Freshwater fish suffer less from heat

Large fish suffocate more quickly than smaller fish in oxygen-poor, warm water. Species with larger body cells are also extra vulnerable in warm water. This is what biologists from Radboud University Nijmegen together with Chilean, British and French colleagues write this week Global Change Biology. Based on this conclusion, it is easier to predict which fish species will be the first to be affected by climate change. Warm water contains less oxygen, while fish need extra oxygen in warm water because their metabolic processes run faster there. The warming is probably primarily a problem for the saltwater fish: they adapt less easily than the freshwater species.

In low-oxygen water, your life as a fish becomes very monotonous

Like other animals, fish need oxygen for their so-called aerobic metabolism, which converts nutrients into energy. The lower the oxygen content in their habitat, the less energy they have to hunt, escape or reproduce. “Then you reach a level where they can still breathe and keep still, but no more,” says lead author Wilco Verberk from Radboud University Nijmegen on the phone. “It makes your life as a fish very monotonous. And it is not conducive to the conservation of fish stocks that it is also not conducive to standing still in one place. It is important to grow and reproduce.”

Less mixing

But especially in fresh water, the percentage of dissolved oxygen – apart from warming due to climate change – is often far from optimal. This can have various reasons. Stratification occurs in lakes in the summer. In addition, the surface water warms up quickly in summer under the influence of high temperatures, and the deeper water remains colder. Little or no mixing of these layers takes place, and as a result, in this case, the deep, cold water becomes deoxygenated. “In addition, ditches and small lakes are often filled with algae, which grow extra quickly at high nitrogen concentrations,” says Verberk. “During the day, these algae provide oxygen, but at night they also consume oxygen themselves, and there is often a shortage of fish.”

Together with colleagues, Verberk analyzed oxygen tolerance data from 195 fish species. Thus they discovered that especially the larger fish and the fish with larger cells suffer from warming water. Also, saltwater fish seem to suffer from shortness of breath more quickly than freshwater fish.

The last is striking. Precisely because freshwater fish are more often exposed to oxygen depletion – through stratification, algae growth, rapid warming – they have been selected in the course of evolution for a greater tolerance to oxygen deficiency. This selection has happened less with saltwater fish, says Verberk. “Additionally, saltwater fish often live in larger bodies of water where they can easily swim away to a place that has sufficient oxygen, an option that freshwater fish often do not have.”

Local level

Although climate change is a global problem, warming water cannot simply be prevented, according to Verberk there are various measures that can be taken at a local level to make the water more habitable for fish. “It starts with reducing algae growth and providing enough clean, flowing water with rich marine life so fish don’t have to spend so much energy searching for food.” In addition, water management can be improved in freshwater areas. “Running water and the influx of cool groundwater can counteract the local effects of the warming. In the Netherlands we are very good at draining water, but not at retaining it. And when the locks dry up, you really have a problem. After all, no fish survives without water, regardless of the amount of oxygen in the air.”

Also read: Hot, acidic and oxygen-free water: life in the sea is having a hard time

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