| As a valuable economic marine fish that is mainly distributed in Bohai Sea and Yellow Sea,Cynoglossus semilaevis represents the Cynoglossidae of Pleuronectiformes and belongs to the demersal inshore fish in temperate water.The cultivation technique for Cynoglossus semilaevis have been started in all parts of Shandong since 2003.Later,it rapidly spread to coastal areas including Hebei,Tianjin,Liaoning,Jiangsu,Fujian and Zhejiang.Meanwhile,fishery workers around different places have adjusted measures to local conditions,conducted various experiments of cultivation mode,obtained multiple breeding experiences and promoted the development of cultivation industry for Cynoglossus semilaevis.However,there are big differences of growth rate between male and female Cynoglossus semilaevis.In a word,the growth rate of female individual is 2-4 times quicker that the male individual.With regard to the group,the proportion of male fish can usually reach 60%-70%.Due to the great differences between the growth of male and female fish and the slowly growth of male individual that has increased culturing cost,it has seriously hindered the mass production and promotion of Cynoglossus semilaevis industry.Thus,it can exert great significance to the seeding promotion of Cynoglossus semilaevis and development of cultivation industry by breeding new varieties with superior-quality,high-yield,strong resistance and high proportion of females.In order to screen fast-growing families,by taking the wild population of Cynoglossus semilaevis in Bohai Sea and artificial breeding group as the base population,the thesis sets up 78 Cynoglossus semilaevis family.After comparing the growth performance of fish cultivated in the same pool,the thesis reaches the conclusion.Namely,different family fishes show significant distinctions in growth speed.In the end,two rapid growth families(14L024 family and 14L032 family)and 5 faster growth families(14L06,14L021,14L035,14L046 and 14L080 family)have been selected.As for the rest 71 fish families,they represent ones with ordinary growth rate.In order to screen high proportion of female fish,according to the statistics toward each family for male and female proportion as well as survival rate of natural breed,the results show six families with high proportion of females(i.e.14L109,14L002,14L064,14L033,14L046 and 14L053).To be specific,the proportion of 14L109 female fish reaches 82.54%,the ratio of 28 family female fishes is between 50% and 65% and the proportion of rest 44 family female fishes is less than 50%.In addition,35 families possess high natural longevity,which is up to 94.25% in 14L075.Meanwhile,the natural longevity of 12 families including 14L075,14L006,14L003,14L080,14L021,14L018,14L074,14L017,14L052,14L028,14L037 and 14L060 reaches 80%.Besides,22 families are ordinary ones in natural longevity and 21 families are ones with low natural longevity.In order to screen strong disease resistance,20 families are selected to respectively infect V.anguillarum and Edwardsiella tarda,and the results show that there are 8 high disease-resistant or disease-resistant families(i.e.1 high disease-resistant V.anguillarum family,3 disease-resistant families,2 high disease-resistant Edwardsiella tarda families,3 disease-resistant families,1 double high disease-resistant family).The study finds that there are big differences of onseting time,settling time and duration time after Cynoglossus semilaevis is artificially infected by V.anguillarum and Edwardsiella tarda.Specifically,the onseting time is 34 h and 132 h respectively,settling time is 142 h and 312 h respectively and the duration time is 108 h and 180 h respectively.Meanwhile,their average survival rates are so close,which are 30.11% ± 13.95% and 31.08% ± 15.53% respectively.In addition,study shows that the survival rates was significantly positively correlated to the growth traits(body weight,length and body width)of each family fish-fry that were infected by V.anguillarum(P<0.01),in which the correlation coefficients are 0.584,0.565 and 0.576.However,there are no significant correlations(P> 0.05)between survival rates and growth traits of each family fish-fry that were infected by Edwardsiella tarda,in which the correlation coefficients are 0.400,0.279 and 0.355 respectively.The study lays a foundation for breeding Cynoglossus semilaevis with the fast growth rate,strong disease resistance and high proportion of female fish,and thus provide new techniques for cultivating high-yield and disease-resistant Cynoglossus semilaevis varieties. |
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