Effects of sexual dimorphism on population parameters and exploitation ratios of blue marlin (Makaira nigricans) in the northwest Pacific Ocean

Blue marlin are sexually dimorphic in size-at-age and other biological characteristics. However, few studies have examined the possible impact of sexual dimorphism on the population parameters and the ratios of fishing to total mortality (the exploitation ratios) for this species. We analyzed sex-specific catch-at-length data for blue marlin collected from the Taiwanese tuna longline fishery in the northwest Pacific Ocean, ranging between 100-311 cm in eye to fork length (EFL) for females and 100-236 cm EFL for males, and show that the proportion of females in the catch (the sex ratio) increases with length, with females reaching larger body sizes than males. Minor differences in fishery sex ratios among months were observed. Growth parameters, length structures, and natural mortality rates were estimated to differ between males and females, while fishing mortality rates were found to be similar. Nevertheless, the exploitation ratio for females was higher than that for males. We suggest that growth parameters and natural mortality rates should be sex-specific when assessments for sexually-dimorphic species such as blue marlin are conducted, and that management of blue marlin fisheries could be developed based on size limit regulations for large individuals.