Comparison of fishery biology between medium-form and dwarf-form of Sthenoeuthis oualaniensis in South China Sea

Based on 780 Sthenoeuthis oualaniensis samples collected from September to October 2012, the fishery biological characteristics of medium form and dwarf form populations were compared and studied, which provided basic data for the management and protection of squid fishery resources in the South China Sea. The mantle length (ML), body weight (BW), gonadosomatic index (GSI) and feeding factor (FF) of the samples were determined according to the marine survey standard, and the number of growth increments of statolith was determined by grinding. The results showed that the ML range of medium form is 79–266 mm, dominant in 111–145 mm, and vary with latitude insignificantly; ML range of dwarf form group is 56–118 mm, dominant in 66–95 mm, and increases slowly with latitude. BW range of medium form is 17.47–957.20 g, dominant in 45.01–120.00 g; BW range of dwarf form is 5.81–76.56 g, dominant in 5.01–50.00 g; BW of medium form and dwarf form both increase with latitude. All of the relationships between ML and BW of female and male of medium form and dwarf form could be described by power functions. Sex ratio (female/male) of medium form is 0.80∶1 and maturity ratio (immature/mature) is 1.12∶ 1; sex ratio (female/male) of dwarf form is 3∶1, and maturity ratio (immature/mature) is 0.83∶1. GSI of medium form is 1.71 and 1.31 in female and male, respectively; GSI of dwarf form is 4.07 and 4.11 in female and male, respectively. FF of medium form is 5.07 and 3.54 in female and male, respectively; FF of dwarf form is 1.89 and 2.75 in female and male, respectively. The range of statolith age of medium form is 30–135 d, and that of dwarf form is 44–81 d. Based on the captured date, hatching periods of medium form and dwarf form are dominant in July and August, respectively. Results indicate that the size of medium form is bigger than that of dwarf form, maturing and hatching are not in synchronization in medium form and dwarf form, and maturing and hatching of female of dwarf form are earlier than those of medium form. The results also indicate that the sea areas at 7°, 14° and 16° north latitude are the feeding grounds of squid populations in the South China Sea.