Effect of climate change on hematotoxicity/hepatoxicity oxidative stress, Oncorhynchus mykiss, under controlled conditions

warming [32]. Dissolved oxygen and water temperature are among the most important vital factors for aquatic organisms. The relationship between these two physical factors includes thermal effects on the amount of oxygen and changes in water viscosity that inhibit the exchange of breathing gases in the gill surface layers [33]. In this study, Hb and RBC decreased significantly due to the high acclimation temperature that may result from the damage of the hematopoietic system under stress conditions. Depending on the adaptability, different species react differently at increased temperature with the different hematological responses. Changes in hematological parameters have been reported to be a common phenomenon as an answer to thermal stress (may be low or high temperature) [34, 35]. In this study, the average value of Hb and RBC was significantly reduced, which is similar to previous findings in Thai pangas and Nile tilapia [36–38]. The size, number and hemoglobin concentration of erythrocytes may be affected by changes in ambient temperature due to stressful conditions and/or insufficient oxygen supply to hematopoietic tissues [39]. On the other hand, WBC content increased significantly at higher temperature. This situa- tion indicates the stress response of rainbow trout. As a healing response during stress expo- sure, high amounts of antibodies are produced and cause WBC content to increase [40–42]. Several studies are similar to our results, with some finding that the number of leukocytes in

pond fish carp increases due to high temperatures [43] Previous research on Nile tilapia showed that RBC and Hb levels decreased significantly, while WBC content increased signifi- cantly at high temperature [15, 44]. Temperature directly affects the physiological process and metabolic demand of fish [45, 46]. Although fish can cope with a series of temperature changes through physiological plastic- ity or evolution, serious illness and death occur when they exceed their heat tolerance [47, 48]. One of the organs involved in which the oxidative state can be affected by thermal stress is the liver [49]. Many studies have found that temperature stress can induce oxidative stress in fish and activate the enzyme-induced and non-enzyme-induced antioxidant defense system [50, 51]. To maximally prevent oxidative damage caused by the production of overactive ROS, the body can upregulate its antioxidant defense system as a survival strategy to eliminate excessive ROS production caused by temperature stress [52]. ROS, which are formed in the process of aerobic respiration, are beneficial for the cell at low concentrations and serve as important sig- naling molecules, being induced in the dysregulation of mitochondrial function due to drastic changes in temperature. If this induction does not occur after a certain reduction, oxidative damage accumulates and cellular function declines [53, 54]. Fish are able to stabilize their body temperature in response to environmental temperature changes or migrate to areas with optimum water temperature. In aquaculture, it is important

to know the effects of sub-optimal water temperatures during the transition periods from spring to summer and/or from fall to winter for fish species. A study reported that water tem- perature changes above optimum trigger thermal stress in fish and reduce feed intake and growth performance of fish. Thermal tolerance of fish as well as other aquatic organisms is a vital criterion in determining their resistance to temperature stress. While some studies have shown that thermal tolerance can be increased by microelements that increase oxygen holding capacity, the general view is that oxygen holding capacity does not increase thermal tolerance in fish. However, investigating how aquatic organisms adapt to the increasing water tempera- ture, how they survive and how they resist against temperature has been one of the most important issues that scientists have emphasized. The unexpected fluctuations in water tem- perature that accompany climate change also cause serious losses in natural populations [55]. The general implication of temperature acclimation studies is that differences in oxidative stress are emphasized at low temperature changes rather than high temperatures [56]. Increased SOD activity due to temperature increase may be an up-regulation of ROS in these PLOS ONE Effect of temperature on fish physiology and biochemistry PLOS ONE | https://doi.org/10.1371/journal.pone.0294656 November 30, 2023 8 / 13