Abstract
Age and growth of the dusky shark, Carcharhinus obscurus, in the western North Pacific Ocean was estimated based on 434 specimens (213 females and 256 males) collected between September 2002 and November 2003 at the Nanfanao fish market, in northeastern Taiwan. Sex-specific relationships between body weight (W) and total length (TL) were estimated as follows: W=2.03 × 10^(-6)TL^(3.216) (n=209, p ＜ 0.01) for females and W=8.01 × 10^(-6)TL^(2.957) (n=294, p ＜ 0.01) for males. The relationship between TL and the vertebral centrum radius (R) for both sexes (＂sexes-combined＂ data) was described using the following equation TL=14.74 + 20.31R (n=399, p ＜ 0.01). Growth band pairs (including translucent and opaque bands) in precaudal vertebrae were determined to form once annually, based on centrum edge analysis and verified by statistical analysis. The maximum number of growth band pairs was 29 for females and 24 for males. The Robertson growth function best fit the observed length-at-age data. Sex-specific growth equations were not significantly different; thus, the combined growth parameters for both sexes were estimated as: asymptotic length (L_∞)=369.4 cm TL, b=0.912, growth coefficient (k_R)=0.126 yr-1 (n=385, p ＜ 0.01). Age at maturity was estimated to be 16.6 yrs for females and 15.5 yrs for males.
Highlights
The dusky shark, Carcharhinus obscurus (Lesueur, 1818), inhabits tropical and warm-temperate continental seas worldwide (Compagno 1984), including the waters of the continental shelf off the coast of northeastern Taiwan, where it is an important species for the local commercial shark fishery
According to catch statistics from the Nanfanao fish market - the largest landing port for the shark fishery in northeastern Taiwan - annual landings of this species fluctuated from 284 metric tons (MT) in 1990 - 193 MT in 2008, with an annual mean of 210 MT, accounting for 11.5% of the total annual shark landings in this area
Dusky sharks caught in the western North Pacific Ocean (Fig. 1) by Taiwanese coastal and offshore longline vessels were landed at the Nanfanao fish market in northeastern Taiwan
Summary
The dusky shark, Carcharhinus obscurus (Lesueur, 1818), inhabits tropical and warm-temperate continental seas worldwide (Compagno 1984), including the waters of the continental shelf off the coast of northeastern Taiwan, where it is an important species for the local commercial shark fishery. As the demand for shark fin has increased in Asian countries, dusky sharks with median sized fins have become one of the major target species. This species is listed as vulnerable (VU) on the IUCN Red List of threatened species, and its population is decreasing
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