The bombay-duck, Harpadon microchir, is one of the important bycatch species of the trawl fishery in the waters off southwestern Taiwan. However, its biological information, especially reproductive biology, is still lacking. In this study, the reproductive biology of the bombay-duck was described based on 1,697 specimens (1,066 females and 631 males) caught by bottom trawl fishery in the waters off southwestern Taiwan from October 2007 to December 2008. The spawning season of this species was estimated to be in May and from August to December with a peak in November based on the macroscopic appearance, gonadosomatic index, histological examination, and group maturity rate. Oocyte development and ovarian development were categorized into nine stages and four stages, respectively based on histological or macroscopic examination. Mean fecundity and mean batch fecundity were estimated to be 446,373 ± 174,858 and 61,680 ± 60,279, respectively. The relationship between fecundity (F) and body weight (BW) was estimated as: F = −46.580 + 9.69 × 103 BW(r 2 = 0.587; n = 50). The sex ratio (female/total), 0.63, was significantly different from 0.5. The sizes at 50% maturity were estimated to be 39.9 and 19.8 cm FL for females and males, respectively.

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