University of Guilan Aquatic Physiology and Biotechnology 2345-3966 13 3 2025 11 22 Effect of different levels of sub-lethal concentration (LC5096h) of heavy metal zinc on growth, survival and some hematological parameters in rainbow trout (Oncorhynchus mykiss) fingerling Effect of different levels of sub-lethal concentration (LC5096h) of heavy metal zinc on growth, survival and some hematological parameters in rainbow trout (<i>Oncorhynchus mykiss</i>) fingerling 59 79 9443 10.22124/japb.2025.29778.1562 FA Mehran Avakh Keysami Associate Professor in Aquatics and Fisheries Department, Mirzakochek Khan Fisheries Science Education Unit, Guilan Research Center, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran Maryam Avakh Keysami M.Sc. Student in Biophysics, Department of Biology, Kharazmi University, Karaj, Iran Afshar Zoughi Shalamani Assistant Professor in Aquatics and Fisheries Department, Mirzakochek Khan Fisheries Science Education Unit, Guilan Research Center, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran Mohammad Rahanandeh Assistant Professor in Aquatics and Fisheries Department, Mirzakochek Khan Fisheries Science Education Unit, Guilan Research Center, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran Journal Article 2025 02 16 This study was conducted to evaluate the effect of heavy metal zinc on growth, nutrition, survival rate and some blood indices of rainbow trout (<em>Oncorhynchus mykiss</em>) fingerlings. A total of 540 fish (20.19±2.45g) were divided into four treatments and three replicates (45 fish per replicate). Treatments 1 to 4 were reared with identical diets and water containing 0, 5, 25 and 50% of LC₅₀96h concentration (9.4mg/L) of zinc for 8 weeks, respectively. The results showed that adding zinc to water had a significant effect on reducing the survival rate of fingerlings (P<0.05). The number of red blood cells in the experimental treatments of 25 and 50% of zinc concentration decreased compared to the treatment of 0% (P>0.05). White blood cells in the experimental treatments of 25 and 50% zinc concentration showed a significant decrease compared to the 0% treatment (P<0.05). In the 5% treatment, red and white blood cells increased compared to the other treatments (P<0.05). Zinc had a positive effect on growth variables and blood indices in the 5% treatment. There was a significant difference between the 25 and 50% treatments in growth variables and the number of red blood cells, hematocrit, hemoglobin and red blood cell indices with the 0 and 5% treatments (P<0.05). In general, exposure of rainbow trout fingerlings to zinc at a level above 5% LC₅₀96h caused changes in the amount of blood indices, growth and survival rate of this fish. Therefore, monitoring water quality in terms of zinc pollution is essential for the economic and hygienic production of this fish. This study was conducted to evaluate the effect of heavy metal zinc on growth, nutrition, survival rate and some blood indices of rainbow trout (<em>Oncorhynchus mykiss</em>) fingerlings. A total of 540 fish (20.19±2.45g) were divided into four treatments and three replicates (45 fish per replicate). Treatments 1 to 4 were reared with identical diets and water containing 0, 5, 25 and 50% of LC₅₀96h concentration (9.4mg/L) of zinc for 8 weeks, respectively. The results showed that adding zinc to water had a significant effect on reducing the survival rate of fingerlings (P<0.05). The number of red blood cells in the experimental treatments of 25 and 50% of zinc concentration decreased compared to the treatment of 0% (P>0.05). White blood cells in the experimental treatments of 25 and 50% zinc concentration showed a significant decrease compared to the 0% treatment (P<0.05). In the 5% treatment, red and white blood cells increased compared to the other treatments (P<0.05). Zinc had a positive effect on growth variables and blood indices in the 5% treatment. There was a significant difference between the 25 and 50% treatments in growth variables and the number of red blood cells, hematocrit, hemoglobin and red blood cell indices with the 0 and 5% treatments (P<0.05). In general, exposure of rainbow trout fingerlings to zinc at a level above 5% LC₅₀96h caused changes in the amount of blood indices, growth and survival rate of this fish. Therefore, monitoring water quality in terms of zinc pollution is essential for the economic and hygienic production of this fish. https://japb.guilan.ac.ir/article_9443_04293219d6237d1a3df958733a037531.pdf