Effect Of Different Feeding Strategies On Growth,feed Utilization And Liver Health Of Largemouth Bass Micropterus Salmoides

Largemouth bass(Micropterus salmoides)is an economically important freshwater fish in China.In 2020,its annual production has exceeded 500 thousand tons.However,the application of formulated feed in largemouth bass is frenquently associated with severe liver diseases in practice.In aquaculture,unscientific feeding strategy is always accomplied by feed wastage,and metabolic disorders and immune stress of fish.It is possible that feeding strategy could affect the liver health status of fish.Nowadays,it is still unclear that whether feeding strategy will affect the liver health of largemouth bass.In this thesis,the effects of feeding ratio and feeding frequency on the growth performance,liver antioxidation status and hepatocyte apoptosis of largemouth bass were explored.The main results of this study are shown as follows:1.Effect of feeding ratio on growth,feed utilization,and liver health of juvenile largemouth bassFeeding restriction(FR)is a scientific strategy to attenuate nutrient excess-mediated liver metabolic syndromes in human beings.However,it is still unclear whether liver health could benefit from FR in cultured animals including fish.In this regard,the regulatory role of FR in liver health of juvenile largemouth bass M.salmoides were investigated in this study using single-molecule real-time sequencing and next-generation sequencing.Groups of fish(20 fish per group)with an initial body weight of 6.9 g were randomly allocated into 12 rectangular tanks(250L each)operating as a recirculating aquaculture system.The satiation(ST)fish was fed to satiation,while ration levels of the moderately restricted(MR)and highly restricted(HR)fish were maintained at 85% and 70% of the ST fish,respectively.All fish were fed a commercial extruded diet containing 49.9% crude protein and 13.2% crude lipid.After a 9-week feeding trial,growth performance of the MR fish was not affected through improving the feed utilization as compared with the ST fish.Even though feed utilization was further improved,growth of the HR fish was still impaired.The MR fish obtained superior antioxidant status in the plasma and liver(based on glutathione or malondialdehyde concentration,and total antioxidant capacity and superoxide dismutase activities)together with better liver integrity(based on HE and TUNEL staining)than the other treatments.A total of 38,909 full-length transcripts were generated from the sequencing.GO enrichment analysis of the differentially expressed genes(DEGs)showed that many terms related to immunization and wound repair were enriched in the feed restricted fish(especially the MR fish).A total of 701 DEGs were identified in the liver of the MR fish as compared with the ST fish,and the apoptosis was enriched as the prominent pathway by KEGG pathway analysis.Our results suggested that the intrinsic apoptosis pathway played an important role in the hepatocyte apoptosis inhibition in the MR fish.Specifically,the activation of AKT might mediate an anti-apoptosis role through inhibiting the release of direct IAP binding protein with low p I(DIABLO)from mitochondria and promoting the transcription of pro-survival genes such as X-linked inhibitor-of-apoptosis protein(xiap),which coordinately activated the inhibitor of apoptosis protein(IAP).Besides the intrinsic apoptosis pathway,our results indicated that the lysosome pathway and endoplasmic reticulum stress pathway might also participate in regulating hepatocyte apoptosis inhibition of the MR fish.2.Effect of feeding frequency on growth,liver antioxidant status,liver function and hepatocyte apoptosis of juvenile largemouth bassGroups of juvenile largemouth bass(17 fish per group)with an initial body weight of 14.3 g were randomly allocated into 9 rectangular tanks(250 L each)operating as a recirculating aquaculture system.Fish were fed with different feeding frequencies(1 T/D,1 time/day;2 T/D,2times/day;3 T/D,3 times/day)for 8 weeks to apparent satiation.The results showed that feed efficiency of largemouth bass was not affected by feeding frequency.However,superior growth performance was obtained in largemouth bass fed 2 or 3 times/day over those fed 1 time/day.Histological examination showed that much clearer hepatocyte boundary was observed in the 2T/D group,while more sinusoidal congestions were observed in the 1 T/D and 3 T/D fish and hepatocyte damage and inflammation were more evident in the 3 T/D fish as compared with the other treatments.The 2 T/D fish obtained superior antioxidant status in the liver(based on glutathione content,total-superoxide dismutase activity and malondialdehyde content in the liver)and better liver function(based on aspartate aminotransferase activity,alanine aminotransferase activity and malondialdehyde concentration in the plasma)than other treatments.Hepatocyte apoptosis was detected in all treatments,but it was not affected by feeding frequency,and the expression of reprentative genes involved with apoptosis confirmed this phenomenon.The results of this study suggested that the growth performance,liver histological integrity,liver antioxidant status and liver function were superior in the 2 T/D fish over the 1 T/D and 3 T/D fish,thus it is recommended that the feeding frequency of 2 times/day is optimal for largemouth bass.