How ocean acidification and ocean warming modify the physiology of coral reef fishes and the migrating temperate fishes

Future shock: Ocean acidification and seasonal water temperatures alter the physiology of competing temperate and coral reef fishes

Angus Mitchell, Chloe Hayes, David J. Booth, Ivan Nagelkerken

Summarized by Shruti Verma, an incoming undergrad student, interested in environmental science, biochemistry and computer programming. She wishes to become a researcher. 

Hypothesis: The purpose of this paper was to assess the relationship between changing marine environment and the physiology of coral reef fish and competing temperate fish, which are migrating into coral reef fish habitats.

What data were used? 60 coral reef fishes (A. vaigiensis)  and 180 temperate fishes (A. strigatus) were collected from a coast in Australia

Scientists measured:

  • Fish’s energy (total lipid content)
  • Fish’s feeding (stomach fullness)
  • Fish’s ability to deal with stress (Malondialdehyde concentration (MDA) and total antioxidant capacity(TAC))
  • Overall health (Fulton’s condition index)

Temperature and pH levels were monitored regularly inside tanks where fish were kept.


  1. Setting up the tanks: Scientists used transparent water tanks with small holes and bubbled pure carbon dioxide (CO2) to increase acidity. Water temperature was altered to mimic future summer and winter conditions. The temperature of  23°C, and pH 8.1 were selected as control conditions to reflect the current winter temperatures in the natural breeding range of coral reef fish populations.
  2. Initializing shoaling: Two groups were created – coral reef fish mixed (N= 60) with temperate fish (N= 60) of varying body sizes (mimicking actual reef conditions), temperate-only mixed with temperate fishes of similar body sizes (to decrease competitive advantage) (N= 120) After 40 days of treatment, fishes were euthanized, after being fed completely, to analyze stomach fullness. 
  3. Assessing water chemistry: Water’s total alkalinity was measured using Gran titration on the 24-25th day of the experiment.
  4. Measuring protein content, MDA and TAC: The TAC kits measured the total concentration of antioxidant macromolecules, antioxidant molecules, and enzymes in the fish’s white muscle tissue. TAC and MDA were then calculated using specific formulas.
  5. Fulton’s condition index: Individual fish were weighed and measured for length before and after the experiment, and their body condition was assessed using Fulton’s condition index, which was calculated from weight and length. Treatment effects on body condition were determined by subtracting the final Fulton’s condition index from the initial index.


Two horizontal panels. The top is 'Coral Reef Fish' and the bottom is 'Competing Temperate Fish'. There are three circles, the middle (yellow) is the current summer temperatures. To the right, there is a red circle indicating future summer conditions with a box to the lower left with the predicted shifting conditions. To the left, there is a blue circle indicating future winter conditions with a box in the lower right with the predicted shifting conditions.
This diagram shows how future water temperatures and ocean acidification could affect coral reef and temperate fish physiology. Arrows indicate significant changes in measured functions, with comparisons made between future winter/summer and current summer conditions. ‘*’ indicates higher response in mixed-species paired temperate fish, while ‘#’ denotes a significant increase in a given function under projected future conditions.

Key takeaways:

  • Lipid content in coral reef fish increased in colder winters (below 20°C) and exhibited decreased physiological performance, suggesting that they struggled to survive in future winter conditions.
  • In hotter summers (above 26°C), temperate fish experience more oxidative damage.

Coral reef fishes will benefit from ocean warming as their habitat range will increase, but they may struggle to survive during future winters. Simultaneously, competing temperate fishes will benefit from the presence of smaller coral reef fishes as temperate fishes will have a competitive advantage due to their greater body size. But this might not be the case when coral reef fish size increases in future summer. In future summer, temperate fishes will experience higher cell damage, and coral reef fishes will have the competitive advantage. Therefore the combined effect of decreased winter and increased summer temperatures on the competition between temperate and coral reef fish is not entirely clear yet. 

Why is this study important? Understanding the future consequences of migration altered physiology and altered shoaling interaction are crucial to determining the likelihood of survival of a fish species in the face of global warming.

Broader Implications beyond this study: It has been well documented that ocean acidification and ocean warming result in migration of fishes between different zones. However, more research needs to be conducted on the effects this behavior has on the shoaling interactions and the physiology of the migrating fish species. Even the smallest changes in environmental conditions can have great physiological impacts on certain fish species as outlined in this study.   

Citation: A. Mitchell, C. Hayes, D.J. Booth, et al., 2023. Future shock: Ocean acidification and seasonal water temperatures alter the physiology of competing temperate and coral reef fishes, Science of the Total Environment.