Fig. 1. Median survival times (±95 % CI) for 7 days postfertilization zebrafish exposed to various concentrations of MS222 in normoxic fresh water. The dashed line indicates the regression equation based on median survival times (ET₅₀) between 50 and 200 mgl^-1. The slope of this line was not significantly different from zero (P>0.05). Values in parentheses indicate the percentage of larvae ventilating their gills at the various MS222 concentrations. 0 % gill ventilation continues as indicated by the horizontal arrow. The vertical arrow indicates <50 % mortality.