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expert reaction to study looking at ocean acidification and coral reef fish behaviour

Research, published in Nature, reports that ocean acidification due to higher carbon dioxide levels may have very little impact on coral reef fish. 


Prof Stephen Widdicombe, Director of Science at Plymouth Marine Laboratory and Work Package Leader for the UK’s Ocean Acidification Research Programme (UKOA), said:

“From our extensive experience of developing ocean acidification experiments on other marine organisms, it has shown that different effects on similar species is not uncommon and I would urge caution against stating that all previous similar studies were flawed based on a single study.  There are numerous reasons why similar experiments on similar species can produce different results, with each study being valid based on the specific set of circumstances under which it is conducted.

“One of the biggest drivers of different results between similar studies is the variability you see between different populations from different locations, particularly in their levels of local adaptation to a particular stressor1.  For example, fish from environments where short periods of high CO2 conditions might be common may have adapted short-term resilience to such fluctuations, but it is often unsustainable due to the long-term energetic cost.  There are a number of examples of experiments on the same species showing different results because the tested individuals were collected from different places or times.  Without information on water conditions at the locations (wild or culture facilities) from which the tested individuals were collected, it is difficult to compare their results to the previous studies.

“Additionally, a null result as reported here can only show that under these specific set of circumstances, no significant response was observed at this time.  However, if an experiment shows a positive response then we can say that there is the possibility for something to happen (e.g. impact on fish senses); maybe not every time or in every set of circumstances but nevertheless, it can happen.  Therefore a single “no effect” experiment does not invalidate the other experiments.

“And finally, experimental design and data analysis can also lead to different results and this is not unique to ocean acidification research but is, in fact, a trait of experimental research on individual species populations.  This highlights the importance of replication and the value of meta-analysis.

“For these reasons, 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.  The new work simply identifies the need to better understand the context of the experiments being conducted and that we should seek to identify the specific differences between the studies that could have led to these different results, to help increase the knowledge base of the current and potential impacts of environmental change across the world.”

1 ‘Species-specific responses to ocean acidification should account for local adaptation and adaptive plasticity’, Nature Ecology & Evolution, Cristian A. Vargas et al, 0084 (2017), DOI: 10.1038/s41559-017-0084


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

“There’s something fishy going on somewhere but we can’t be sure where yet.  Experiments by Timothy Clark and colleagues using high CO2 levels were unable to replicate the scale of fish behavioural changes observed in many other studies by several research groups.

“The new study is well-described and apparently thorough in its attempt to repeat previous studies.  Yet without direct collaboration with the other groups, subtle yet important differences in protocols and design may have occurred.  Biological variability occurs over a range of scales, and behaviour is an especially dynamic parameter.

“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.  The authors also consider the results of previous work to have a probability of 0 out of 10,000 – that is either a typographical error or statistically meaningless.

“The authors of the new study emphasise that they videoed their experiments while others didn’t.  There may have been errors in previous research, but not necessarily.  Other expert responses by those directly involved should hopefully help explain the discrepancies.

“Until then, ‘low confidence’ would seem an appropriate way to summarise the evidence on the sensitivity of coral reef fish to ocean acidification.  That happens to match the last detailed assessment by the Intergovernmental Panel on Climate Change in 2014, which said “confidence levels [for impacts of ocean acidification] for fishes were converted from medium to low in light of uncertainty on the longterm persistence of behavioural disturbances.”  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.”


Prof James Crabbe, Fellow, Institute of Marine Engineering, Science & Technology, said:

“Because fish have such a good pH control system it was considered surprising by a number of coral reef specialists that previous studies had found ocean acidification to alter fish behaviour so much.  This study has looked at many species and uses transparent methodology and raises the important issue of reproducibility of scientific results.  Ocean acidification is a relatively new topic of scientific research and it’s clear further studies will be required to truly understand the impact of ocean acidification.

“However, despite what appears to be relatively good news for coral reef fish behaviour the study doesn’t definitively proof that some species in some environments won’t suffer in some way.  Ocean acidification is only one aspect of climate change that could influence fish behaviour.  A greater one is temperature.  Unlike corals, fish can move, and are likely to move with changes in sea surface temperature and will have to adapt to new environments which may not offer the same feeding or safety opportunities they had before.”


Prof Alex Ford, Professor of Biology, University of Portsmouth, said:

“This study highlights an important element of reproducibility in behavioural studies and science studies in general.  The effects of environmental factors such as various pollutants on wildlife behaviour is currently a hot topic.  Whilst it is becoming apparent that human pressures on the environment can alter wildlife behaviour, also becoming evident is the need for more carefully planned experimental designs, statistical analyses and knowing the underlying baseline behaviours of the study organisms.  Without such it is possible to researchers to come to wrong conclusions.

“This is good quality research.  The press release accurately reflects the science and does well to highlight that ocean acidification is still likely to have effects on wildlife, just not in the study scenario conducted.

“This work disputes much of the existing evidence which has shown effects on coral reef fish of ocean acidification at the turn of the century levels.  The implications for the real word are positive in the sense that fish (some fish at least) are unlikely to suffer the effects predicted from the acidification of the oceans caused by raised CO2.  The authors have been careful not to suggest that ocean acidification won’t have effects at the predicted turn of the century levels on other wildlife, but just that the existing studies on the effects of ocean acidification on fish behaviour can’t be backed up by their very thorough experiments.”


