Research interest in CO 2 -driven ocean acidification has been centered on certain groups of calcifying marine organisms, but knowledge on the possible impacts of ocean acidification on fish is limited. Our survey of the existing literature on the effects of increased pCO 2 on fish (total of 116 papers) revealed that few studies were conducted under pCO 2 conditions relevant to the future scenarios of ocean acidification. Information is nearly absent on reproduction, early development, and behaviour of marine fish. The short experimental durations of these studies preclude forecasting of how mortality and growth of marine fish would be affected by future increases in seawater CO 2 . Fish have been shown to maintain their oxygen consumption under elevated pCO 2 conditions, in contrast to declines seen in several marine invertebrates, in spite of possible additional energetic costs incurred by higher pCO 2 . Impacts of prolonged CO 2 exposure on reproduction, early development, growth, and behaviour of marine fish are important areas that need urgent investigation. There is also a need to rapidly advance research into possible acclimation of marine fish to high pCO 2 environments, endocrine responses to prolonged CO 2 exposure, and indirect influences through food availability and quality on fish growth, survival and reproduction. Useful guidance could be gained from the rich literature on the effects of freshwater acidification.

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[1] Fish are important members of both freshwater and marine ecosystems and constitute a major protein source in many countries. Thus potential reduction of fish resources by high-CO 2 conditions due to the diffusion of atmospheric CO 2 into the surface waters or direct CO 2 injection into the deep sea can be considered as another potential threat to the future world population. Fish, and other water-breathing animals, are more susceptible to a rise in environmental CO 2 than terrestrial animals because the difference in CO 2 partial pressure (PCO 2 ) of the body fluid of water-breathing animals and ambient medium is much smaller (only a few torr (1 torr = 0.1333 kPa = 1316 matm)) than in terrestrial animals (typically 30-40 torr). A survey of the literature revealed that hypercapnia acutely affects vital physiological functions such as respiration, circulation, and metabolism, and changes in these functions are likely to reduce growth rate and population size through reproduction failure and change the distribution pattern due to avoidance of high-CO 2 waters or reduced swimming activities. This paper reviews the acute and chronic effects of CO 2 on fish physiology and tries to clarify necessary areas of future research.

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