Reproductive biology of the black marlin, Istiompax indica, off southwestern and eastern Taiwan

• Sex ratios were skewed in favor of females in both sampling areas.
• Ovarian maturation criteria were established through histological methods.
• Size-at-50% maturity and batch fecundity was estimated for female black marlin.
• Southwestern Taiwan is an important spawning ground for black marlin.
• The estimated spawning season for black marlin was March–April.

The reproductive biology of the black marlin, Istiompax indica, was assessed by examining 4762 fish (458 ovary samples) caught by offshore longliners off southwestern and eastern Taiwan during 2004–2007. The overall sex ratio (0.67 off southwestern and 0.86 off eastern Taiwan) was significantly different from 0.5 (p < 0.01), with females predominant in both sampling areas. Monthly variation in the gonadosomatic index, mean monthly oocyte diameter, and histological analysis indicated that black marlin spawned off southwestern Taiwan during March and April. Although black marlin aggregated for feeding purposes off eastern Taiwan, no evidence of spawning was found in samples from this location. The size-at-50% maturity was estimated at ∼195 cm eye-to-fork-length (EFL) (∼5.4 years old) for female black marlin. Fully mature female black marlin were estimated to spawn once every ∼2.8 days using the hydrated oocyte method. Batch fecundity was estimated from 19 females, and varied 10-fold from 3.2 to 32 million eggs, with an average of 15.2 ± 1.9 (±S.E.) million eggs. The average relative batch fecundity was estimated at 105.7 ± 12.0 (±S.E.) eggs per gram of body weight. The relationships between batch fecundity (BF, million eggs), EFL and round weight (i.e., whole fish weight; RW, kg) were BF = 2 × 10−12EFL8.05 (r2 = 0.43) and BF = 8 × 10−4RW1.95 (r2 = 0.37), respectively. The parameters estimated in this study are key information for stock assessments of black marlin in the western Pacific Ocean and will contribute to the management, conservation and sustainable yield of this species