HKU marine biologist and international team unveil impacts of heatwave on reef fishes

The University of Hong Kong

With elevated temperatures during a marine heatwave this cardinalfish species (Cheilodipterus quinquelineatus) shows the least changes in gene expression and appears to be more tolerant. Credit: @The University of Hong Kong
With elevated temperatures during a marine heatwave this cardinalfish species (Cheilodipterus quinquelineatus) shows the least changes in gene expression and appears to be more tolerant. Credit: @The University of Hong Kong

The marine heatwave of 2016 was one of longest and hottest thermal anomalies recorded on the Great Barrier Reef in Australia, influencing multiple species of marine ectotherms, including coral reef fishes.

Dr Celia Schunter from School of Biological Sciences and the Swire Institute of Marine Science (SWIMS), The University of Hong Kong (HKU) and a team of international scientists conducted a study attempting to understand the molecular response of five species to the 2016 heatwave conditions that killed a third of the Great Barrier Reef corals. This is the world-first study tracking how wild fish populations respond to a severe marine heatwave. The results of the study were published in the journal Science Advances.

Marine heatwaves (MHWs) are elevated extreme temperatures in the oceans for an extended period of time, similar to an atmospheric heatwave. These elevated temperatures can have a significant impact on marine life, possibly pushing the thermal limits of many organisms. With the frequency and intensity of heatwaves predicted to increase in the future, this could have greater impacts on the performance of ectotherms, when compared to slight thermal increments over years or decades.

“To understand the challenges fish face under such conditions we used a molecular approach to evaluate how acute warming events directly affect reef fish communities in nature,” said Dr Celia Schunter. “We chose to work with five different species which are commonly found on the reef to be able to understand differences in reactions among fish species with different life histories to get a broader overview of the reaction and impact.”

“Our study shows that reef fishes are directly affected by heatwaves, but their responses vary greatly among species,” said co-author Associate Professor Jodie Rummer from the ARC Centre of Excellence for Coral Reef Studies at James Cook University (Coral CoE at JCU). Dr Rummer was part of the international team that studied changes in the expression of thousands of different genes in five species of coral reef fish, collected at different points before, during and after the 2016 heatwave.

“Changes in gene expression can tell us how an animal responds physiologically to an environmental shock, such as a heatwave,” said Dr Celia Schunter from HKU School of Biological Sciences and SWIMS, one of the lead authors in the study. “We measured RNA levels in livers in the fish. This can control when proteins are made and in what amount, and these proteins dictate how the cells of the body function. We saw many genes change expression levels across the timepoints of a heatwave revealing important functions such as cellular stress response and changes in metabolic functions.”

Through these genetic analyses, the team identified species-specific physiological responses to the heightened temperatures. “Fast water warming causes an increase of the metabolic demands in fishes, which are similar to what happens to an athlete doing intense exercises. When water temperature increases, fishes have a higher demand for energy and oxygen, which leaves a signal that is measurable with genetic techniques. This higher energy demand at warming can affect their reproduction, swimming and development, and that is why it is important to understand the response to warming.” said Dr Moisés A Bernal, co-author of the study from Auburn University.

Interestingly, “these patterns of gene expression also changed with the duration of the heatwave,” said Dr Rummer. “This suggests that the physiological mechanisms the fish use to cope with the warmer waters changed as the heatwave progressed. The results suggest fish populations are influenced by both the intensity of a heatwave and how long it lasts.” This signals potential long-term consequences for the health of fish populations as extreme heat events increase in frequency, duration and magnitude under human-induced climate change.

At a species level, Dr Rummer says the responses varied in intensity. Some fish struggled less than others. “The spiny damselfish responded strongly to the warmer conditions, with changes in the expression of thousands of genes, suggesting it is particularly sensitive to heatwaves. Other species appear to be more tolerant, with fewer changes in gene expression.” said Dr Rummer . Two of the five studies species studied can also be found in waters around Hong Kong and Southern China as well as many more closely related fish species providing also some context for possible effects for waters around Hong Kong.

