Acid overload: The far-reaching effects of ocean acidification on reef fishes

Written by Samantha Cavallaro

Edited by Cassandra Wilson

Introduction

It is no secret that oceans are becoming more acidic, which is a serious threat to the health and future of coral reefs. The extreme damage done to a coral’s calcium carbonate skeleton has been well studied over the last few decades. However, the impact of a lower pH goes farther than the strength of coral – it poses both direct and indirect risks to the health of fish populations by inhibiting their senses and ability to survive. How much longer can these crucial ecosystems survive with a threat coming from every angle?

Crippling damage to corals and their communities

As fossil fuel emissions increase the concentration of atmospheric carbon dioxide, it is absorbed by the ocean and reacts with water to form carbonic acid. This in turn lowers the pH and the amount of calcium carbonate used by corals to build a strong skeleton. Calcium carbonate is becoming so scarce that corals cannot calcify and are too weak to survive (Hoegh-Guldberg et al. 2007). Unfortunately, this damage has far greater effects than what first meets the eye. 

Studies have shown a positive correlation between the amount of live coral coverage and the population of reef fishes. This means that a decrease in the amount of coral will coincide with a decrease in both the number of fish and their diversity (Jones et al. 2004). Recent research suggests that the number of live corals may have a stronger impact than even fishing pressure does on fish abundance (Russ et al. 2020). With reef fish populations dependent on the health of coral reefs, and ocean acidification becoming increasingly severe, the future of these species may not be bright.

Bleached coral reef
Figure 1. Ocean acidification leaves corals so brittle that they crumble into a field of lifeless, broken branches. Source: https://thecommonwealth.org/ocean-acidification

You can swim but you cannot hide

One of the many reasons fish rely on coral reefs is for protection from predators. Fish are able to hide within branching corals as well as camouflage themselves against vibrant corals (Boström-Einarsson et al. 2018). However, with ocean acidification looming on top of temperature-related bleaching, these corals become so brittle that their branches break and they often die. When there is a lack of healthy coral cover to hide within, fish actually swim away from corals when confronted by a predator. Being out in the open then leaves them at a higher risk of exposure. The dead coral will instead provide more space for algae to flourish, which also contributes to the reef fish being deterred from sheltering inside (2018). With ocean acidification leading to less coral cover, and therefore more mortality from predation, this indirect effect on fish populations is a serious issue to consider for conserving both corals and fish. 

A screenshot of a video game

Description automatically generated with medium confidence
Figure 2. Even thriving corals cannot withstand severe ocean acidification. Degraded reefs provide little shelter for the fish who once relied on them to stay alive. 
Source: https://science.sciencemag.org/content/318/5857/1737/F5 

Compromised ability to hear and smell 

Not only does ocean acidification negatively affect the shelter used by fish to avoid predation, it also affects their hearing and smell. For example, in a study, researchers exposed fish to the high level of dissolved carbon dioxide and resulting low pH expected by 2100, and the fish’s ability to smell predators nearby was significantly impaired, so much so that they often swam towards a predator (Munday et al. 2010). Less shelter, plus the decrease in a fish’s ability to detect threats is a recipe for disaster. 

Ocean acidification has also been shown to hinder a fish’s ability to hear, which is key for young reef fish to find a suitable habitat to settle. The intensity of “reef noise,” generated by reef organisms, positively correlates with the health of a reef. Therefore, free-floating larvae will navigate towards a louder, and therefore healthier, reef over other, quieter ones (Piercy et al. 2014). However, a lower pH inhibits the ability of reef fish larvae to hear reef noise. This in turn decreases their ability to detect an appropriate reef to settle on and increases their chance of predation in the open water (Radford et al. 2021).

This is not a drill

With atmospheric carbon dioxide at the highest concentration seen in hundreds of millennia — and expected to continue to increase —  the impacts on the diversity and survival of coral reef communities are likely to be devastating (Hoegh-Guldberg et al. 2007). Because carbon dioxide concentrations are slated to increase throughout the century, fish will continue to lose their much-needed shelter and will therefore face higher rates of death — that is if their larvae can manage to locate a reef in the first place before being eaten by predators they could not detect. Without serious actions immediately taken to decrease carbon emissions, declines in healthy coral cover and reef fish populations may soon be facing the point of no return.

References 

Boström-Einarsson, L., Bonin, M.C., Munday, P.L., & Jones, G.P. Loss of live coral 

compromises predator-avoidance behavior in coral reef damselfish. Scientific Reports 8: 7795 (2018). https://doi.org/10.1038/s41598-018-26090-4
Hoegh-Guldberg, O., Mumby, P.J., Hooten, A.J., Steneck, R.S., Greenfield, P., Gomez, et al. Coral reefs under rapid climate change and ocean acidification. Science 318: 1737-1742 (2007). https://doi.org/10.1125/science.1152509 

Jones, G.P., McCormick, M.I., Srinivasan, M. & Eagle, J.V. Coral decline threatens fish 

biodiversity in marine reserves. PNAS 101: 8251-8253 (2004). https://doi.org/10.1073/pnas.0401277101

Munday, P.L., Dixson, D.L., McCormick, M.I., Meekan, M., Ferrari, M.C.O., & Chivers, D.P. 

Replenishment of fish populations is threatened by ocean acidification. PNAS 107: 12930-12934 (2010). https://doi.org/10.1073/pnas.1004519107

Piercy, J., Simpson, S.D., Codling, E.A., Smith, D. & Hill, A.J. Habitat quality affects sound 

production and likely distance of detection of coral reefs. Marine Ecology Progress Series 516: 35-47 (2014). https://doi.org/10.3354/meps10986 

Radford, C.A., Collins, S.P., Munday, P.L. & Parsons, D. Ocean acidification effects on fish 

hearing. The Royal Society (2021). https://doi.org/10.1098/rspb.2020.2754Russ, G.R., Rizzari, J.R., Abesami, R.A. & Alcala, A.C. Coral cover stronger driver of reef fish trophic biomass than fishing. Ecological Applications 31: e02224 (2021). https://doi.org/10.1002/eap.2224

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