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What is thermal stress?
When the water warms up, the fish feels the effects. And unlike us, there?s nothing it can do about it. Its body temperature depends directly on the temperature of its environment. Every additional degree therefore has an immediate effect on its body, altering its metabolism, vital functions, and behavior.
This phenomenon has a name: thermal stress. A meta-analysis published in 2023 in the *Journal of Fish Biology*, covering 33 species, shows that the closer a fish gets to its thermal limit, the more its cortisol levels (the stress hormone) increase. This physiological reaction has far-reaching consequences.
Minor variations
Fish, in fact, perceive temperature with remarkable precision, capable of detecting even the slightest variations. This sense is crucial: digestion, growth, and nervous activity all depend directly on water temperature. For example, the growth rate of largemouth bass doubles between 20 and 23 °C, while pike digest their food in two days in the summer compared to nearly two weeks in the winter.
But this advantage quickly turns into a disadvantage. Above a certain threshold (around 25 °C for trout, 31 °C for pike), the fish?s body goes into overdrive. The fish then experiences heat stress.
The first phase is active: the fish becomes restless, moves around more to find cooler areas, and eats less. If there is no way out, a second phase sets in: the fish becomes lethargic, swims very little, may lose its balance, and breathes more rapidly. Finally, in the final stage, only essential movements remain. Even when returned to suitable conditions, the fish generally does not recover.
In addition to this thermal constraint, there is an aggravating factor: oxygen. The warmer the water, the less oxygen it contains, while microorganisms consume more of it. As a result, even a temperature that is theoretically tolerable can become suffocating.
Tangible consequences for the fishing industry
For the angler, the consequences are very real. A fish that is already weakened tires much more quickly during a fight. This extra exertion can be fatal, even after it is released back into the water.
Under these circumstances, adapting your fishing practices becomes essential. Knowing when to call it a day during heat waves?or to cut a session short?is an integral part of responsible fishing. Preserving fish today means ensuring the quality of our aquatic environments and fish populations tomorrow.
