Population structure of purpleback flying squid (Sthenoteuthis oualaniensis) in Nansha area in spring

Research on fish population is foundamental for studies on fishery biology. Based on the survey data of purpleback flying squid (Sthenoteuthis oualaniensis) collected by Chinese falling-net vessels in Nansha area of the South China Sea during April to May in 2011, we studied the population composition, mantle length distribution, sex ratio and maturity of S. oualaniensis in that area. The results show that two populations of S. oualaniensis existed in Nansha area. The medium-form and dwarf-formindividuals accounted for 88. 92% and 11. 08% of the total catches, respectively. The dwarf-formpopulation was less abundant but occurred throughout the survey area. Mantle length ( ML) of S. oualaniensis ranged from69 mm to 231 mm for medium-form population and from 51 mm to 125 mmfor dwarf-formpopulation, respectively. The average ML of females was higher than that of males for two populations. The females accounted for 48. 48% and 61. 88% in the medium-formand dwarf-formpopulations, respectively. The variation of sex ratio by different ML groups was similar for two populations. The sex ratio of females to males was approximately the same among young squids with increasing ML, i. e. , males were first dominant, then were females and finally there were all females. The maturity stage of dwarfformpopulation was higher than that of medium-form population. The maturity stage of males was higher than that of females for two populations. By fitting Logistic function, the MLs at first maturity for females and males of medium-form population were 179. 7 mm and 125. 6 mm, respectively; and those for females and males of dwarf-formpopulation were 94. 8 mmand 81. 1 mm, respectively. It is suggested that the variation of sex ratio is closely related to the growth and maturity of males.