Effects Of Different Weaning Strategies On The Survival Rate And Digestive System Structure And Function Of Larvae And Juvenile Chinese Sucker, Myxocyprinus Asiaticus

Chinese Sucker(Myxocyprinus asiaticus) is a rare and endemic in China, owing to the serious shortage of resources and the human impact has been listed as national level2 protected animal, with artificial breeding technology becoming more mature, also become an important economic species of freshwater fish. At present, despite technical advance in fishes larval rearing, most hatcheries still rely on live feed such as rotifers and brine shrimp for fish larvae in the early stage. However, live feeds exists some disadvantages such as high production cost, unstable of output and quality and carrying diseases etc. Therefore, it is desirable to wean fish larvae on to micro-diets at a certain early stage. The Chinese sucker belongs to no stomach type and have problems in weaning stage as the most marine fishes, which seriously influence the survival rate during the rearing of fishes larval. We study the growth and survival rate of M.asiaticus under different wean strategies to provide some reference for improving the survival rate of mullet larvae in the early stage and generating appropriate weaning strategies.And so to further improve the M.asiaticus breeding benefit.Two weaning strategies were adopted to feed the M.Asiaticus, which is from larvae(9.50±0.84) mg to 60 days post hatching(dph). The main contents are as follows:1.Three groups were divided depending on different weaning starting points, 15dph(W15), 20dph(W20) and 25dph(W25), respectively, and each groups were performed in triplicate.The results showed that:(1) The survival rates of all the three weaning groups were more than 80%. The W20 group had the highest survival rate(91.21±1.93%), but there was no significantdifference among them. The specific growth rate(SGR) of each group was decreasing.In 35-60 dph period that is all groups fed micro-diet until the end of the experiment, the SGR of W20 and W25 groups was significantly higher than W15 group. The difference between the two groups was not significant. The W25 group had the highest total length and body weight, which was significant higher than that of W20 group. The body weight of the W20 group and W25 group didn’t show significance with each other;(2) Different starting points of weaning have significant influence on the activity of digestive enzymes(P>0.05). The larvae Chinese sucker in 15 dph have detected higher amylase activity and the amylase activity in three group were with the extension of breeding cycle showed decline, then had a slightly increased then decreased trend in30 dph. There is no significant difference between the three groups in 40 dph(P>0.05).The trypsin activity of three groups correspondingly began to rise after weaning and turning from live feed to micro-diet. The peak appeared in 30 dph or 35 dph respectively,then began to decline. The lipase activity of this three groups are present the downward trend with the extension of breeding cycle. The lipase activity of W15 and W20 group was significantly higher than that in W25 group(P<0.05), and is elevated to a higher level; In 30 dph, three groups’ lipase activity decreased to the lowest, there was no significant difference with each other(P>0.05). The alkaline phosphatase activity of this three groups was showed higher before they fall and then rise in law, but due to different starting point of weaning, the value was different. The alkaline phosphatase activity of W15 and W20 group both in 20 dph elevated. The enzyme activity of W15 group decreased in 30 dph, and began to rise in 35 dph. And the enzyme activity of W20 group decreased in 35 dph, then began to rise in 40 dph. The enzyme activity of W25 group elevated in 30 dph, reduced in35 dph, and began to rise in 40 dph.(3) We found that weaning at different time points can effect the density of liver cells significantly(P<0.05). The liver density of W15 group was the highest,(5213.90±27.05)individual/mm2, significantly higher than the other two groups(P<0.05), the lowest group was W25 group for(3670.15±188.99) individual/mm2.Compare the liver density of the three groups,we could find that the later the smaller liver density the larvae Chinese sucker wean. And the long diameter and short diameter of liver cells were greater. The liver cells of each group neat rowed and the boundaries were visible, but there have been some different degree of pathological changes, such as lack of hepatic nuclear or nuclear dissolved. The W15 group of lack of nucleus of liver cells is one of the most.