Sex-specific yield per recruit and spawning stock biomass per recruit for the swordfish, Xiphias gladius, in the waters around Taiwan

Sex-specific yield per recruit and spawning stock biomass per-recruit analyses were conducted to evaluate the population status of swordfish in the waters around Taiwan. Estimates of the total mortality rate, obtained from length composition data by catch-curve analysis, are on the order of 0.30 year−1 for females and 0.35 year−1 for males. Pauly's empirical method indicates natural mortality rates of 0.25 and 0.27 for females and males, implying that the current fishing mortality rate on either sex is less than 0.1 year−1. This is well below the estimates for the target reference points F40% (0.12 year−1) and F0.1 (0.20 year−1), suggesting the swordfish in the waters around Taiwan are not over-exploited. The spawning potential ratio associated with the current level of exploitation was estimated to be 65% of the pristine level. Isopleths of yield per recruit and spawning stock biomass per recruit generated with various mortality rates suggest that both the yield and spawning biomass per recruit could be increased by raising the age at first capture to a value greater than the age at the onset of sexual maturity.