimates were derived from 652 specimens (356 females and 296 males, 9.5–33.8 cm in disc width) using vertebral band counts and verified by length-frequency analysis of 1443 specimens (768 females, 675 males) collected between October 2004 and December 2007 in waters off northeastern Taiwan. The sexes-combined relationship between body weight (W) and disc width (DW) was W = 0.0193 × DW3.0144 (n = 1399, p < 0.05). Edge analysis indicated that a single growth band pair (comprising translucent and opaque bands) is formed each year, with up to 11 band pairs for females and 10 band pairs for males. The Robertson growth function had the best fit for both sexes and the growth parameters including 95% confidence intervals were estimated as follows: asymptotic disc width (DW∞) = 32.4 (31.3–34.0) cm DW, growth coefficient (k) = 0.367 (0.315–0.426) yr−1 for females; and DW∞ = 28.8 (27.6–30.3) cm DW, k = 0.365 (0.311–0.424) yr−1 for males. Age at maturity was estimated to be 7.2 and 6.8 yrs for females and males, respectively, while longevity was estimated to be 13.2 and 13.3 yrs, respectively. Length-frequency analysis produced similar results, supporting the vertebral aging estimates. Total mortality was estimated to be 0.85 and 0.86 yr−1, while natural mortality was 0.32 and 0.32 yr−1 for females and males, respectively. Our study indicates that it is likely that the sharpspine skate is at the margin of optimal exploitation and therefore close monitoring and seasonal closure management of trawl fishing are recommended to ensure sustainable utilization of this stock.