The effect of a sudden temperature decrease on selected physiological indices in the common carp

The objective of the study was to evaluate stress invoked by a temperature decrease in the common carp (n = 56). After acclimatisation to laboratory conditions at a temperature of 27.8 ± 1.0 °C, the fish (n = 28) were transferred to 16.8 ± 1.0 °C water (group T2; temperature difference Δ -11 °C); the rest of the fish were kept at the original water temperature (group T1). The examination of individual fish was made after 6, 12, 24 and 48 hours (h). The impact on the haematological and biochemical indices was evaluated. Furthermore, the effect on the food content passage speed was investigated and a histopathological examination was performed. The changed haematological indices in the T2 group returned to the T1 values 48 h after a temperature decrease, except for the neutrophil-lymphocyte ratio which was found to be higher in all the T2 samplings. Most of the monitored biochemical indices decreased in the T2 group and some of them remained that way at 48 hours. On the contrary, the liver enzymes increased in the T2 group at 48 hours. The passage of food through the digestive tract was mostly finished after 48 h for both groups. However, in the warmer water, the intestine was empty in 71.4% cases after 24 h already, while in the T2 group, the food content passage was only completed in 14.3% of the fish at the same time. Local gill and skin necrosis, and mononuclear cell infiltration were found in the T2 group. A sudden change in the temperature can affect various parameters of the common carp, however, these changes can mostly be eliminated within 48 hours.