Fish have a kind of stone in their ear that scientists can read like tree rings. My team’s new research found a way to decode the chemicals in these stones to measure how much energy they used when alive. What we learned could help bluefin tuna survive the climate crisis.

There is still so much we don’t know about how animals respond when their habitat suddenly changes. Temperature is one of the most important puzzle pieces, as it affects the rates of the chemical reactions that define life.

For animals, rising temperatures act like inflation. Rising prices mean housing and food take up more of our budget, leaving less money for luxuries. More heat means more of an animal’s bodily resources, like food and oxygen, are needed to fuel basic functions, like breathing and moving, leaving less energy for growth and reproduction.

However, heat changes don’t affect all animals the same way. Just as the wealthy can use their large cash reserves to weather inflation, animals differ in how close they are to their energy “ceiling”.

Warming waters

Animals living in temperatures in the middle of their species’ range can increase the rate of their metabolism, meeting the extra cost of living in warmer waters. Those on the warm edge of their species’ range might be closer to their limits, where increases in temperature push them into a form of energy debt.

Reserves that might have been used for growth must be diverted to maintain essential life processes. Rising temperatures, through their effects on metabolism, force species to adapt, move somewhere new or die.

Measuring energy expenditure in wild animals is no easy task. Fortunately, metabolic reactions leave chemical traces in the body.

The otolith is a stony lump in the fish ear. Otolith rings, much like tree rings, reveal a fishs’s age. At the University of Southampton we have developed a technique to decode the chemistry of otoliths.

Different forms or isotopes of oxygen in the otolith indicate the temperature the fish experienced when it was alive. Carbon isotopes reveal how quickly food was converted into energy. Fish carry their fitness trackers in their ears.

Studying how animals’ energy needs shift with temperature can help us predict which animals are most at risk from rising temperatures. Juveniles, for instance, which need to grow quickly so they are strong enough to evade predators, might be more vulnerable to the effects of global warming.

Recently, we applied this new technique to Atlantic bluefin tuna. These fish can grow to two metres long and can swim at 40mph. They also have a high metabolism which allows them to thrive in colder waters than most other tuna species.