Water temperature and fish welfare

The water temperature in aquaculture systems has a major impact of fish welfare. Fish are warm-blooded animals, which means that their body temperature is not constant, but adapts to the environment. Depending on the species, there are different tolerance ranges. In the following, we explain at which water temperatures different fish species feel comfortable and how changing water temperatures in closed recirculation systems has an influence on the animals.

Fish species and their temperature sensitivity

In general, there is a certain dichotomy in Europe regarding fish species that are adapted to either cold or warm water. For example, fish from the carp family prefer warmer waters, while fish species such as trout or salmon live exclusively in colder waters.

Globally, there is a wide range of temperatures that fish can cope with:

Some species, such as icefish from the Antarctic, are absolute cold specialists and can cope with temperatures below freezing thanks to special antifreeze proteins.
Other fish species, such as the Julimes desert carp, love the heat and can be found in a hot spring in Mexico at up to 45 °C.
Many exotic fish species such as the Yellowtail Kingfish, Barramundi or Red Snapper live in warm water of 26 °C and above.
Shrimps feel comfortable at tropical temperatures of around 30 °C.

But there are also temperature experts in Europe:

The crucian as a carp fish, can persevere in the frozen mud without oxygen for several months in winter.
A Romanian rudd species also likes to inhabit warm springs and can cope with as much as 35 °C.
The common food fish sea bass and dorado tolerate temperatures between 18 and 28 °C.
Salmon prefer temperatures between 9 and 17 °C in the adult stage.
Rainbow trout also tolerate temperatures between 10 and 18 °C, but 20 °C should not be exceeded.

Effects of water temperature on fish welfare

As warm-blooded animals, fish are very dependent on water temperature. It determines their activity and directly influences important processes such as feeding and reproduction. The tolerance for temperature changes depends on several factors. These are mainly:

the stage in the life cycle
the physiological state
the speed of temperature changes

Especially embryos in the egg as well as adults ready to spawn have a very limited tolerance. Well-fed fish have a high chance of surviving a temperature change, while sick and starving fish reach their limits early.

If temperature changes occur very suddenly and to a great extent, fish experience stress. In addition, their activity is reduced in extreme water temperatures. Furthermore, the appetite of the animals is reduced. The greater proliferation of bacteria in warm water also has a direct effect on their health. Fish with a weak immune system or open spots on the skin can be attacked by pathogens and further weakened or even die.

Fish and the climate crisis

Historically, climate has always had a major influence on water temperatures and fish. The current distribution of freshwater fish species in the temperate and subpolar zones is largely the result of the large-scale extinction of fish species during the last ice age. Tropical and subtropical waters are richer in species compared to our native waters, because there was no extinction of fish species due to the cold.

Nowadays, we see a trend in the opposite direction. Global warming is the great challenge of our time and is also a huge problem for fish. It affects fish and their aquatic ecosystems in many ways. For example, one direct effect of rising temperatures is increasing evaporation of surface water. In some regions, this can lead to the drying up and disappearance of still and flowing waters and their fish. Conversely, flooding and high water can also cause an imbalance. They flush fish out of their natural habitat, and there is spatial displacement of populations, up to and including extinction of those that cannot find sufficient food in other waters or cannot tolerate the living conditions. Another negative effect is the increase in extreme weather (floods, droughts, heat waves, cold waves, storms), which inevitably causes unrest in natural waters and can lead to species loss in the long term.

Some studies have identified fish species that are particularly vulnerable to climate change (tropical marine fish are thought to be especially at risk) and are already documenting effects of climate change on fish fauna (population declines, increased growth, and temporal changes in migration and spawning behavior). Nevertheless, it is still difficult to predict when and where major changes in fish fauna will occur. In particular, the interactions of the climate crisis with other problematic human-induced processes such as aquatic eutrophication, the spread of invasive species, and environmental pollution and degradation are elusive. They have the potential to further worsen the precarious situation of scaled species.

So it’s up to all of us to stop climate change and ensure that the diversity of aquatic life is preserved for future generations to experience. Sustainable recirculating aquaculture can be part of the solution.

Further information

Discover more about innovative recirculating aquaculture systems.

SEAWATER Cube

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Bundesverband Aquakultur

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