Abstract：The high temperature is an important threat to aquaculture of rainbow trout (Oncorhynchus mykiss). In the present research, the effects of heat stress and recovery on non-specific immunity parameters were investigated in rainbow trout with average weight of 350 g under the laboratory conditions. Samples for detections of respiratory burst (RB), complement protein 3 (C3) content, superoxide dismutase (SOD) activity, malondialdehyde (MDA) content, cortisol (COR) content, catecholamines (CA) content, alkaline phosphatase (AKP) activity and lactate dehydrogenase (LDH) activity were taken before heat stress (18 ℃, 0 h), 2, 4, 8 and 12 h after stress (25 ℃) and heat stress recovery (18 ℃, heat stress recovery(HSR)), respectively. The results showed that compared with before stress, the stress group had the following significantly (P＜0.05) increased parameters: RB activity at 8 and 12 h; serum SOD activity at 2~12 h; MDA level at 4 and 12 h; C3 level at 2 and 12 h; AKP activity at 2, 8 and 12 h; the content of COR and CA at 8 h. The activity of LDH in the serum decreased significantly (P＜0.05) at 8 h after heat stress. Compared with the stress group, the HSR had the following variations: RB activity had no significant change (P＞0.05); MDA and LDH level were significantly lower than that of 4 and 12 h under heat stress (P＜0.05); C3 level was lower than that of 2 and 12 h of stress (P＜0.05); SOD activity was lower than that of 12 h under heat stress (P＜0.05); COR and CA level were lower than that of 8 h under heat stress (P＜0.05); AKP was at the same level as that of 2, 8 and 12 h under heat stress (P＞0.05). Compared with data before stress, the HSR had the following variations: RB, SOD, COR and AKP increased significantly (P＜0.05); C3, MDA, CA and LDH recovered to the level before stress (P＞0.05). These results indicated that 25 ℃ heat stress significantly affected the non-specific immunity of rainbow trout. The increase of RB and AKP indicated that heat stress could cause inflammation to rainbow trout. Most parameters decreased in the HSR group, especially, C3, MDA, CA and LDH reduced to the levels of pre-stress, which deduced that rainbow trout could restore normal function gradually under optimal temperature after 25 ℃ heat stress. However, the content of MDA significantly increased after heat stress and the high level in HSR group of AKP indicated that the fish cells were damaged. The death fish after only 2 h heat stress, showed that 25 ℃ approached to the upper limit of tolerance to temperature for rainbow trout. The above results provide basic data for resistance regulation to heat stress in rainbow trout and help producers taking good preventive measures based on better understanding about immune physiology and the adverse effects assess under heat stress.
