Publisher Summary Increased atmospheric concentrations of CO2 not only cause global warming but also raise the partial pressure of CO2 and lower pH of oceanic water. These environmental alterations are likely to affect shallow-water marine organisms. On the other hand, feasibility studies recently suggested that sequestration of anthropogenic CO2 in the deep ocean could help reduce atmospheric CO2 concentrations. However, implementation of this strategy could have significant environmental impacts on deep-sea life. Hence there is an urgent need for detailed studies on the effect of CO2 on marine organisms. Fish are a central component of the marine ecosystem, and constitute an important protein source, particularly for countries such as Japan. The majority of work concerning effects of hypercapnia on fish has focused on freshwater species and little is known for marine species. This chapter provides the basic information useful for assessing the impacts of ocean sequestration of CO2 on marine fish. It also studied acute CO2 tolerance during early life stages, long-term effects of sublethal hypercapnia, physiological responses of adults to hypercapnia, and compared the effects of hypercapnic water and acidic water. These experiments clarified tolerance levels of elevated CO2 and gave insights into compensatory mechanisms at different developmental stages.