| Antarctic krill(Euphausia superba Dana)place an important role in Southern Ocean,mainly distribute in Southern of Atlantic and are second only to copepods.Lots of researchers pay high attention to the species for their high biomass,rich nutritional value and they are the main predators for other crustaceans.It is well-known having the basic biological information of the species,generally including the age structure and growth,feeding,the natural mortality rate and the growth rate,should be considered firstly to get further insight into the fishery management and assessment.The age determination of Antarctic krill is very difficult.Recently ageing Antarctic krill has made a breakthrough based on the eyestalk sections,however,the age determination of this species preserved on formalin is still unresolved.Our study mainly researches on the age of this species.Beside,there were few studies on the feeding of Antarctic krill,which was focused on by many researchers.Antarctic krill is an important food resources for penguins,seals,whales and fishes,which links primary producers and secondary consumers.Based on the age of this species,we analyzed the feeding of them using fatty acid analysis and stable isotopic analysis,in order to get further insight into Antarctic krill diets during austral autumn to early winter.In our study,the samples preserved on 5% formalin were collected by Kaili fishery vessel during May to June 2012 and 2015,including 67 males,21 females and 16 unknown.And the samples without any chemicals directly preserved on the refrigerator(-85?)were also researched in our study,which were collected by Longda,Longteng and Mingkai vessel during April to May 2015 and 2016,including 13 males,15 females and 2 unknown.Von Bertalanffy(VBGF)and General Linear Model(GLM)were applied to confirm the relationship between the total length and the growth bands and test the relativity between sexual stages and the growth bands,respectively.The age distribution of the total Antarctic krill and in the different survey areas were analyzed,respectively.Three parameters,Total Concordance(TC),Average Percentage Error(APE)and Coefficient of Variation(CV)were used to evaluate counting precision of different readers.SPSS was used to analyze the age difference of Antarctic krill preserved on different methods.Apart from the age structure,the lack of the researches on the diet of Antarctic krill was attached importance to focus by more and more researchers.This species is also an important food sources for penguins,seals whales,some kinds of marine mammals and seabirds,linking the primary producers to the secondary consumers.Fatty acid and stable isotopic analysis were used to research on the feeding of Antarctic krill,respectively,which the samples were collected by Longda,Longteng and Mingkai vessel during April to May 2015 and 2016 and by Kaili fishery vessel during May to June 2012 and April to May 2015,respectively.The conclusions are as follows,1)Age determination:(1)Total Concordance,Coefficient of Variation and Average Percentage Error are 64.43%,7.58%(<10%),5.30%,respectively.(2)Different survey areas have different dominant age groups.The samples are 3,4 and 5 years old off the South Shetland Islands and 4-year-old individuals are dominant and the percentage is 70.37%.3 and 5-year-old occupy 16.67%,12.96%,respectively.Compared to off the South Shetland Islands,the lift span of age groups in the Bransfield Strait is wider,including 2,3,4 and 5-year-old.However,3-year-old individuals occupied 51.43%,have advantage over 2,4,and 5-year-old individuals.Nevertheless,the number of 2 years samples is rare,which accounts for 10.67%.(3)The range of the total length is from 29.56~58.89 mm(46.73mm+6.83)and is significantly correlated with the number of growth bands.The VBGF function of Antarctic krill is as follows: Lt=64.57*(1-e-0.38*(t-0.16)).(4)The sexual stages of males have significantly related to the number of growth bands(n=80,P=9.51E-17<0.0001).Also females are slightly relevant to the number of growth bands(n=36,P=2.82E-06<0.0001)(5)The age structure of Antarctic krill preserved on different methods(frozen in the refrigerator directly,5% formalin)has no differences(P=0.238>0.05).2)Feeding of Antarctic krill based on fatty acid analysis:(1)The main fatty acids of the Antarctic krill are in the total of 26.C16:0,C14:0,C18:1n9c,C16:1n7,C20:5n3,C22:6n3 have high advantage over other fatty acids,accounting for 21.84 %+ 3.22,8.65%+ 2.16,10.18%+ 3.18,4.48% +1.49,18.60% +6.34,14.29%+ 4.13 mg·g-1,respectively.The contents of C20:5n3,C22:6n3 are after C16:0 that is the highest concentrations in 26 fatty acids.C20:1 and C22:1 account for 1.03% +0.33,0.95%+ 0.40,which indicates copepods are one of the food sources for Antarctic krill and some of Antarctic krill are carnivorous.The total length has negative relationships to C20:1,C22:1n9.However,there is significantly positive relationships between the total length and C22:6n3.It further illustrates krill switches to heterotrophic food sources such as copepods even though phytoplankton is rare in the autumn-winter.It's possible to feed some the low trophic heterotrophic flagellates.(2)?15+?17 fatty acids contains C17:0,C15:1n5,C17:1n7 in our study and the total contents of them account for 1.28%.Planktonic bacteria is also one kind of food sources for Antarctic krill.(3)The contents of C14:0,C16:0,C16:1n7,C18:1n9c,C20:5n3 and C22:6n3 are high,but the content trends are different.3)Feeding of Antarctic krill baed on stable isotope analysis:(1)The ?13C and ?15N values in the tissue of Antarctic krill ranged from-30.0‰ to-22.4‰(26.3‰+1.66),and 3.1‰ to 5.6‰(4.6‰+0.62)respectively.The ?13C and ?15N values of krill tissue from the two survey areas were not related to the standard length.It showed a steep increase in ?13C values of the individuals from April to May,but a reduction from May to June.There had no difference in ?15N values and it showed an increasing trend from April to June.Both ?13C and ?15N values of krill off the South Shetland Islands were larger than those of krill in the Bransfield Strait.The latest eyestalk sections method is composed of dissecting the eyestalks,immersing,embedding,polishing,viewing with Olympus compound microscope and counting the number of growth bands,which is the first study that analyzes the age of Antarctic krill preserved on formalin around the world.Then we will get the date and analyze them.Beside,compared to other traditional methods,eyestalk sections are further proved to be a more precise and direct method.It can provide the Antarctic krill management with scientific basis and be helpful to the acoustic measurement study of this species.Antarctic krill is the main food for other animals and occupies an important position in the South Ocean.Fatty acids can provide lots of information about the feeding,growth and reproduction.?13C,?15N values can be used to interpret the food sources and be the trophic indicators,respectively.The feeding of Antarctic krill were analyzed by fatty acids and stable isotopic analysis in our study.It can reduce the effects those were took by the traditional methods on the krill feeding.It's helpful to further study on the Antarctic krill diets,provide better evidence of material source of the ecosystem and get insight into the function of this species in the ecosystem. |
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