The study provides a possible approach for predicting which fish species are most at risk under repeated heatwave conditions, said another co-author Professor Timothy Ravasi, from the Marine Climate Change Unit at the Okinawa Institute of Science and Technology Graduate University (OIST). “This has ramifications for policy makers and for the fishing industry, because not all species will be equally affected. We need to screen a large number of species to predict which will be sensitive and which will be more tolerant to warming waters and heatwaves.”

“Over time, the fish may adapt to rising temperatures, or even migrate to cooler waters,” Professor Ravasi said. “But these heatwaves are happening now, and it’s necessary to understand and consider the immediate consequences.”

In 2015 the South China Sea experienced a heatwave of a similar magnitude than the heatwave on the Great Barrier Reef studied here. The coastal waters of Hong Kong and the South China Sea are predicted to experience more frequent and intense marine heatwave events as seen on the global scale. It is now clear that these extreme events can have far-reaching effects on marine fishes, but also economic implications on aquaculture and fishing industries Dr Celia Schunter urges the need for more research into the impacts of such events in the marine waters of Hong Kong to avert the potential collapse in the marine ecosystem and the industries relying on it.


The paper:

‘Species-specific molecular responses of wild coral reef fishes during a marine heatwave.’ in Science Advances by Moisés A Bernal, Celia Schunter, Robert Lehmann, Damien J Lightfoot, Bridie J M Allan, Heather D Veilleux, Jodie L Rummer, Philip L Munday and Timothy Ravasi.’

Link of journal paper:

For more information about Dr Celia Schunter’s research, please visit:

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Terry Harvey
March 23, 2020 10:16 am

James Cook University – now why does that ring a bell? Oh yes.

March 23, 2020 10:18 am

So, what actually happened to the fish in the wild? It would be good to compare that with the molecular effects observed in the lab.

Terry Harvey
March 23, 2020 10:21 am

James Cook University – now why does that name ring a bell? Oh yes.
It would be good to have some idea of the “extreme heat” they are claiming but they seem reluctant to give figures.

Bob Turner
Reply to  Terry Harvey
March 23, 2020 1:02 pm

Have you read the paper? Or just the press release?

Ron Long
March 23, 2020 10:27 am

I am solidly in favor of studying the effect of heat on fish, and am searching for research grants to support my proposed study: FISH-Catch Them, Cook Them, Eat Them. The research part is determining what wine goes best with what species of fish. I’m thinking this study will, of necessity, because, you know, the sciencey aspect, include visits to tropical reefs.

March 23, 2020 10:52 am

Not being a genetic expert, is it correct to correlate high change in gene expression with a negative outcome? I would think change in gene expression could also be an indicator of individual/species adaptation. A smaller change in gene expression might be a less stressed species or simply a species with less capacity for change.

Also from the paper:
“The major caveat of this study is that we lack a reference for the measures of liver gene expression throughout nonheatwave years…..we did not have a preestablished baseline of summer gene expression patterns for the study species. Ideally, we would have a complete understanding of how liver gene expression fluctuates throughout the year for the five analyzed species, as liver gene expression can be influenced by multiple other environmental and biological factors [e.g., reproduction, diet (43)]…..One additional challenge is that seasonal food availability and trophic changes associated with the heatwave ….could have influenced the patterns of gene expression we observed. “

Reply to  MJB
March 23, 2020 12:41 pm

Exactly, MJB – change in gene expression is obviously adaptation. So the paper has got it backwards. They do correctly say “it’s necessary to understand and consider the immediate consequences” but the way they are setting about it they will never understand.

Reply to  Mike Jonas
March 23, 2020 3:35 pm

James Snook: I do agree with you about the fish, but I was thinking about the researchers: I would argue that it’s necessary for them to understand a bit more about nature’s adaptability. It isn’t necessary for anyone to interfere, so that’s another thing they appear to have got backwards.

Reply to  Mike Jonasm
March 31, 2020 10:39 am

1. How much heat are we talking about in an oceanic heat wave? How long is long? Are there other populations of these fish in warmer areas which are thriving?

