| Sthenoeuthis oualaniensis is one of the main catches of light-attraction falling net inthe South China Sea. In order to explore the relationship between S. oualaniensis andmarine oaganism, we use traditional stomach contents analysis to study the prey category,the degree of stomach fullness from S. oualaniensis, and the feeding intensity withmantle length, gonad maturation and seasonal change. In these study,The technology ofcarbon and nitrogen stable isotope were also used to analysis δ15N and δ13C valueschange with growth, and so on. comparing the influence of δ15N and δ13C values changebefore and after the degreasing process, further to confirm its food resources and itstrophic position, exploring the carbon and nitrogen stable isotope techniques in oceaniccephalopods of predatory research and the application of trophic levels, providing a basicdata for further research for S.oualaniensis feeding habits in the South China Sea.Based on the monthly sampling in the South China Sea from February2012toJanuary2013, the specimen were mainly from Nanwan fishing port of Guangxi. The seaarea of catch sampling was located from108°to118°E，and from9°to20°N accordingto correlated date from the CATSAT remote sensing system of fisheries. A total of3543individuals of S.oualaniensis were collected, ranging from60mm to302mm inmantle length and7.5g to1162.0g in body weight. there were about85.24%(3020)which have prey in stomach, and the percentage of empty stomach was only14.76%(523)among them. The results of this study are as follows:1. By the biology measurement data analysis, there are some results we achieved.(1)mantle length and body weight showed a power function relationship:W=2.4759×10-6ML3.5614(R2=0.9744, n=1755)； W=1.4167×10-6ML3.6856(R2=0.9561,n=1788).(2)The sex ratio was0.98:1showed a non-significant difference which wastested by chi-square test, and also with the monthly chang（eF=1.516, P=0.249）. We useddifferent group of mantle length (sex maturity stage Ⅲ,Ⅳ,Ⅴ) to fit Logistic curve, it isconcluded that the S.oualaniensis first mantle length of gonad maturation (ML50%) was154mm of female and103mm of male, respectively. Gonad somatic index (GSI)analysis of S.oualaniensis suggested a significant relationship with mantle length and sex maturity stage（P<0.05）.2. The results of stomach contents analysis showed that:(1)The mainly feedingcomposition of S.oualaniensis was fish, cephalopods and lessl crustaceans, fish wereindentified from otolith, squama and residual vertebrae, comparison analysis suggestedthe S.oualaniensis mainly feeding on Myctophidae, Carangidae, and Exocoetidae.Cephalopod showed as rostral, broken tentaclese and residual muscle mass, such asThysanoteuthis elegans and multihamata, ect, and a obvious phenomenon of cannibalismat the same time, crustaceans are only sporadic appendages vestigital, mainly for thepelagic red crab, etc.(2)Cephalopods account for50.82%of all the prey, while fishoccupied33.98%, and the rest of unidentifiable account for15.20%, the frequency ofoccurrence of fish was436, cephalopods was52and crustaceans66, respectively.3. Prey category and its change.(1)fish and cephalopods were the mainly prey withthe individual maturity, stage Ⅰ individuals had a high degree of digestion from thestudy, the prey unrecognizable stomach contents of Ⅱ-Ⅳ Individuals showed adecreased while Ⅴ was increased. And with sexually mature, fish in stomach contentsshowed a little change, but the percentage of cephalopods in stomach contents increasedwith individuals maturity at stageⅠ-Ⅳ, specially, and was to get the maximum (67.99%)at stage Ⅳ. However, the frequency of fish appear in each period were higher thancephalopods and crustaceans.(2)The stomach weight of S.oualaniensis increased withthe individual growth, exponential growth with the mantle length:Stomach weight=7.75×10-4ML2－0.139ML＋6.45, R2=0.973.(3)The percentage of preyweight showed that all the individuals under70mm were empty stomach, digestedstomach contents accounted for69.39%ranging the mantle length from71mm to80mm,the proportion of fish in stomach contents was not changed obviously(R>0.05, n=19)with mantle length while cephalopods significant(R<0.001, n=19).4. The degree of stomach fullness and its change.(1)The level of femalepredominantly stage of3and4, while the main level of male was2and3. One-wayANOVA test suggested that the degree of stomach fullness of female changed withmantle length significantly(P<0.01), however, there was unsignificant change of male(P>0.05).(2)The levels focus on stage2and3throughout the year,24.47%and25.71%respectively. The percentage of empty stomach of female was the highest in stageⅠ(22.59%), while the level in the stage Ⅱ-Ⅴwas arising gradually, the proportion of malewith an empty stomach decreased with gonad maturity. As a consequence, the levelsvariation of both male and female with the gonad development was found no obvious(P>0.05). 5. Feeding intensity and its change.(1)The mean repletion index of individuals under70mm was0, thus vacuity coefficient was the maximum value of100, the change of RIincreased obviously with the mantle length(F=12.84, P<0.05), conversely, VC decreasedwith mantle length significantly(F=11.48, P<0.05).(2)The RI of female were higher thanmales in each season, while VC were lower, however, the RI and VC of both male andfemale changed non-significant with seasonal variation(P>0.05).(3)The RI of femaleincreased with individual maturity, but VC did not change significantly with gonadmaturation(F=1.73, P>0.05), whereas RI of male increased with gonad maturation at thestage of Ⅰ-Ⅳ, but VC decreased(F=33.29,0.01<P<0.05).6. Carbon and nitrogen stable isotope results.(1)δ15N value ranged from7.100‰to12.429‰, and δ13C ranged from-22.334‰to-18.807‰.(2)The value of δ15N changedwith mantle length significantly(P<0.01), and showed an exponential growth relationship(Y=9.085e0.014ML, R2=0.982), and δ13C value also changed with mantle length are verysignificant(P<0.01). C/N ratio ranged from3.181to3.515during every mantle lengthgroup, the ratio decreased with the mantle length increase(F=18.76, P<0.01).(3)Defatting process: adout30samples of muscle was defatted, the Pared-samples T testshowed that both the δ15N value and δ13C value were significantly increased(P<0.05)before and after degreased.7. Trophic level and its change: Mimachlamys nobilis was used as the baseline tocalculate the Trophic level of Sthenoeuthis oualaniensis, the results ranged from2.37to3.94, and the average was3.29. While the trophic level and δ15N of Bigeye tunacombined with the δ15N of Sthenoeuthis oualaniensis, it showed an different but similarresults from2.41to3.98, the average Trophic level was3.33. The Trophic level ofSthenoeuthis oualaniensis was increased with its mantle length. |
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