Fish might become “fearless” as a result of ocean acidification

The rising level of atmospheric CO2 has led to several studies focussing on changes in terrestrial and water ecosystems that might occur as a consequence. Ocean acidification, which is caused mainly by the increasing amount of CO2 dissolved in sea water, is at the forefront of marine research topics. However, most of the studies focus on calcifying organisms (e.g. corals, crustaceans) and only a few looked at fish and changes in fish behaviour so far.

A recently published paper in Functional Ecology, a British Ecological Society journal, is aiming to fill in a knowledge gap by examining the links between ocean acidification and changes to coral reef fish behaviour based on visual effects. Behavioural studies have demonstrated already that elevated CO2 levels can cause chemosensory and auditory impairment.

Fish use various information to avoid predators. Coral reef fish use auditory and chemosensory cues to decide where to settle. Once settled, chemical and visual information become more important. The study by Ferrari et al. tested the changes in visual risk assessment by exposing juvenile damselfish to the sight of a predator, a spiny chromis. The chromis was put into a watertight bag to avoid the damselfish receiving chemosensory signals.

The researchers found that damselfish showed reduced antipredator behaviour when exposed to 850 µatm CO2 which is equivalent to the CO2 level projected by the end of this century. However, no difference in behaviour was detected for smaller amounts of CO2 (440, 550, 700 µatm) in this study; previous studies showed altered chemosensory and auditory responses at 700 and 600 µatm, respectively. This led to an interesting conclusion that visual risk assessment cannot entirely compensate for the loss of chemosensory or auditory assessment.

Current projections indicate that atmospheric CO2 will reach 500 µatm by mid-century and 850 µatm by the end of this century. This study shows therefore that the latter concentration can alter the behaviour of coral reef fish and may eventually change the dynamics of predator-prey interactions and biodiversity of coral reefs.

The researchers highlight the need for longer term experiments to answer questions about adaptation and selection. However, they proved an extra point on why we should act immediately on reducing CO2 emissions.

Ferrari, M.C.O., McCormick, M.I., Munday, P.L., Meekan, M.G., Dixson, D.L., Lonnstedt, O. & Chivers, D.P. 2012 Effects of ocean acidification on visual risk assessment of coral reef fishes. Functional Ecology. DOI: 10.1111/j.1365-2435.2011.01951.x

The original Open Access paper can be downloaded here.