Even fish suffer from the heat

Fish suffer greatly from abnormal heat, and this is for two essential reasons: the lack of dissolved oxygen in the water and the acceleration of their metabolism.

There are also other negative effects on the life of fish when the water temperature increases:

• They get stressed more easily: they may become less active, breathe more rapidly, or seek cooler, deeper areas.
• By effect of the stress increased, the fish become more irritable, and also more nervous.
• They lose their memory and they risk becoming stupid.
• They may have difficulty feeding and reproducing: prolonged high temperatures compromise the growth and reproductive success of many species.
• Increases the risk of disease: some parasites and pathogens proliferate more easily in hot weather.
• In extreme cases they can dieDuring heat waves, especially in lakes, ponds, and rivers with little flow, fish kills can occur due to the combination of high temperatures and oxygen deficiency.

A 2019 study , published in ScienceDirect , found that, in the case of zebrafish, both too low and too high temperatures (34°C compared to the optimal 26°C) impair learning in a maze and alter brain proteins involved in nerve transmission, causing the fish to become forgetful. Another 2024 study shows that in hot weather, fish have a higher metabolism and greater energy needs; this can lead to less selective, more impulsive, and more nervous behavior when searching for food. And above 32°C, they can find themselves in a maze and lose their sense of direction underwater.

In general, the fish that suffer most from heat are those adapted to cold, well-oxygenated waters. For them, even a slight increase in temperature can pose a problem. Cold-water fish (such as trout or lota) begin to suffer at temperatures above 24°C, while temperatures above 30–34°C can prove fatal even for many lake, river, or aquarium species.

Among the fish most sensitive to excessively high temperatures are:

• Trout brown trout: It thrives between about 10 and 16°C. When the water temperature exceeds 20–22°C for prolonged periods, stress increases significantly.
• Rainbow trout: It tolerates heat a little more than the brown trout, but it also suffers from high temperatures.
• Arctic char: It is among the European fish most sensitive to water warming.
• Grayling: requires fresh, oxygen-rich rivers.
• Whitefish: It lives in deep, cold lakes; warming of the surface layers may reduce its habitat.

In the sea, the species that suffer most from heat are those adapted to cold waters or those with a rather narrow temperature range. As the sea warms, many of these species are moving to higher latitudes or greater depths, while others struggle to find suitable habitats.

Among the most sensitive are:

• Atlantic cod: It is one of the most studied cases. Populations in the southernmost areas of its range are particularly vulnerable to warming.
• Haddock: prefers cool waters of the North Atlantic and may be sensitive to rising temperatures.
• Atlantic Halibut: It lives in cold, deep waters, where warming is reducing favorable habitats.
• Atlantic herring: Heat can alter their breeding areas and the availability of plankton they feed on.
• Capelin: small Arctic fish very sensitive to temperature changes.

In the Mediterranean, the situation is different: the sea is already relatively warm, so many species are adapted to high temperatures. However, some still suffer from marine heatwaves, including:

• European hake, especially in the warmer areas of its range.
• Mediterranean cod, where present, it may be affected by heating.
• Some species that live predominantly in deep, fresh waters, such as various scorpionfish and cartilaginous fish, may be indirectly affected.

Conversely, species typical of warmer waters, such as the amberjack, the Mediterranean barracuda, and the dolphinfish, are in many cases increasing their presence.

Saltwater fish that are least sensitive to heat are generally those that evolved in warm temperate or tropical waters. Many of them tolerate temperatures above 25–28°C well, as long as there is sufficient oxygen and the warming is not extreme.

In the Mediterranean, among the most tolerant species are:

• Amberjack: It grows well in warm waters and is often most abundant during very hot summers.
• Dolphin fish: typical species of warm waters, especially frequent in summer and autumn.
• Barracuda Mediterranean: in recent decades it has expanded its diffusion in many areas of the Mediterranean.
• Gilt-head bream: It tolerates high temperatures well and also lives in lagoons, where the water can become very warm.
• Bass: It has a good adaptability and tolerates wide variations in temperature and salinity.
• Mullet: It is among the most resistant fish and often lives in warm, shallow coastal waters.

Among the tropical species that are becoming increasingly common in the Mediterranean are:

• Rabbit fish, very resistant to high temperatures.
• Silver pufferfish, also adapted to warm waters (although it is an invasive and toxic species).

In the Mediterranean, climate change is favoring warm-water species, while those associated with cooler waters are declining or moving northward or deeper into the sea. This is progressively altering the composition of the sea's fish communities.

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