Early chronic hypoxia does not impact rainbow trout behaviour later in life

In trout aquaculture, tight control of water quality is crucial, especially for young life-history stages. This study examined whether chronic exposure to moderate hypoxia during the fry stage affects the rainbow trout's behavior during later development. One month after yolk-sac resorption, young trout were exposed for 25 days to hypoxia, obtained by decreasing water flow and bubbling nitrogen gas into the tank (Hypoxia group, H: mean O-2: 4.85 mg/L; min: 3.83, max: 6.87 mg/L). A control group was maintained in optimal conditions (C: mean O-2: 10.41 mg/L; min: 10.12, max: 10.69 mg/L). At 4 days post-hypoxia, basal (resting) whole body cortisol levels were significantly higher in the H group (166.84 +/- 20.67 ng/g) as compared to the C group (103.71 +/- 8.78 ng/g), but no post-stress differences were evident (values similar to 175 ng/g). Long-term effects of hypoxia on various behavioural traits were analysed between 3 and 4 months after the return to normoxia and compared to the C group. The exposure of fry to chronic hypoxia did not impact the fish's behavioural repertoire (activity, stereotypic behaviour, aggression) in their home tanks after re-acclimation to normoxia. Moreover, basal and post stress plasma cortisol levels, fear-related behaviours (novel-tank test), and risk-taking behaviours (emergence test) did not differ between the C and H groups. However, this early challenge resulted in an enhanced posthypoxic growth rate in these fish. Our results show that early exposure to moderate hypoxia does not induce any behavioural changes in rainbow trout after several months of return to normoxia.