Dr Bev Mackenzie, Chartered Marine Scientist, Fellow of the Marine Biological Association, and Fellow and Policy Director of the Institute of Marine Engineering, Science & Technology, said:

“The paper looks at a very specific aspect of ocean acidification which is fish behaviour, and whilst the methodology and results may be sound it fails to account for the much wider potential consequences of ocean acidification on the reef such as habitat degradation and impacts on other species at lower trophic levels – which in turn may impact the behaviour of the fish more than the acidification itself.

“Whilst the study appears to present a glimmer of hope for coral reef fish it is important that any positive news from this study doesn’t have the unintended consequence of distracting from efforts to reduce the amount of greenhouse gas entering the atmosphere and ocean or to undermine efforts to protect the world’s coral reefs.

“Independent to the potential impacts of ocean acidification on fish behaviour coral reefs remain under threat from a multiple stressors over different timescales, from warmer waters and bleaching through to more direct impacts such as over fishing and pollution.  There are no excuses for not affording the greatest of protection to these significant ecosystems.”


Prof Kevin Flynn, Professor of Marine Biology, Swansea University, said:

“The paper seems sound and the analysis looks robust.  It’s also good to see a high profile journal publishing what could be interpreted as a null result, recognising the importance of replication of results, rather than simply emphasising negative impacts of climate change.

“However, while the results from this work may be interpreted in a good light, it must not be read as indicating in any way that ocean acidification (which is an insidious consequence of CO2-induced climate change) is not a problem for these fish or for the ecosystems they live in.  We know from our own work, and from others, that ocean acidification affects various members of the food chain, from the microscopic plankton on upwards, that ultimately support growth through to fish.  Damage to such organisms supporting growth of all stages of the life of these fish will result in ocean acidification indirectly affecting them even if there is no clear direct link to behaviour as this paper reports.

“This paper serves to remind scientists, publishers and the public of the importance of replication, and of the complexities of climate change research.”


Dr Michael Steinke, Senior Lecturer in Marine Sciences, University of Essex, said:

“There is no doubt that climate warming from rising CO2 levels will adversely affect many of our planet’s living creatures but scientists still debate about the impact of ‘the other CO2 problem’: ocean acidification.

“Clark and his colleagues present a compelling and exceptionally thorough study that concludes that some behaviours of tropical reef fishes are not impaired by high CO2 concentrations expected for the turn of the next century.  This study fits with the general view that organisms from coastal ecosystems will show relatively little response to ocean acidification in comparison to the potentially more vulnerable open ocean environments where natural daily and seasonal CO2 fluctuation is typically much smaller.

“Since the process of ocean acidification has only begun and will not stop with the year 2100 (a short 80 years from today), apart from the well-documented deleterious warming, it is still uncertain how marine ecosystems may respond to the excessive addition of CO2 from human activities in the long-term.”


Prof John Sumpter, Professor in Ecotoxicology, Brunel University, said:

“This is an excellent paper in all respects.  It is very thorough and very comprehensive.  It is also very balanced.  It is by far the best environmental science paper I have read for a long time.

“It contradicts a series of papers that reported truly sensational – and very worrying – results about coral reef fishes.  Those results were published in very respected, influential journals and should have been questioned at the time.

“It’s important to note however that this study does not mean ocean acidification is something we can forget about, nor does it mean that rising CO2 levels aren’t having an impact on our oceans.  My only criticism of the paper is that if it was my paper I would have added the word ‘anticipated’ as the first world of the title (to make it ‘Anticipated ocean acidification does not impair the behaviour of coral reef fishes’).  I say this because ocean acidification COULD affect marine organisms: it all depends on how severe the acidification becomes.  Acid rain, which had dramatic effects on the pH of many freshwater bodies, definitely had dramatic effects on freshwater organisms.  So if oceanic pH changed dramatically, then I think effects on biodiversity would occur.  But if ocean acidification develops as predicted for the remainder of this century, then this paper suggest that major effects of ocean acidification on coral reef fish are unlikely.

“Taking a step back from the topic at hand, this paper could have wide implications across all areas of research, as it highlights the issue of reproducibility.  It raises serious questions about why sensational results appear to be so often accepted by both reviewers and editors of top-notch journals.  Irreproducible research pervades all of science, and risks undermining confidence in science.  Not enough is being done to address the issue.  The authors of this paper have done a great service to science in general.”


‘Ocean acidification does not impair the behaviour of coral reef fishes’ by Timothy D. Clark et al. was published in Nature at 18:00 UK time on Wednesday 8 January 2020.

DOI: 10.1038/s41586-019-1903-y


Declared interests

Dr Phillip Williamson: “Dr Phil Williamson is Honorary Reader at the University of East Anglia.  He was previously Science Coordinator for the UK Ocean Acidification research programme (2010-2015), co-funded by NERC, Defra and DECC.”

Prof James Crabbe: “No conflicts of interest from me.”

Prof Alex Ford: “No interests to declare.”

Dr Bev Mackenzie: “No conflict of interest.”

Prof Kevin Flynn: “I was the lead PI in the UKOA commercial-fisheries project funded by NERC, Defra, DECC a few years ago.  That project did include some OA-fish behaviour work.”

Dr Michael Steinke: “I do not have a conflict of interest.”

Prof John Sumpter: “I do not know the authors, and have no connections with any of them.”

None others received.

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