Experimental fish were of the same age (1 year) but originated from two different strains, a fast growing strain (FGS; mass¼290 g) and a slow growing strain (SGS; mass¼120 g). Comparison of heat and hypoxia tolerance in these strains showed that individuals from FGS displayed higher tolerance to reduced oxygen availability, whereas those issued from SGS were more tolerant to increased water temperature. For instance, Fig. 3 shows that the time required to cull 50% of the population (T50) during HCT was 260 min and 200 min for the FGS and SGS respectively, whereas during TCT, T50 values were 394 min and 264 min. Also worth noticing is the fact that the range of within strain variation in tolerance to hypoxia and hyperthermia was very similar to that observed at the species level i.e., CV¼7 and 7% (hypoxia) and 4 and 1% (temperature) for FGS and SGS respectively.