| Fish are ectotherms and their physiological activities are influenced by water temperature.In recent years,some weather disasters like cold waves have been frequently occurred,and the sudden drop in temperature has had a negative impact on the growth and immunity of farmed fish,which has caused major losses to our aquaculture industry.Rainbow trout(Oncorhynchus mykiss)as a popular cold-water farmed fish,it has growth optima at 16-18℃,and it is not tolerant to higher than 22°C,therefore,most studies on the temperature of rainbow trout are focused on heat stress,but few studies have been reported on low temperature.In this experiment,rainbow trout(mean body weight: 830.12 ± 16.23 g,mean body length: 35.29 ± 2.36 cm)were cultured for 8 weeks in a low temperature group(8 ± 0.5°C)and a moderate temperature group(17 ± 0.5°C)to investigate the effects of cold temperature on growth performance,nutritional composition,intestinal flora and immune levels of rainbow trout.The main findings of the study are as follows:1.Compared to the moderate temperature group,a significant reduction in feeding rate and weight gain rate were observed in rainbow trout at low temperature treatment(P <0.05),which led to inhibition of growth and a significant reduction in growth hormone receptor ghr1 transcript levels in muscle(P < 0.05).Low temperature markedly increased the moisture of rainbow trout muscle tissue(P < 0.05),but significantly reduced crude protein(P < 0.05),as well as total essential amino acids,total semi-essential amino acids and total amino acids(P < 0.05),and the hardness,gelatinousness and chewiness of muscle also showed significant reductions at low temperature(P < 0.05),but there was no significant change in fatty acid type and content.2.There was no significant effect of hypothermia on the mucosal muscle layer,lamina propria and folds of the rainbow trout intestine,but the height of the folds was significantly reduced(P < 0.05),and the number of goblet cells in the rainbow trout intestine was significantly increased in the low temperature group.Lipase and pepsin activities in the stomach of rainbow trout were significantly lower in the hypothermic group(P < 0.05),but there were no significant changes in the activities of various digestive enzymes in the intestine.The dominant species of intestinal flora in the low temperature treated group were Proteobacteria and Desulfomicrobium at the phylum level and genus level respectively,while the moderate temperature group was Bacteroidetes and Flavobacterium respectively,meanwhile,there was an increase in microbial species and abundance in the gut of rainbow trout at low temperatures group.3.Compared to the moderate temperature group,there was a significant increase in MDA and T-AOC activity in serum and liver of rainbow trout in the low temperature treated group(P < 0.05),but T-AOC and MDA showed a significant decrease in the muscle(P <0.05).In addition,the transcript levels of HSP70 were significantly reduced in the spleen(P < 0.05),and significantly reduced the transcript levels of spleen and kidney antioxidant enzyme-related genes(GPx and Trx R3b)through the Keap1/Nrf2 signalling pathway(P <0.05).AKP activity was significantly decreased(P < 0.05)and LZM activity was significantly increased(P < 0.05)in the liver of rainbow trout in the low temperature group.The expression of TNF-α and IRAK4,the related receptor genes in the Toll-like receptor signalling pathway,were significantly upregulated in the spleen(P < 0.05).In summary,the results of this study show that:(1)8°C inhibits growth and reduces nutritional quality in rainbow trout,(2)compared to the appropriate temperature(17°C),low temperatures reduce the height of the intestinal folds,inhibit digestive enzyme activity,affect the intestinal flora,and reduce the ability of rainbow trout to digest and absorb,(3)low temperature stress produces oxidative stress damage to the rainbow trout organism,along with an inflammatory response,but stimulates the enhancement of the immune level of the fish. |
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