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Fig. 6 Acclimation to chronic hypoxia reduced the aquatic surface respiration (ASR) response to acute hypoxia. Adult zebrafish previously acclimated to hypoxia for various timepoints (24 h, 48 h, 72 h, or 7 days) were exposed to successive bouts of progressively more severe acute hypoxia (100, 50, 30, and 15 mmHg). Data points for all groups were connected with a continuous line for clarity. Acclimation to hypoxia for 48 h, 72 h, and 7 days significantly reduced mean ± SEM cumulative time in ASR during acute hypoxic exposure (squares) at Po2 values below 30 mmHg, compared with unacclimated zebrafish (circles; Mann–Whitney U test; p < .01; N = 8 in control and acclimated groups for each Po2 and each period of acclimation).