(4) Different starting point of weaning for the Chinese sucker larvae can effect the intestinal tissue morphological indexes significantly(P<0.05). According to the results of intestinal tissue section, in W15 group, the lumen diameter of intestinal is smaller,and the number and height of intestinal folds significantly less than the other groups in60dph(P<0.05).2. Begin to wean at 20 dph, we divided three groups depended on different weaning transition time, 5d(W20-1), 10d(W20-2) and 15d(W20-3) in W20 group,respectively. The results showed that:(1) At the end of the experiment, the survival rate of W20-2 and W20-3 groups was(95.73%±0.60%) and(91.21%±1.93%), respectively, there was no significant difference(P>0.05). The survival rates of this two groups were higher than this of W20-1 group. The SGR of each group was decreasing. In 35-60 dph period that was all groups fed micro-diet until to the end of the experiment, the SGR of W20-2 and W20-3groups was significantly higher than W20-1 group(P<0.05). The difference between the two groups was not significant(P>0.05). The length and body weight of W20-3 group were significantly higher than those of the other two groups(P<0.05).(2) The different transition time of weaning can effect the activity of digestive enzymes significantly(P<0.05). The amylase activity of Chinese sucker larvae in 3groups showed declining trend with the extension of breeding cycle, then slightly increased then decreased in 30 dph or 35 dph. The amylase activity of three groups correspondingly began to rise after weaning and turning from live feed to micro-diet,then decreased. The trypsin activity of 3 groups with the extension of breeding cycle presents the overall upward trend. The trypsin activity of W20-3 group was significantly higher than the other two groups in 25 and 35dph(P<0.05). In 40 dph, the W20-2group’s trypsin activity was significantly higher than the other two groups(P<0.05).The lipase activity of 3 groups were present the declines with the extension of breeding cycle,then decreased after rising trend. The W20-3 group’s lipase activity were significantly higher than that of W20-1and W20-2 groups in 25dph(P<0.05). During the period of 30 to 40 dph, the lipase activity of 3 groups were in a increased then decreased process. The alkaline phosphatase activity of these three groups were present a down after rising first and then upward with the extension of breeding cycle trend. The alkaline phosphatase activity of W20-3 group were significantly higher than other two groups all the time in addition to the 25 dph, and elevated to the maximum value in40 dph.The W20-1 group had a significantly higher alkaline phosphatase activity thanW20-2 group in 35dph(P<0.05), there was no significant difference between two groups at other time points(P>0.05).(3) Different transition time of weaning can effect the density of liver cells significantly(P<0.05). The liver density of W20-1 group was the highest,(5673.24±162.16) individual/mm2, significantly higher than the other two groups(P<0.05), the lowest group was W20-3 group for(2330.91±115.39) individual/mm2.Compare the liver density of the three groups,we could find that the longer of transition time the smaller liver density the larvae Chinese sucker wean. And the long diameter and short diameter of liver cells were greater. The liver cells of each group neat rowed and the boundaries were visible, but there have been some different degree of pathological changes, such as lack of hepatic nuclear or nuclear dissolved. The W20-1group of lack of nucleus of liver cells is one of the most.(4) Different transition time of weaning for the Chinese sucker larvae can effect the intestinal tissue morphological indexes significantly(P<0.05).This experiment mainly through the lumen diameter, height of intestinal fold and muscular thickness three indicators on the intestinal morphology for evaluation. According to the results of intestinal tissue section, in W20-1 group, the lumen diameter of intestinal is smaller, and the number and height of intestinal folds significantly less than the other groups in60dph(P<0.05).Aaccording to the above research results, some suggestions about the strategy of weaning are put forward: on the stage of weaning, the Chinese sucker larvae can start to wean at 20 dph and co-feeding with live feeds and micro-diets. Then gradually increase the quantity of micro-diets and reduce the amount of live feeds until completely fed for micro-diets. The transition time of weaning for the Chinese sucker is 10 d.