Background
The authors note that "fish are important members of both freshwater and marine ecosystems and constitute a major protein source in many countries."  Hence, they say the "potential reduction of fish resources by high-CO2 conditions due to the diffusion of atmospheric CO2 into the surface waters ... can be considered as another potential threat to the future world population."

What was done
In response to this concern, Ishimatsu et al. conducted a survey of the scientific literature with respect to the potential negative consequences of atmospheric CO2 enrichment on the health of fish that could arise from continued anthropogenic CO2 emissions.

What was learned
As in all good surveys of the potential threats of the ongoing rise in the air's CO2 content, the authors found a number of possible dire consequences.  Focusing on hypercapnia - a condition characterized by an excessive amount of CO2 in the blood that typically results in acidosis, a serious and sometimes fatal condition characterized in humans by headache, nausea and visual disturbances - they say their survey 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."

Although this conclusion sounds dire indeed, it represents an egregious flight of the imagination in terms of what could realistically be expected to happen anytime in earth's future.  Ishimatsu et al. report, for example, that "predicted future CO2 concentrations in the atmosphere are lower than the known lethal concentrations for fish," noting that "the expected peak value is about 1.4 torr [just under 1850 ppm] around the year 2300 according to Caldeira and Wickett (2003)."

So just how far below the lethal CO2 concentration for fish is 1.4 torr?  In the case of short-term exposures on the order of a few days, the authors cite a number of studies that yield median lethal concentrations ranging from 37 to 50 torr, which values are 26 and 36 times greater than the maximum CO2 concentration expected some 300 years from now!

In the case of long-term exposures the results are even better.  To cite just a few examples, Ishimatsu et al. report that Fivelstad et al. (1999) observed only 5 and 8% mortality at the end of 62 days of exposure to CO2 concentrations of 5 and 9 torr, respectively, for freshwater Atlantic salmon smolts, while mere 1 and 5% mortalities were found for seawater postsmolts of the same species at 12 and 20 torr after 43 days (Fivelstad et al., 1998).  In addition, they say that Smart et al. (1979) found little difference in mortality for freshwater rainbow trout reared for 275 days at 4 to 17 torr, and that no mortality occurred by the tenth week of exposure of juvenile spotted wolf fish to 20 torr (Foss et al., 2003).

Fish embryos and larvae, however, are often more vulnerable to environmental stresses than are adult fish.  Yet even here, the authors report that the 24-hour median lethal concentration of CO2 on both eggs and larvae of several marine fish studied by Kikkawa et al. (2003) "ranged widely from 10 torr to 70 torr among species," with the smaller of these two values being over seven times greater than the CO2 concentration expected 300 years from now.

But how about something short of death, such as growth reduction?  Ishimatsu et al.'s review reveals growth reductions of 24 to 48%; but, again, the CO2 concentrations needed to induce these growth reductions ranged from 17 to 20 torr, or 12 to 14 times more than the CO2 concentration expected 300 years from now.

What it means
Clearly, the scientific literature review of Ishimatsu et al. suggests that earth's fish, both freshwater and marine, will likely never experience any discomfort or ill effects from the direct consequences of the elevated atmospheric CO2 concentrations caused by human activities.

References
Caldeira, K. and Wickett, M.E.  2003.  Anthropogenic carbon and ocean pH.  Nature 425: 365.

Fivelstad, S., Haavik, H., Lovik, G. and Olsen, A.B.  1998.  Sublethal effects and safe levels of carbon dioxide in seawater for Atlantic salmon postsmolts (Salmo salar L.): Ion regulation and growth.  Aquaculture 160: 305-316.

Fivelstad, S., Olsen, A.B., Kloften, H., Ski, H. and Stefansson, S.  1999.  Effects of carbon dioxide on Atlantic salmon (Salmo salar L.) smolts at constant pH in bicarbonate rich freshwater.  Aquaculture 178: 171-187.

Foss, A., Rosnes, B.A. and Oiestad, V.  2003.  Graded environmental hypercapnia in juvenile spotted wolfish (Anarhichas minor Olafsen): Effects on growth, food conversion efficiency and nephrocalcinosis.  Aquaculture 220: 607-617.

Kikkawa, T., Ishimatsu, A. and Kita, J.  2003.  Acute CO2 tolerance during the early developmental stages of four marine teleosts.  Environmental Toxicology 18: 375-382.

Smart, G.R., Knox, D., Harrison, J.G., Ralph, J.A., Richards, R.H. and Cowey, C.B.  1979.  Nephrocalcinosis in rainbow trout Salmo gairdneri Richardson: The effect of exposure to elevated CO2 concentrations.  Journal of Fish Diseases 2: 279-289.