Climate Change Makes Fish Act Strangely... or Does it?

Up until now, the research has been clear. Fish exposed to high levels of CO2 in the water behave really strangely: they appear to swim towards predators, rather than away from them.

A high level of CO2 in the water seems to affect how the fish detect chemical cues from predators or unfavourable habitats, making them more vulnerable to predation [2,3]. As climate change continues to increase levels of CO2 in the oceans and make them more acidic, an increase in predation could have devastating consequences for fish populations over the next century.

Or will it? In early 2020 a new study published in Nature, from Australian researcher Timothy Clark and colleagues, suggested that fish behaviour might not be as strongly affected by climate change as we'd previously thought [1].

The Research

The team tried to replicate previous studies, and concluded that earlier results must have simply been down to chance and random variation.  In this new study, which used a much larger sample size, the scientists reared multiple fish species and kept them in tanks with either 'normal' or 'high' CO2 levels, representing CO2 levels today and those predicted for the end of the century, respectively.

During the experiment, they placed the fish in a tank with two streams of water: one regular seawater, and the other with water that had previously contained a predator. They then measured how long each fish spent in either stream of water. Surprisingly, both the fish from the ’normal’ and the ‘high' CO2 levels spent most of their time in the regular seawater. This result was opposite to most existing research, which showed fish from high CO2 conditions spend much more time in predator-cue water, suggesting they can no longer tell when a predator is nearby.

So why the different outcomes?

The scientists stand by their results, as the lead author, Tim Clark says in a 'Behind the Paper' article published in Nature Ecology & Evolution [4]:"We knew our science was robust, and we had full confidence in our results. We also felt a responsibility to inform the global scientific community that we were unable to replicate dramatic effects published [in previous journals]. How is it possible that our results are so dramatically different from a decade of publications by another research group? We cannot provide a satisfactory answer to that question at the moment, but we hope that our research stimulates thoughtful discussion and reflection among interested readers in the scientific community."

Why is this study unusual?

It’s considered fairly uncommon for so-called ‘negative' results to be published in high-impact journals, especially a top-tier journal such as Nature. Replication studies tend to be quite costly for scientists, and are less likely to win them extra funding than if they’d performed more 'novel' research. However, this paper has come as a breath of fresh air for many, who see it as a step in the right direction towards making science more open and rigorous.

Why the Controversy?

A large proportion of the previously-existing research on fish behaviour and ocean acidification comes from the lab of Professor Philip Munday, at James Cook University. When asked by Times Higher Education [5] about the replication study from Dr. Clark’s team, he said: “You can hardly say you’ve repeated something if you’ve gone and done it in a different way."

Professor Munday expressed doubts about the team’s methodology, referring to differences in the species and life-stages of fish used in the experiments. He pointed out over 70 studies which had shown that elevated carbon dioxide levels could affect fish behaviour, saying: “There’s a lot of evidence, not just from my group. There’s been work on salmon, sharks [and] eels from different locations, by different groups, measured in different ways.”

Ask the Audience

When a new publication like this comes out, which challenges the existing view, it’s often useful to hear from scientists within the area of research. The Science Media Centre reached out to multiple experts in the field [6], to get their opinion on how sound the science was in this study. Some scientists supported the study, praising the good methods used (like videotaping the experiments), and the large sample sizes.  However, others expressed concerns about whether the blunt headline might deter further research into this area, or discourage future researchers from looking at other effects of ocean acidification on fish behaviour.

Prof Stephen Widdicombe, Director of Science at Plymouth Marine Laboratory said:

"I would urge caution against stating that all previous similar studies were flawed based on a single study. [...] a null result as reported here can only show that under these specific set of circumstances, no significant response was observed at this time. [...] I think this paper is an interesting addition to the field but does not invalidate the significant body of research that have shown different results."

Dr Phillip Williamson, Honorary Reader, University of East Anglia, said:

“The inconsistencies in the evidence overall in this area do not justify the unambiguity of the paper’s title, that ‘ocean acidification does not impair the behaviour of coral reef fishes’. Indeed, in some of the experiments in this study, there were significant changes in fish responses. [...] Whichever way these issues are eventually resolved – and the authors here do call for further replication – it would be over-simplistic and incorrect to conclude that ocean acidification has now been proved harmless for fish.”

(Responses edited for brevity and clarity.)

So What's the Answer?

We should never rely on just a single scientific study to give us the answer to a question, and this case is no exception. Due to the large amount of evidence suggesting ocean acidification does negatively affect fish, more research is clearly needed, paying close attention to the variables like age of the fish and where in the ocean it was brought from.

Personally, I’d like to see some research into the other potential side-effects of ocean acidification on fish. Does it affect breeding behaviour? Reproductive success? Overall life-span? These are all really important factors that might influence how well fish will adapt under a changing climate.

Finally, whether climate change still has fish 'in hot water' remains to be seen, but hopefully this study is accurate, and things might not end up so bad for them after all.

Sources for the Curious

1. Clark, T. D., Raby, G. D., Roche, D. G., Binning, S. A., Speers-Roesch, B., Jutfelt, F., & Sundin, J. (2020). Ocean acidification does not impair the behaviour of coral reef fishes. Nature. https://doi.org/10.1038/s41586-019-1903-y
2. Munday, P. L., Dixson, D. L., McCormick, M. I., Meekan, M., Ferrari, M. C. O., & Chivers, D. P. (2010). Proceedings of the National Academy of Sciences of the United States of America, 107 (29), 12930–12934. https://doi.org/10.1073/pnas.1004519107
3. Nilsson, G. E., Dixson, D. L., Domenici, P., McCormick, M. I., Sørensen, C., Watson, S. A., & Munday, P. L. (2012). Near-future carbon dioxide levels alter fish behaviour by interfering with neurotransmitter function. Nature Climate Change, 2(3), 201–204. https://doi.org/10.1038/nclimate1352
4. Behind the Paper: Is ocean acidification driving fish bonkers? (Nature Ecology & Evolution website)
5. Ex-Judge to Investigate Controversial Marine Research (Times Higher Education website) Retrieved 07May20