2. Isn’t it just as likely that the extra warmth is a plus for these fish, so that the molecular changes really are a sign of an improved environment? Consider a fish living at the cold extreme of its range, which suddenly (how long is suddenly? 1 day, 1 week, 1 month?) has its environment warm up 1 degree – toward that species’ middle temperature range. Isn’t the extra heat a benefit to those fish.

3. The plural of fish is fishes. Or does Australian English differ that much from American English?

Reply to  MJB
March 23, 2020 6:07 pm

In other words: We don’t know what happened or why because, despite being at a University focusing on climate change, we didn’t do our homework.

So we have to make conjectures about what we found and foretell what it might mean. The major caveat of this release is that we don’t know anything useful yet.

March 23, 2020 10:54 am

The other HKU paper

(Pls scroll down to where it says

Paul Johnson
March 23, 2020 11:08 am

This reads like a study intended to create some great panic over potential reef fish extinctions. Unfortunately, the actual data failed to support catastrophic results. In the end all they could salvage was an uncalibrated “tool” for future well-funded studies.

March 23, 2020 11:09 am

It would be interesting to have Peter Ridd’s opinion of this study.

March 23, 2020 11:36 am

“..more frequent and intense marine heatwave events as seen on the global scale. It is now clear that these extreme events can have far-reaching effects on marine fishes..” Why is a heatwave classified as an extreme event? Where I live in New England, a heatwave can be anything above zero in January, and we welcome it.

Right-Handed Shark
March 23, 2020 11:36 am

Funny how they never mention that water cannot be heated by warm air from above in open atmosphere. Whatever causes a “marine heatwave”, if such a thing exists, it ain’t caused by human activity. Grant application refused.

Reply to  Right-Handed Shark
March 23, 2020 12:42 pm

Right on I have said it many times here on other silly studies. I guess since a marine biologist doesn’t study thermodynamics that allows them to engage in silliness.

Reply to  Right-Handed Shark
March 23, 2020 1:12 pm

OK, I’ll bite. A hot cup of coffee put outside in January will cool down faster in Svalbard Norway (cold of winter) than in Marble Bar Australia (heat of summer). Without being the source of energy, the temperature of the air can influence the rate of energy loss for the water. So all else being equal, if the lower atmosphere this summer is hotter than last summer, the water will be some increment hotter as well. Where’s my mistake?

Reply to  MJB
March 23, 2020 1:55 pm

Magnitude. The heat content of equal volumes of water and air are vastly different. The water has about 3000x that of air. You are also talking about water radiating to the air not the other way around. What you talking about is typical of El Nino events where the ocean heats the air. Also the scale is vastly different oceans not cups. To put it more simply if you’ve got 1 Liter of water and 1 Liter of air the water weighs 1000 grams and the air is 1+ gram. That is you have a lot more stuff. Also the water molecules stick to one another air not so much. The stickiness needs to be overcome with heat to get them moving.

Right-Handed Shark
Reply to  MJB
March 23, 2020 1:58 pm

You answered your own question. In the coffee cup scenario, the heat is already in the liquid, it wasn’t heated by the air. Try heating a bucket of water with a blowlamp from above. Report back when you have raised the temp by any significant amount.

Alasdair Fairbairn
Reply to  Right-Handed Shark
March 24, 2020 4:39 am

Yes precisely. This why the oceans never get much above 35C in spite of tens of thousands of years of the Sun’ blowlamp. It is also why we will never get any runaway heating without some catastrophic collision or the like. Water provides a very effective thermostat where extra energy is involved.; but not so good if the energy is sparse. Hence more ice ages rather than interglacials.
It is due to negative feedback; but the IPCC hasn’t yet tumbled to that.

For info: For every kilogram of water evaporated from the surface some 694 Watthrs of energy gets pumped up into the clouds and beyond to space for dissipation.

