Background
With respect to earth's 30,000 species of teleost fish, which include virtually all of the world's important sport and commercial fishes, the authors write that several of them have previously been shown to be able to "fully compensate extra cellular fluid pH," as well as "maintain oxygen consumption rates and growth performance under ocean acidification conditions (e.g. Larsen et al., 1997; Foss et al., 2003; Fivelstad et al., 1998, 2003; Deigweiher et al., 2008)," but they note that there have been no studies of these phenomena that have lasted for more than a few days.

What was done
To rectify this situation, Melzner et al. maintained a group of Atlantic Cod (Gadus morhua) for four months in a re-circulating aquaculture system of 15 cubic meters volume at an atmospheric CO2 partial pressure of 0.3 kPa (~3,000 ppm) and another group for twelve months at a CO2 partial pressure of 0.6 kPa (~6,000 ppm), after which the fishes' swimming metabolism was investigated in a swim-tunnel respirometer, and tissue samples of their gills were taken for various chemical analyses, including gill Na⁺/K⁺-ATPase capacity, which serves "as a general indicator for ion regulatory effort."

What was learned
The six German scientists report that "motor activity in adult Atlantic Cod is not compromised by long-term exposure to water PCO2 levels of 0.3-0.6 kPa," which are "scenarios exceeding the 0.2 kPa value predicted for surface ocean waters around the year 2300 (Calderia and Wickett, 2003)."

What it means
In light of what they learned from their study, Melzner et al. conclude that "adults of active fish species with a high ion regulatory capacity [which is employed to eliminate metabolic CO2] are well equipped to cope with prospected scenarios of global climate change," even those far beyond what could likely be produced by the burning of all fossil fuels in the crust of the earth.

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

Deigweiher, K., Koschnick, N., Portner, H.O. and Lucassen, M. 2008. Acclimation of ion regulatory capacities in gills of marine fish under environmental hypercapnia. American Journal of Physiology-Regulatory Integrative and Comparative Physiology 295: R1660-R1670.

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., Asgard, T., Baeverfjord, G., Rasmussen, T., Vindhelm, T. and Stefansson, S. 2003. Long-term sublethal effects of carbon dioxide on Atlantic salmon smolts (Salmo salar L.): ion regulation, haematology, element composition, nephrocalcinosis and growth parameters. Aquaculture 215: 301-319.

Foss, A., Rosnes, B.A. and Oiestad, V. 2003. Graded environmental hypercapnia in juvenile spotted wolfish (Anarhichas minor Olafson): effects on growth, food conversion efficiency and nephrocalcinosis. Aquaculture 220: 607-612.

Larsen, B.K., Portner, H.O. and Jensen, F.B. 1997. Extra- and intracellular acid-base balance and ionic regulation in cod (Gadus morhua) during combined and isolated exposures to hypercapnia and copper. Marine Biology 128: 337-346.