Predation in high CO2 waters: prey fish from high-risk environments are less susceptible to ocean acidification

Most studies investigating the effects of anthropogenic environmental stressors do so in conditions that are often optimal for their test subjects, ignoring natural stressors such as competition or predation. As such, the quantitative results from such studies may often underestimate the lethality of certain toxic compounds. A well-known example of this concept is illustrated by the marked increase in the lethality of pesticides when larval amphibians are concurrently exposed to the odor of potential predators. Here, we investigated the interaction between background levels of environmental predation risk (high vs. low) and ocean acidification (ambient vs. elevated CO2) in 2 × 2 design. Wild-caught juvenile damselfish, Pomacentrus amboinensis, were exposed in the laboratory to the different risk and CO2 conditions for 4 days and released onto coral reef patches. Using a well-established field assay, we monitored the in situ behavior and mortality of the damselfish for 2 days. We predicted that juvenile fish exposed to elevated CO2 and high-risk conditions would display more severe behavioral impairments and increased mortality compared to fish exposed to elevated CO2 maintained under low-risk conditions. As expected, elevated CO2 exposure led to impaired antipredator responses and increased mortality in low-risk fish compared to ambient CO2 controls. However, we failed to find an effect of elevated CO2 on the behavior and survival of the high-risk fish. We hypothesized that the results may stem from either a behavioral compensation or a physiological response to high risk. Our results provide insights into the interactive nature of environmental and natural stressors and advance our understanding of the predicted effect of ocean acidification on aquatic ecosystems