The Oreochromis niloticus, a warm-water fish, thrives in temperatures ranging from 28 to 32 ℃.However,low temperatures become a common challenge for this species during the colder seasons.This study aimed to identify O.niloticus strains that display enhanced cold tolerance.The O.niloticus Egyptian strain(NE strain), Egyptian Nile Resistance strain(NER strain), Xianghu strain(NX strain), "New Gift" strain(NNG strain), and Gift strain(NG strain) were selected for investigation.The study monitored changes in blood bio-chemical and enzymatic indices of the five O.niloticus strains when the water temperature naturally dropped from the optimal growth temperature of 28 ℃ to the critical death temperature of 11 ℃ to evaluate the impact of tem-perature change.The survival rates of each strain were continuously observed for one week once the water tem-perature reached 11 ℃.Additionally,Kaplan-Meier curves were used to compare the cold tolerance perform-ance of the five strains.The results demonstrated significant changes in serum biochemical indexes,activities of liver function-related enzymes, antioxidant enzymes, and other non-specific immune enzymes in all five strains of O.niloticus in response to chronic low-temperature stress.Among the strains,the NER strain exhibited the highest survival rate.Glucose levels significantly increased in the NX and NER strains,while total protein levels significantly decreased in the NER and NG strains.The albumin/globulin values did not change signifi-cantly or substantially across all O.niloticus strains.Superoxide dismutase,lysozyme, and lactate dehydrogenase levels significantly or highly significantly increased in all strains, with the NER strain showing the least increase in lactate dehydrogenase.Alanine aminotransferase and aspartate aminotransferase levels significantly increased in the NX,NNG, NE, and NG strains, while only aspartate aminotransferase showed a significant increase in the NER strain.These findings indicate that the five O.niloticus strains exhibit varying levels of resistance to low-temperature damage and can adapt to chronic natural cooling to some extent.However,the NER strain demon-strated the least liver function and muscle damage and the highest survival rate after the water temperature dropped to 11 ℃ for one week.Therefore,the NER strain exhibited the strongest cold resistance among the five strains of O.niloticus.This study provides valuable data for the cultivation of new germplasm of cold-tolerant tila-pia by assessing the changes in blood biochemical and antioxidant enzymes(or immune enzymes) of the five strains under natural cooling conditions and calculating the survival rate of each strain.
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