Predation in High CO 2 Waters: Prey Fish from High-Risk Environments are Less Susceptible to Ocean Acidification

Abstract

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.

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