Food ration does not influence the effect of elevated CO2 on antipredator behaviour of a reef fish

The appropriate behavioural response to predation risk is critical to survival; however, behavioural responses can be subjected to trade-offs. For example, individuals may engage in riskier foraging behaviour to secure sufficient energy if resources are limited. Additionally, elevated CO2 can influence foraging and antipredator behaviour of marine organisms. Yet, how the availability of energetic resources may influence antipredator behaviour in an elevated CO2 environment is unknown. We tested the effects of food ration (low and high: 4 and 8% of body weight per day, respectively) on antipredator behaviour at ambient (489 µatm) and elevated (1022 µatm) CO2 in juvenile Amphiprion percula at 50 d post-hatching. Juveniles were from parents held at either ambient or elevated CO2, as parental exposure can influence phenotypic response in offspring. Antipredator behaviour was severely impaired by elevated CO2, with juveniles reared at elevated CO2 exhibiting no change in feeding rate in the presence of the predator cue compared with a >67% reduction in feeding rate in ambient CO2 fish. By contrast, food ration had a minor effect on the change in feeding rate in response to the predator cue, with only a 2.3% difference between high and low food ration fish. The effect of elevated CO2 on antipredator behaviour of juveniles was not influenced by food ration. Parental exposure to elevated CO2 influenced the baseline feeding rate and exhibited a small carry-over effect in elevated CO2 juveniles. These results suggest that reef fish could exhibit riskier behaviour at elevated CO2 levels, regardless of the energetic resources available