Effect of low water temperature on the stress, innate immune, and antioxidant responses of pacu, Piaractus mesopotamicus, a sub-tropical fish

Abstract

The sustainability of aquaculture is subject to environmental changes, such as changes in temperature, which affect the physiological homeostasis of fish. Thermal shock (natural or anthropogenic) is a stress condition that results in a sequence of physiological responses and even mortality. Considering the impact of sudden reductions in temperature on the biological systems and that this occurrence is common in regions where pacu (Piaractus mesopotamicus) is farmed, we evaluated the stress, immune, and antioxidant responses of the species exposed to temperature drop. Two groups of fish were exposed to 28 °C or 16 °C for 24 h, and then fish were kept at 28 °C to recover and be sampled: before the temperature drop (baseline); after 24 h of cold exposure; and 3, 8, and 24 h after the return to 28 °C (recovery). The exposure of pacu to 16 °C did not alter the stress biomarkers after 24 h, but it activated the innate immune system (leukocyte respiratory activity, hemolytic activity of the complement system, and serum lysozyme concentration). It also promoted oxidative stress without altering most of the antioxidant system indicators, except for an initial reduction of glutathione peroxidase (GPx) activity after exposure to 16 °C and a late activation, besides the late activation of glutathione transferase (GST), probably as a compensatory process to reduce tissue damage. Pacu was sensitive to temperature changes, but it was able to activate immune and compensatory responses to mitigate oxidative stress. The results will help producers in the management of pacu during periods of thermal changes, such as in autumn and winter.