March 23, 2020 12:20 pm
March 23, 2020 12:26 pm

The spiny damselfish responded strongly to the warmer conditions, with changes in the expression of thousands of genes, suggesting it is particularly – adaptive

….what agenda driven morons they are……

Reply to  Latitude
March 23, 2020 3:45 pm

Isn’t that called epigenetic ?

March 23, 2020 2:07 pm

Did anyone ever do a real survey of the GBR to determine if one third of the corals were killed as they assert in this press release? I seem to recall that the research on that consisted of some people finding a few reefs that had been exposed at high tide and extrapolating that to one-third of the reef dead. If that is their starting point, I discount all other conclusions in the study.

March 23, 2020 2:26 pm

When you see the usual grant-seeking Alarmist nonsense on coral reefs and fish emerging, there is a better than even chance that it emanates from James Cook University. This place is established as being notorious for academic fascism.
This time it is the turn of Associate Professor Jodie Rummer from the ARC Centre of Excellence for Coral Reef Studies at James Cook University to pen the grant application.

March 23, 2020 3:08 pm

“Centre of Excellence for Coral Reef Studies at James Cook University”

Isn’t that an oxymoron? I agree with Oldseadog, it would be interesting to see what Peter Ridd made of this study.

I came across this article from the Guardian yesterday, see link. Apparently James Cook University are conducting studies at the moment on The Great Barrier Reef, regarding recent coral bleaching. I seem to remember Jennifer Marohasy put out a challenge about naming the place and she would go and check it out after she put out Beige Reef video.

They claim to have discovered areas of coral bleaching in the Northern part of reef and have yet to study the South. They are conducting these studies from the air! Is that even possible?

March 23, 2020 3:31 pm

Report itself in Conclusions section acknowledges “… suggested … correlation between thermal tolerance & geographic range … [as] … species with broader physiological niches … occupy larger areas ….” Thus the authors’ data shows different responses in the case study & a 1.5*C differential is not sensational news, rather non-lineal adaptation.

March 23, 2020 3:51 pm

So was this study done on the living reef, or was it in the labouritery. If the latter did they allow for the fact that in the real world any body of water is subject to currents, thus the water is changing all of the time.


Crispin in Waterloo
March 23, 2020 7:54 pm

“The coastal waters of Hong Kong and the South China Sea are predicted to experience more frequent and intense marine heatwave events as seen on the global scale.”

Predicted by whom? What is the record of their predictions so far? What does their predictive model have to say about the coming global cooling?

Read well down the page, Dr Schunter. Why should coastal waters experience more frequent and more intense heatwave events? Is anyone suggesting and then explaining how El Nino events are created by CO2 in the atmosphere? That one paper I would like to read.

Selwyn H
March 23, 2020 7:56 pm

Associate Professor Jodie Rummer is the marine biologist from JCU who spent over a month in 2016 at the Lizard Island research station on the northern Great Barrier Reef. In an article in The Maritime Executive magazine she reported that “This year the combination of El Nino, climate change and an extended period of hot summer days when the tide was exceptionally low has caused many of the corals that survived last years cyclone to lose their symbiotic algae and start bleaching”. These low tides were what Jim Steele was talking about in his essay about El Nino “Falling Sea Level” and what Professor Terry Hughes ignored in his aerial surveys of the reefs.

March 24, 2020 2:50 pm

Half of the Great Barrier Reef has been bleached to death since 2016. Mass coral bleaching, a global problem triggered by climate change, occurs when unnaturally hot ocean water destroys a reef’s colorful algae, leaving the coral to starve. The Great Barrier Reef illustrates how extensive the damage can be: Thirty percent of the coral perished in 2016, another 20 percent in 2017.
Is it possible to fact check these claims?

Reply to  Robber
March 24, 2020 3:31 pm

Robber I commented on this post, my concerns about a story put out by the Guardian a few days ago. I suggested that Peter Ridd and or Jennifer Marohasy might refute it.

old construction worker
March 25, 2020 2:59 am

Amazing, Just amazing. Going back to the last ice age when the oceans were how many feet lower the Great Barrier Reef survived changes with the climate. But, somehow, this time is different we are told. What a bunch of bull.

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