Effects Of Ocean Acidification On Fish Predator-Prey Interactions

Abstract

The paper presents an overview of the phenomenon of ocean acidification and the reasons for its occurrence before delving deeper and exploring the specific implication to fish predator prey relationships. To analyze these effects, several previous studies are cited and interpreted, and their findings are discussed so as to unveil the possible outcomes and final consequences of the phenomena studied. Finally, there is the discussion of possible solutions to these issues to try to diminish negative impacts and restore balance to the ecosystem.

Introduction

The process of ocean acidification has been a controversial issue ever since it started being discussed, since it warns of the danger of carbon emissions and therefore relates to furiously debated topics such as the effect of the ocean in human ecosystems and anthropogenic impact on the environment at large. It has long been relevant because of the important role the ocean plays, since it absorbs roughly 30 to 40% of all CO2 in the atmosphere. As a result, the uptake in fossil fuel burning and the impact of rising carbon dioxide on various biological relations become more significant. The substance’s presence in the air can greatly affect the ocean’s biome in several ways that weren’t predicted. The most prominent effect is the acidification caused by the production of carbonate, bicarbonate and free hydrogen ions.

After carbon dioxide from the air has been taken up by the ocean, the water and CO2 molecules react leading to H2CO3, or carbonic acid. That, in turn, disassociates into free hydrogen and bicarbonate ions, which prompt the water to become more acidic, lowering the pH. The bicarbonate ions can even disassociate into free hydrogen atoms and carbonate ions in some instances.

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The most commonly studied consequence of this acidification is the impact it has on corals, clams, oysters, mussels and sea urchins, who cannot build their external structures like calcareous skeletons and protective shells in too acidic environments. However, another negative effect that has not been explored so much is the influence of lower pH in fish predator prey interactions. In the few studies that have been made on this matter, there has been found a correlation between acidic waters and fish having more difficulty detecting predators due to various repercussions to fish’s organisms.

The aim of this study is to analyze how ocean acidification affects fish behavior to the point of causing an imbalance in the prey-predator relationship, which, in turn, may impact the overall well-being of the environment in which these fish are included.

Body

To first assess the changes to fish behavior in acid water, Munday et al. (2009) chose to experiment with different predator stimuli and orange clownfish. The research team raised four groups of orange clownfish: one in control (390 ppm CO2) seawater and the others in water with different levels of acidification by CO2. They were kept in these environments from when their eggs were laid up to 11 days after the larvae hatched. The scientists allowed the subjects to flow freely within a two-channel choice flume – a container with two defined sources providing differing substances into the water, there is a clear division between the two areas so that researchers can be clear that the fish intended to follow a specific stimulant. In this study, one channel provided water containing a common predator’s odor cues and the other provided water without that stimulus.

Going into the study, Munday et al. had the data and previous knowledge to confirm that clownfish larvae are born with the ability to detect predators by smell. With that piece of information (which was further proved by the experiment performed), they could contrast and compare statistics with the behavior of the clownfish who were bred in acidic water and then exposed to predator olfactory cues. What they found was that the fish which grew up in control seawater with normal levels of pH avoided the predatory signals at all times. Meanwhile, the fish raised in acidified water (with 550ppm, 700ppm and 850ppm of CO2) were more attracted to predatory cues as the pH levels of the water they were raised in lowered.

The researchers found that fish exposed to waters with a pH equal to or below that of 7.8 lost the ability to detect and the inclination to avoid predators, instead being more attracted to them. This outcome represents severe consequences to the species of clownfish and maybe others if this is a recurring trend, because of a possible fatal increase in mortality rates.

The larvae of numerous reef fish species are used to settling at night when there is little to no light to guide them, so they must direct themselves by smell. That, for example, explains why the olfactory sense is so important, especially during the critical settlement stage (when larvae are going into adulthood, are about to become fertile and produce offspring to further the species). Disturbing the functionality of this vital faculty could have catastrophic consequences.

Above all, life in an environment like a coral reef means interacting with many different species and having to differentiate between potential predators and other organisms of similar size or appearance. That ability is indispensable for survival and if it is taken away by acid water, it could also damage the continuity of the species.

In addition to discovering that certain fish act differently in water with lower pH levels, Munday et al. (2011) found that predator behavior also changes under acidic circumstances. After exposing both predators and prey water with high and control levels of carbon dioxide (one predatory reef fish and a group of eight small or large young damselfishes from four different species were left to interact for 24 hours to try and simulate a ‘coral reef setting’), the scientists observed that predation rates were higher under elevated levels of CO2 (an acidic environment) rather than under control conditions. As to which organisms were most affected, it was found that predators did not show a preference to one specific species of prey, since they consumed approximately each type equally.

