For the past three years, Trent researchers have been testing a long-standing theory about how fish and other freshwater and marine species may be able to withstand increases in water temperatures, a result of climate change and warming global temperatures. 

Associate professor of Biology at Trent University Dr. Graham Raby, along with undergraduate and graduate students from Trent University and scientists from around the world, tested the oxygen and capacity limited thermal tolerance hypothesis, investigating the role of oxygen supersaturation in protecting aquatic animals from heat stress.  

“This has been hotly debated in the animal physiology world over the last 15 years. Basically, it suggests that fish underperform at high water temperatures because of limited oxygen,” said Professor Raby.  

The study, now published in PLOS Biology, found minimal effects of increased oxygen on stretching the CTmax of aquatic species, which is the max temperature at which an animal maintains control of its motor system and is considered to be at its limit of thermal tolerance. 

Prof. Raby and the research team, which included Leroy Reynolds, a Trent Biology alum and former recipient of the Roy L. Edwards Scholarship, tested nearly 1,500 animals across 14 aquatic species in labs at Trent, and in French Polynesia, Sweden, and Norway. The animals were used in one of two tests, normal oxygen levels or high oxygen levels, to compare any differences in their heat tolerance between the two conditions. Aquatic species turn upside down when they exceed their limit of thermal tolerance, a physiological response similar to fainting.  

“In our study, while we saw subtle evidence of higher thermal tolerance for a couple of species, the majority of experiments showed nothing to that effect,” said Prof. Raby. 

The authors also highlighted that naturally occurring oxygen saturation in shallow, coastal waters may not protect aquatic species from the effects of extreme heat.  

“The main thing is we were able to get some insight into the role of oxygen, which will be helpful for conservation planners in determining which lakes and rivers to prioritize for species protection,” said Prof. Raby. 

Casting a wider net

While the study ultimately led to null results, the findings still offer a helpful set of data for the pursuit to better understand the role of oxygen on thermal tolerance. 

“Publishing null results can be helpful for guiding future research,” said Prof. Raby. “We did find some evidence for a small effect in crustaceans. It’s possible there may be something about their cardio-respiratory system and physiology that allows them to benefit a little from extra oxygen during heat stress, so this might be something for researchers to explore in the future.” 

Learn more about Biology at Trent University.