Fig. 2. (A-C) The effects of acute hypercarbia (indicated by the shaded areas) on (A) water P_CO2 (PW_CO2; same data as Fig. 1A), (B) ventilation amplitude (V_AMP) and (C) ventilation frequency (fG) in rainbow trout Oncorhynchus mykiss previously subjected to sham extirpation (controls; open circles; N=16) or bilateral extirpation (filled symbols; N=12) of the first gill arch. Values are means ± 1 S.E.M.; significant differences (P<0.05) from pre-hypercarbic values (time=0) are denoted by horizontal lines, whereas significant differences (P<0.05) between control and extirpated fish are indicated by daggers. (D) Relationships between water P_CO2 (PW_CO2) and V_AMP in intact (open circles; r²=0.97) and gill-extirpated (filled circles; r²=0.84) rainbow trout. The data represent mean values for 2 min intervals immediately prior to, and during, the 20 min period of hypercarbia. The non-linear regression (double rectangular four-parameter hyperbola) for the intact fish was generated using iterative curve-fitting software (Sigmaplot 2001; SPSS Inc.). 1 mmHg=0.133 kPa.