Physiological effects on fishes in a high‐CO₂ world

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 ( P CO 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 μatm)) 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.