Evaluation of global warming effects on juvenile rainbow trout: focus on immunohistochemistry and osmoregulation

The negative effects of global warming also directly affect aquatic populations. Consequences such as evaporation due to chronic temperature increase, increase in salinity, and increase in stock density per unit volume are potential stress factors. While creating the trial design, an attempt was made to simulate the effects of global warming, especially on species living in salty and brackish water biotopes. In this study, changes in the gills of rainbow trout (Oncorhynchus mykiss) acclimated to 0, 20, and 38 ‰ of saline in the laboratory were examined histologically and immunohistochemically and blood serum osmolarity. In addition, the water temperature was changed, and experiments were carried out at 16, 19, and 22 °C for each salinity group in parallel with the increase in salinity. However, to simulate the decrease in water volume and intensive stocking due to the potential impact of climate change, the study was carried out using 15 fishes in low-volume aquariums (45 L). Tap water that had been kept for at least 3 days was used in the aquariums. To protect the water quality, independent aquariums with sponge filters were used, and since the aim was to keep dissolved oxygen low, no ventilation system other than the sponge filter was used. In order to minimize the deterioration in water quality during the trial, a 15% water change was performed by performing a bottom flush every 4 days and water of the same temperature and salinity was added as much as the reduced volume. In addition, since increasing stock density due to temperature increase and water decrease will cause the amount of dissolved oxygen to decrease, pure oxygen was not entered into any tank throughout the experiment, and the concentration was requested to be at a low level (7 ± 0.13 mg/L) in all groups. The trials were terminated at the end of the 71st day. Increased serum osmolarity values were observed due to the increase in salinity, and the highest serum osmolarity value was measured at 644 mOsm/kg in the 38 ‰ salinity group. Differences between the groups were found to be statistically significant (p < 0.05). It was observed that the number of cells containing Na⁺/K⁺-ATPase increased depending on salinity. Also, the number of chloride cells reached the maximum level in the 38 ‰ salinity group. Due to increasing salt levels, an increase in mucus cells, limited onset hyperplasia, aneurysm, lamellar separation, and necrosis were observed in the gill tissue.