CO2 ocean storage by which liquefied CO, is injected into the deep-sea to mitigate the climate change would increase the CO2 concentrations of the surrounding seawater. The biological impacts of such dynamic CO2 environments are, however, unknown. We examined the acute toxicity of temporally changing seawater CO2 concentrations on juveniles of Sillago japonica. Step-wise increases in ambient CO2 to fCO(2) (fractional CO2 concentration of the gas mixture bubbled into seawater) levels of 7% and 9% resulted in mortalities of 0.15 and 0.40-0.67 after 18 It, respectively. In contrast, one-step increases to these CO, levels killed all fish within 15 min. Further, a sudden drop of fCO(2) from 9-10% CO2 to normocapnia (0.038%) killed all the surviving fish within a few minutes. These results demonstrate that impacts of ocean CO2 storage need to be examined under conditions mimicking the dynamic changes in CO2, levels expected to occur by the CO2 injection procedure. (c) 2005 Elsevier Ltd. All rights reserved.