Estimation of Age,Growth and Maturation of Purpleback Flying Squid,Sthenoteuthis oualaniensis,in Bashi Channel,Central Pacific Ocean

The age, growth and maturation of Sthenoteuthis oualaniensis were determined with statolith data collected with a light purse seine from the Bashi Channel of central Pacific Ocean. The estimated longevity of the squid was no more than 6 months for females, and no more than 5 months for males. Growth in mantle length(ML) was best described by logistic models for both females and males, while growth in body weight(BW) was best fitted by power curves. The maximum absolute growth rate(AGR) and instantaneous growth rate(IGR) in ML or BW both occurred at 91–105 days for females and 76–90 days for males. Back calculated hatching dates were from October to January, with a peak in December, although the short duration of sampling date might have had an influence on the result. The lower percentage of mature females(37.2%) suggested that the study area during the sampling date was not a spawning ground for the species. Size and age at first maturity were 183 mm ML and 136 days for females, whereas they were 156 mm ML and 85 days for males.

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.

Statolith Morphology of the Sthenoteuthis oualaniensis in the Central South China Sea

The morphology, microstructure and demographic parameters of statolith of Sthenoteuthis oualaniensis were studied from 90 individuals captured in the central South China Sea in September and October, 2012. The marphologic results showed that the statolith has great lateral dome and wing dome, small dorsal dome, and long narrow rostrum dome. S. Oualaniensis could be divided into three populations by statolith morphology in the central South China Sea. Five most important featured parameters including total statolith length （TSL）, wing length （WL）, maximum width （MW） were described by logarithmic functions with mental length （ML） （P 〈 0.05 ） ; rostnnn length （ RSL）, dorsolateral length （DLL） were described by linear functions with ML（ P 〈 0.05 ）. With the squid growth, the size of statolith gradually develops, while the ratio of each featured parameter to ML slowly decreases, the ratio of DLL, LDL, RSL and WL to TSL almost remain the same level, corresponding to 48.05%, 60.05%, 35.44%, 67.59%, and the ratio of LDL and WL to SL show little higher.

Climate-related changes in seasonal habitat pattern of Sthenoteuthis oualaniensis in the South China Sea

A habitat suitability index model(HSI)was established to examine seasonal habitat pattern of Sthenoteuthis oualaniensis in the South China Sea and assessed the impacts of the El Niño event on habitat variations based on sea surface temperature(SST),chlorophyll-a(Chla)and sea surface height(SSH).Environmental conditions and habitat patterns showed significant seasonal and spatial variations.Spatial pattern of favorable environmental conditions played important roles in regulating the ranges and longitudinal and latitudinal distributions of suitable habitats(areas with HSI≥0.6)of S.oualaniensis across seasons.Significant positive correlation was found between SST and HSI,whereas negative relationship was revealed between HSI and Chla as well as SSH by correlation analysis.Comparing to normal climate condition in spring 2014,the El Niño events in spring 2015 and 2016 yielded lower SST and higher Chla and SSH,which were unfavorable for the formation of high-quality habitats.Favorable ranges of environmental conditions showed decreasing trend from spring 2014 to 2016.Consequently,the suitable habitats in spring 2015 and 2016 largely contracted and became discontinuity.Our findings indicated that seasonal habitat patterns of S.oualaniensis in the South China Sea were strongly affected by the El Niño events and local environmental conditions.