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Warmer water!
Fish are cold-blooded animals: their metabolism is directly influenced by water temperature. The higher the temperature, the greater their energy needs. As a result, they consume more oxygen.
The problem is that warm water contains less oxygen. This paradox creates a real biological dilemma: fish need more oxygen at the very moment when it is becoming scarcer. In extreme cases, this can lead to episodes of asphyxiation.
Several recent events bear this out. During the heat waves of 2003, 2018, and 2022, massive die-offs of trout and grayling were observed in many European rivers, particularly in France.
Growth, Nutrition: Lasting Effects
Beyond respiration, heat profoundly alters the functioning of aquatic ecosystems. Fish feeding patterns are disrupted, their growth is slowed, and their reproductive cycles are desynchronized.
In the Mediterranean, research by Ifremer has revealed a striking phenomenon: the average size of sardines has decreased over the past few decades, from about 15 cm to 11 cm, due to changes in plankton (their main food source) and an energy imbalance linked to warming waters.
In some species, egg-laying periods are also changing. This shift may seem insignificant, but it can jeopardize the survival of the larvae if their emergence no longer coincides with the availability of food.
Different reactions depending on the species
When it comes to heat waves, not all fish are created equal. Some species experience sharp declines in population, while others manage to adapt... or flee.
Modeling studies indicate that marine heatwaves can lead to significant migrations toward cooler areas, as well as a rapid decline in biomass.
However, the picture is not uniform. A study published in 2023 in the journal *Nature* indicates that certain heat waves do not systematically lead to an overall decline in demersal fish populations in the North Atlantic and the Northeast Pacific. Some species are more resilient and may even temporarily fare better.
A quiet but very real crisis
While the effects of heat waves are visible on land, they often remain invisible underwater. Yet they are very real. For many species, these heat waves represent a major source of stress, affecting their breathing, feeding, and reproduction.
Behind the apparent tranquility of a summer river, a silent struggle sometimes lies hidden. A struggle for oxygen, for energy, and, ultimately, for survival.