However, when it comes to size, it was detected that smaller damselfish suffered from higher mortality because they were being sought after by predators more. Due to that piece of data, study authors suggest that maybe larger fish are less affected than smaller ones when it comes to predation in acidic environments. On the other hand, they propose that smaller species may compensate for this vulnerability with better speed and agility.

In regards to how much different levels of CO2 affect behavior and the replenishment of the population differently, Munday et al. (2010) analyzed the effect of predator cues on different larval groups raised in waters with the different predicted levels of carbon dioxide for the next hundred years. Different species’ larvae groups were exposed to CO2 levels of control (390 ppm) water, 550 ppm, 700 ppm or 850 ppm through the two channel choice flume mechanism.

The first two strongly avoided the predator cue at all times, leading scientists to believe that the 550 ppm level of carbon dioxide has no effect on fish’s responses to predators. Regarding the subjects kept at 700 ppm CO2 water, even though they initially avoided the predator signals, after four days of exposure they were already spending half of their time in the channel with the cues.

Even worse, 850 ppm CO2 larvae seemed to be completely unable to sense and then avoid predators from the start, since they were attracted to the predator smells from the beginning of the experiment. Overall, larvae exposed to more acid water were five to nine times more likely to die from predation than control fish, indicating that there is a positive relationship between CO2 concentration and mortality.

Besides the changes in how fish react to predatory signals, researchers also saw that larvae exposed to higher levels of CO2 became more active, exhibiting riskier behavior and swimming farther away from the reef. That attitude suggests a decrease in risk sensitivity, which is significant since it indicates that elevated CO2 levels can affect general physiological processes that interfere with fish larvae behavior. That development might represent a threat for the overall fish population in the future, especially since it is so vital that fish stay alive and near the coral throughout the transition from youth to adulthood, when they become fertile and reproduce.

Conclusion

The process of ocean acidification has consequences for the ecosystem far more complex than first expected. While it can deeply affect the external structures of organism like mussels, corals, oysters and clams, there are also significant impacts for dynamic beings like fish, which can then have an even grander effect on the environment.

Levels of dissolved CO2 in the range of 700–850 ppm can make it difficult for larvae brought up in those waters to respond to predator odors. The attraction to predators that adult fish and especially larvae feel after being brought up in carbon dioxide acidified water could result in increased mortality rates and a decline in overall population for several species beyond the clownfish if this proves to be a growing trend – which can deeply affect marine biodiversity.

According to the studies mentioned, larvae could exhibit a fatal attraction to predators at CO2 and pH levels that could occur in our oceans by 2100. This is important because transition between larval and juvenile life could be impaired, causing reduced replenishment of the population and impacting marine sustainability, which would then affect human ecosystems dependent on fish such as those in small villages.

Results show that even moderate increases in CO2 concentrations dissolved in the ocean can impact the sustainability of fish populations because of changed individual behavior and increased mortality at critical life-history transitions, such as youth to adulthood (the settlement stage right before organisms are mature and ready to reproduce) when it is vital that the fish stay alive for the continuation and evolutionary success of the species.

References

1. Dixson, D. L., Munday, P. L., & Jones, G. P. (2010). Ocean acidification disrupts the innate ability of fish to detect predator olfactory cues. Ecology letters, 13(1), 68-75.
2. Ferrari, M. C., McCormick, M. I., Munday, P. L., Meekan, M. G., Dixson, D. L., Lonnstedt, Ö., & Chivers, D. P. (2011). Putting prey and predator into the CO2 equation–qualitative and quantitative effects of ocean acidification on predator–prey interactions. Ecology letters, 14(11), 1143-1148.
3. Figure 2. Correlation between rising levels of atmospheric of carbon dioxide, rising levels of carbon dioxide in water and the lowering of ocean pH. Adapted from the Alaska Ocean Acidification Network, 2013. Online source: https://www.aoos.org/alaska-ocean-acidification-network/about-oa/what-is-ocean-acidification/
4. Figure 3. Duration of exposure to elevated CO2 required to cause olfactory impairment in (A) laboratory-reared settlement-stage clownfish and (B) wild-caught settlement-stage damselfish. Adapted from ‘Replenishment of fish populations is threatened by ocean acidification,’ by Munday, P. L., Dixson, D. L., McCormick, M. I., Meekan, M., Ferrari, M. C., & Chivers, D. P., 2010. Online source: http://www.pnas.org/content/107/29/12930
5. Introduction to Ocean Acidification. Central and Northern California Ocean Observing System. Retrieved from https://www.cencoos.org/learn/oa/intro
6. Munday, P. L., Dixson, D. L., McCormick, M. I., Meekan, M., Ferrari, M. C., & Chivers, D. P. (2010). Replenishment of fish populations is threatened by ocean acidification. Proceedings of the National Academy of Sciences, 107(29), 12930-12934.