We all know that the world is getting warmer. Global temperatures are rising and extreme heat events are happening more often, which is particularly bad news for ectotherms (cold-blooded animals) that adapt to the temperature of their environment. For these organisms to survive extreme heat, some scientists say the ability to take up and use oxygen during these events is key. The absolute hottest temperature an ectotherm can tolerate is thought to be limited by how well they can get oxygen to the tissues (where it is needed in crisis mode). Factors such as heart condition, breathing rate and even blood chemistry could help (or hinder) how well oxygen is moved through the body. But what if you could train to improve your oxygen transport abilities? Could you then handle the heat?
These are the questions Daniel Gomez Isaza from Murdoch University, Australia, and Essie Rodgers from the University of Canterbury, New Zealand, asked. Exercise has been shown to increase heart size and the ability of blood to carry oxygen, so the scientists predicted that fish could improve their thermal tolerance by increasing exercise. As salmon are active fish, migrating long distances and often living in fast-flowing water, the duo trained young Chinook salmon every day for 1 h by taking them for a ‘group run’ in a swim tunnel, which acts as a fish treadmill. The researchers set the water flow rate so that fish were swimming at about 60% of their maximum sustainable speed; think easy recovery runs instead of sprint workouts. Then, after 3 weeks the researchers put the salmon to the test. Would exercise training improve the fish's oxygen transport and their thermal tolerance in turn?
To find a fish's upper thermal limit, Gomez Isaza and Rogers warmed the fish's tank gradually until it reached a temperature where the fish toppled over and couldn't right itself again. Immediately following this, they took a blood sample to see whether the combination of the high temperature stress with the exercise regime had affected the blood's ability to carry oxygen, and they also collected samples of the fish's heart and spleen (a red blood cell factory).
However, the duo found that exercise training did not actually impact the fish's ability to transport oxygen around their bodies. There were no changes in the amount of red blood cells or the protein that carries oxygen (haemoglobin), nor were there increases in heart or spleen size. Only immediately after the thermal tolerance test did they see an increase in the blood's oxygen carrying abilities, which was similar in exercised fish and fish that simply relaxed in the tunnel each day. With no differences in oxygen transport between the exercised and control fish, it was not surprising to find that the thermal tolerances of both groups were similar.
That's not to say that exercise might not still be beneficial. The exercised fish were able to tolerate slightly higher temperatures than their unexercised counterparts, managing to remain upright at temperatures up to 28.60°C, while the unexercised fish toppled over at 28.24°C. This might not be ecologically relevant if a heatwave of 29°C occurs, but it gives us some insight into the effects of exercise training. It seems that many factors could affect not only the oxygen transport abilities but also the thermal limits of the fish. And it is possible that exercising at 60% of maximum swim speed for an hour might not have been the ideal training plan for juvenile Chinook salmon, even though it has increased swim performance, active metabolism and energy stores in other species. Perhaps a longer exercise regime might benefit the Chinook's thermal tolerance. Many questions about the impact of exercise remain to be explored.
