The capacity for marine fishes to perform aerobically (aerobic scope) is predicted to control their thermal tolerance and, thus, the impact that rapid climate change will have on their populations. We tested the effect of increased water temperatures on the resting and maximum rates of oxygen consumption in five common coral reef fishes at Lizard Island on the northern Great Barrier Reef, Australia. All species exhibited a decline in aerobic capacity at elevated water temperatures (31, 32 or 33 1C) compared with controls (29 1C); however, the response was much stronger in two cardinalfishes, Ostorhinchus cyanosoma and O. doederleini, compared with three damselfishes, Dascyllus anuarus, Chromis atripectoralis and Acanthochromis polyacanthus. Aerobic scope of the two cardinalfishes was reduced by nearly half at 31 1C compared with 29 1C, and virtually all capacity for additional oxygen uptake was exhausted by 33 1C. In contrast, the three damselfishes retained over half their aerobic scope at 33 1C. Such differences in thermal tolerance between species, and possibly families, suggest that the community structure of reef fish assemblages might change significantly as ocean temperatures increase. Populations of thermally tolerant species are likely to persist at higher temperatures, but populations of thermally sensitive species could decline on low-latitude reefs if individual performance falls below levels needed to sustain viable populations.

Concerns about the impacts of ocean acidification on marine life have mostly focused on how reduced carbonate saturation affects calcifying organisms. Here, we show that levels of CO 2 -induced acidification that may be attained by 2100 could also have significant effects on marine organisms by reducing their aerobic capacity. The effects of temperature and acidification on oxygen consumption were tested in 2 species of coral reef fishes, Ostorhinchus doederleini and O. cyanosoma, from the Great Barrier Reef, Australia. The capacity for aerobic activity (aerobic scope) declined at temperatures above the summer average (29°C) and in CO 2 -acidified water (pH 7.8 and ~1000 ppm CO 2 ) compared to control water (pH 8.15). Aerobic scope declined by 36 and 32% for O. doederleini and O. cyanosoma at temperatures between 29 to 32°C, whereas it declined by 33 and 47% for O. doederleini and O. cyanosoma in acidified water compared to control water. Thus, the declines in aerobic scope in acidified water were similar to those caused by a 3°C increase in water temperature. Minimum aerobic scope values of ~200 mg O 2 kg -1 h -1 were attained for both species in acidified water at 32°C, compared with over 600 mg O 2 kg -1 h -1 in control water at 29°C. Mortality rate increased sharply at 33°C, indicating that this temperature is close to the lethal thermal limit for both species. Acidification further increased the mortality rate of O. doederleini, but not of O. cyanosoma. These results show that coral reef fishes are sensitive to both higher temperatures and increased levels of dissolved CO 2 , and that the aerobic performance of some reef fishes could be significantly reduced if climate change continues unabated.KEY WORDS: Global warming · Ocean acidification · Hypercapnia · Marine fish · Metabolism · Aerobic scope Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 388: [235][236][237][238][239][240][241][242] 2009 ture and elevated CO 2 levels interact to affect the performance of individuals (Rosa & Seibel 2008), and ultimately how this interaction affects their capacity to sustain viable populations.Atmospheric CO 2 concentrations have increased from an estimated 280 ppm around 1700 AD to over 380 ppm today, causing earth average surface temperature to increase by ~0.7°C (Brohan et al. 2006). Depending on future CO 2 emission scenarios, the average surface temperature is projected to increase by 1.1-6.4°C by 2100, with best estimates placing the range between 2.0 and 4.5°C (Meehl et al. 2007). Average sea surface temperature increases at a slower rate than average air temperature, but is still predicted to increase by up to 3°C in tropical and temperate seas within the next 100 yr (Poloczanska et al. 2007, Lough 2008, Munday et al. 2009). Although increases in average temperature may be greater in temperate regions than in the tropics, it is thought that tropical species might respond more strongly to climate change because they have evolved in a more thermally sta...