Ocean acidification is the name given to describe the process by which, in response to rising carbon dioxide (CO₂) emissions, more and more CO₂ is taken out of the atmosphere and dissolved into the surface waters of the world’s oceans, thus lowering their pH values. Such reductions are hypothesized by many scientists to harm marine life. Citing the review paper of Briffa et al. (2012), for example, Jutfelt and Hedgärde (2015) state that “a number of reports in recent years have suggested that the behavior of coral reef fish, including their activity level, boldness, behavioral asymmetry (lateralization), and responses to olfactory and auditory cues, may be affected by ocean acidification.” Such reports have promulgated widespread concern that ocean acidification may well become a major threat to marine fishes in the not-too-distant-future; and in light of this concern, the two Swedish scientists decided to conduct a study of their own to learn more about this important topic.

The subject of Jutfelt and Hedgärde’s analysis was Atlantic cod (Gadus morhua), which they describe as an “ecologically and economically important species that has a history of being exposed to overfishing (Rose, 2004) and cod populations may therefore be sensitive to the effects of additional stressors such as ocean acidification.” For their analysis, the two researchers reared juvenile Atlantic cod for 30 days in control water (~500 μatm) or water with elevated CO₂ levels (~1,000 μatm), during which time the juveniles were subjected to three separate behavioral experiments: (1) swimming activity, measured by the number of lines crossed per minute (12-19 days after exposure), (2) emergence from shelter, assessed by how long it took the fish to exit a shelter after a disturbance (26 days post exposure), and (3) lateralization, measuring turning side preference and the strength of behavioral symmetry (29-30 days post exposure). The purpose of the experiments was to determine whether or not these specific behaviors were affected by exposure to elevated CO₂, as they “were previously reported to be affected by CO₂ exposure in tropical reef fish.”

When all was said and done, however, Jutfelt and Hedgärde report they found no effect of CO₂ treatment on any of the behaviors tested, writing that “behavioral effects of CO₂ are not universal in teleosts” and that “the behavior of Atlantic cod could be resilient to the impacts of near-future levels of water CO₂.” Ruminating on why this may be the case, the authors note that Atlantic cod have been observed to forage in deep waters with low pH, and, therefore, may be “physiologically adapted to be tolerant to high environmental CO₂ levels.”

Whatever the reason, one thing is clear. As stated by the two researchers in the final sentence of their paper, “the results obtained in this study complicate the prediction of future effects of ocean acidification on fish, suggesting that behavioral effects could be negligible in some species and that we might not be able to make good predictions until more species from representative geographical and phylogenetic groups are tested and published.” Ocean acidification alarmists—take note!

References

Briffa, M., de la Haye, K. and Munday, P.L. 2012. High CO₂ and marine animal behaviour: potential mechanisms and ecological consequences. Marine Pollution Bulletin 64: 1519-1528.

Jutfelt, F. and Hedgärde, M. 2015. Juvenile Atlantic cod behavior appears robust to near-future CO₂ levels. Frontiers in Zoology 12:11, DOI 10.1186/s12983-015-0104-2.

Rose, G.A. 2004. Reconciling overfishing and climate change with stock dynamics of Atlantic cod (Gadus morhua) over 500 years. Canadian Journal of Fisheries and Aquatic Sciences 61: 1553-1557.