Natural mortality estimation and rational exploitation of purpleback flying squid Sthenoteuthis oualaniensis in the southern South China Sea

Based on the biological data of purpleback flying squid(Sthenoteuthis oualaniensis)collected by light falling-net in the southern South China Sea(SCS) during September to October 2012 and March to April 2013,growth and mortality of 'Medium' and 'Dwarf' forms of squid are derived using the Powell-Wetherall,ELEFAN methods and length-converted catch curves(FiSAT package).Given a lack of commercial exploitation,we assume total mortality to be due entirely to natural mortality.We estimate these squid have fast growth,with growth coefficients(k) ranging from 1.42 to 2.39,and high natural mortality(M),with estimates ranging from 1.61 to 2.92.To sustainably exploit these squid stocks,yield per recruitment based on growth and natural mortality was determined using the Beverton-Holt dynamic pool model.We demonstrate squid stocks could sustain high fishing mortality and low ages at first capture,with an optimal fishing mortality >3.0,with the optimal age at first capture increased to 0.4-0.6 years when fishing mortality approached optimal levels.On the basis of our analyses and estimates of stock biomass,we believe considerable potential exists to expand the squid fishery into the open SCS,relieving fishing pressure on coastal waters.

Review on the Biology of Purpleback Flying Squid in South China Sea

Purpleback flying squid （Sthenoeuthis oualaniensis） is an important marine economic species, studying its biological characteristics is very important for the resource assessment and management. This paper summed up domestic and international information of squid biology, and showed some details on this subject. According to the squid life history ：egg, larvae, juveniles and adult fish stage, it summarized from the age of growth, population structure, migration, spawning, vertical distribution, habitat, and status of resources. There is little research on the squid larvae and the signs of juveniles period ended are not clear. Adult fish is divided into four main populations and two minor popular ons, the large group, medium-sized group, small group and mini-group, while medium-sized groups can be divided into medium uniaxial group and medium-sized biaxial according to the characteristics of the inner shell. The distribution of purpleback flying squid has the ＂island correlation＂, it usually ascend to the surface at night and drop to the deep-water during the daytime. Purpleback flying squid plays an important role in the ecosystem. It is the main food of many fish, and marine mammals ; also it is the predators of crustaceans and small fish. It has the self-feed characteristics. Purpleback flying squid widely distributes in the 38 °N to 40°S tropical and subtropical waters, especially in the South China Sea, the catchability is （130 -200）×10^4 tons in one year.

Insights into the divergent evolution of the oceanic squid Sthenoteuthis oualaniensis(Cephalopoda:Ommastrephidae)from the Indian Ocean

Sthenoteuthis oualaniensis is known for its complex population structure with three major transoceanic forms(viz.middle-sized,dwarf,and giant forms)whose taxonomic status has been disputed for decades.This integrated taxonomic study examines these prevenient morphotypes gathered on cruises in the Indian Ocean to ascertain their status in the evolutionary history of the species.Molecular analyses employing mitochondrial(COI,ND2)and nuclear(H3)markers revealed four genetically distinct and novel lineages of the species in the Indian Ocean,representing three morphotypes from the Arabian Sea and one from the Southern Indian Ocean.The mitochondrial-based phylograms revealed two distinct clades in the species:“dwarf forms+giant form”and“middle-sized forms,”which further branch into geographically structured evolutionary units.Species delimitation analyses recovered five distinct clades,namely,the Arabian Sea giant and dwarf forms,Equatorial,Eastern Typical,and Other Middle-sized forms,representing the consensus molecular operational taxonomic units.H3 being heterozygous could not resolve the phylogeny.Haplotype network and AMOVA analysis of mtDNA genes indicated explicit phylogeographic structuring of haplotypes,whereas these outputs and PCA results were incongruent with the morphological grouping.Phenetic features distinguishing the morphotypes were sometimes plastic and mismatched with the genotypes.The giant form was genetically close to the dwarf forms,contradicting the earlier notion that it descended from the middle-sized form.It may be assumed that the dwarf form evolved following sympatric speciation and adaptation to warm equatorial waters,while the focal features of the Western Arabian Sea guide toward allopatric speciation of the giant form.