Study On Large-scale Breeding Of Brachionus Plicatilis In Environment Under Low-salinity Condition

The large-scale cultivation of freshwater fish larvae and juvenile opening biological bait has always been one of the main problems that plagued the development of freshwater fisheries.At present,the fish fry mainly feed on soybean meal,egg yolk and artificial compound feed,which may cause problems such as low feeding rate,nutritional deficiency and low survival rate.Brachionus plicatilis,one of the common saltwater species,has the advantages of small size,fast reproduction,rich nutrition,strong adaptability and good palatability.Open bait has important economic value in aquaculture,but B.plicatilis has a short survival time in fresh water and the feeding rate of fry is low.Therefore,low-salt domestication and large-scale cultivation are of great significance for reducing production costs,increasing their survival time in water,and further improving the feeding rate and survival rate of freshwater larvae.Therefore,in this experiment,the low-salt domesticated B.plicatilis was used as the experimental object,and the related researches on the large-scale cultivation of low-salt domesticated B.plicatilis were carried out.The main contents and corresponding conclusions are as follows:1.To study the salinity 23 ppt and its low-salt acclimation to salinity B.plicatilis at different temperatures,different feed（Chlorella）densities and salinity 6 ppt.Life history characteristics under low-salt stress.the result shows:（1）At the feed density of 1.0×10⁴,1.0×10⁵,1.0×10⁶,1.0×10⁷and 1.0×10⁸cells/m L,with the increase of feed density,the reproductive stages of rotifers with salinity 23 ppt and salinity 6 ppt,the reproductive duration,average life span,spawning quantity,life expectancy at hatching（e₀）,net reproductive rate（R₀）,generation time（T）and intrinsic rate of population increase（r_m）all showed a significant increase at first and then a significant decrease（P<0.05）.The duration of juvenile B.plicatilis under salinity of 6 ppt under three food densities of 1.0×10⁶,1.0×10⁷and1.0×10⁸cells/m L was smaller than that under food densities of 1.0×10⁴and 1.0×10⁵cells/m L（p<0.05）,and the duration of juvenile B.plicatilis under salinity of 23 ppt under food densities of 1.0×10⁶and 1.0×10⁷cells/m L was smaller than that under food densities of 1.0×10⁴,1.0×10⁵and 1.0×10⁸cells/m L（p<0.05）.The R₀and r_mof B.plicatilis under salinity of 6 ppt were lower than those under salinity of 23 ppt in food density of 1.0×10⁴cells/m L.The e₀of B.plicatilis under salinity of 6 ppt was significantly higher than that of B.plicatilis under salinity of 23 ppt in food density of 1.0×10⁵cells/m L.When food density is above 1.0×10⁶cells/m L,the oviposition quantity,R₀and r_mof B.plicatilis under salinity of 6 ppt were significantly higher than those under salinity of 23 ppt.The research shows that the optimal density control range of salinity 6ppt rotifers is between 1×10⁶～1×10⁷cells/m L.（2）Under the temperature of 18℃,20℃,22℃,24℃,26℃,28℃,when the feed density is1×106cells/m L,the life expectancy and generation time of rotifers with salinity 23 ppt and salinity 6 ppt increase with temperature,the increase of temperature,the e₀,T,R₀,r_m,,average lifespan and spawning quantity all showed an overall increasing trend and had a significant impact（P<0.05）;e₀,average lifespan and T of rotifer salinity 6 ppt were the largest at 26℃,while R₀,intrinsic growth rate and spawning quantity were the largest at 28℃;e₀and average lifespan were not significantly different from those of salinity 23 ppt rotifers（P>0.05）.Except for 20℃,the net reproductive rate of salinity 6 ppt rotifers was significantly lower than that of salinity 23 ppt rotifers（P<0.05）.There was no significant difference in generation time between the two groups of salinity（P>0.05）;there was no significant difference in r_mof rotifers between the two groups of salinity between 18 and 20℃（P>0.05）,and the other temperature groups had significant differences（P>0.05）.there was no significant difference in the number of spawning quantity laid by rotifers between the two groups at 20 and 22℃（P>0.05）,while the number of spawning quantity laid by rotifers in the other temperature groups with salinity 23 ppt was significantly higher than that at salinity 6 ppt rotifers（P<0.05）.Studies have shown that the large-scale production of rotifers with salinity 6 ppt is the best in the temperature range of 26～28℃.（3）Life history characteristics of rotifers with salinity 6 ppt under low-salinity（salinity 5ppt,4 ppt,3 ppt,2 ppt,1 ppt）stress at a feed density of 1×10⁶cells/m L and a temperature of25±1°C,respectively.The results showed that the reproductive duration,average life span,spawning quantity,e₀,R₀,T and r_mof salinity 6 ppt rotifers under salinity 4 ppt and salinity 5 ppt environment were significantly higher than those under salinity 1 ppt,salinity 2 ppt and salinity3 ppt environment（P<0.05）.the reproductive duration,average life span,spawning quantity,e₀,R₀,T and r_mof rotifers under salinity 3 ppt environment were significantly higher than those under salinity 1 ppt and salinity 2 ppt environment（P<0.05）.Studies have shown that the rotifer reproduction effect is better between the salinity 4 ppt and 6 ppt in the low salinity variation range,and the salinity 6 ppt rotifer still has the ability to reproduce under the salinity 3 ppt environment.salinity 3 ppt provides data support.2.On the basis of the study on the life history characteristics of B.plicatilis under low-salt domestication,further research on the large-scale breeding of B.plicatilis was carried out in different low-salt（salinity 6 ppt and salinity 3 ppt）domestication.Through the collection of data related to two-level expanded culture and large-scale culture,the following results were obtained:（1）Through the first-level expansion,when the rotifers are initially cultured to a density of6/m L,the salinity is 3 ppt（1）,the salinity is 3 ppt（2）,the salinity is 6 ppt（1）,and the salinity is6 ppt（2）.)reached a high peak after 39 days,41 days,11 days and 14 days,respectively,and their mean densities were 92/m L,76/m L,99/m L and 80.7/m L,respectively.The study showed that the average density of salinity 6 ppt rotifers reached peak time about 18 days shorter than that of salinity 3 ppt rotifers.（2）Because the time for reaching the peak period of salinity 3 ppt rotifers is too long,the second-level expansion cannot meet the actual production needs,so only the second-level large-scale expansion of salinity 6 ppt rotifers is carried out.In the expansion culture,the microfluidic water was used to increase oxygen.The results showed that the average daily proliferation rate of rotifers was 16.15%,the peak time was 25 days,and the average density was68.2/m L.3.The nutritional value of B.plicatilis cultured with different low salts（salinity 6 ppt and salinity 3 ppt）was compared.The results showed that the content of basic nutrients in rotifers with salinity 6 ppt was higher than that in rotifers with salinity 3 ppt;except for manganese and potassium,the contents of other detected minerals in rotifers with salinity 6 ppt were significantly higher than those in rotifers with salinity 3 ppt（P<0.05）;The content of essential amino acids in rotifers with salinity 6 ppt was significantly higher than that in rotifers with salinity 3 ppt（P<0.05）;essential fatty acid linolenic acid was detected in rotifers at both salinity levels,but essential fatty acid linoleic acid was detected in rotifers with salinity 6 ppt acid.It can be seen that the nutritional value of salinity 6 ppt rotifers is more comprehensive than that of salinity 3 ppt rotifers.