Fish can’t mate when it’s hot: global warming is to blame

In a new meta-study, experts from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) published groundbreaking findings on the effects of climate change on fish stocks around the world. According to scientists, the risks to fish are much higher than previously thought, especially considering that at certain stages of development they are especially sensitive to rising water temperatures. The results of the study were published in the journal Science.

One of the critical places in the life cycle of fish is their low resistance to heat during mating. In other words, the water temperature in spawning grounds largely determines how successfully they reproduce. This makes fish particularly vulnerable to the effects of climate change – not only in the ocean but also in lakes, ponds, and rivers. According to scientists, if this process is not controlled, climate change and rising water temperatures will adversely affect the reproduction of up to 60% of all fish species.

Organisms must breathe so that their bodies produce energy; this is equally true for humans and fish. In addition, the energy needs of people and animals depend on temperature: when it gets warmer, the need for energy increases exponentially, and with it the need for oxygen. It follows that organisms can adapt to a rise in temperature, providing their bodies with a large amount of oxygen. But there are certain species limitations of this ability; if these limits are exceeded, it can lead to cardiovascular collapse.

Armed with this knowledge, experts from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) conducted a meta-study to determine in which phases of life marine and freshwater fish around the world are most sensitive to heat. To this end, biologists collected scientific data on 694 species of fish and analyzed the temperature ranges in which they can survive as adults ready to mate; embryos in the eggs; larvae and adults during the mating season.

Our results show that both embryos in eggs and adults ready for mating, fish are much more sensitive to heat than at the larval stage or as adult sexually mature individuals outside the breeding season. On average, for example, adults outside the mating season can survive in water that is 10 ° C warmer than adults who are ready to mate or lay eggs.

Fleming Dalke AWI Marine Biologist



The reason for this temperature deviation lies in the anatomy of the fish: fish embryos do not have gills that would allow them to receive more oxygen. In contrast, fish that are ready to mate produce an egg and sperm. This additional body mass must also be supplied with oxygen, so even at lower temperatures, their cardiovascular system is under tremendous stress.

The research results are applicable to all types of fish. They also clarify why fish are sensitive to heat, especially during mating and in their infancy. Accordingly, at the second stage, the researchers analyzed the extent to which the water temperature may increase due to climate change in the spawning areas of the studied species. For this purpose, they used new climate scenarios.

Fish evolution

The findings of scientists confirm that every degree of Celsius of warming creates more problems for world fish stocks.

If people can successfully limit climate warming to 1.5°C by 2100, only 10% of the fish species we have studied will be forced to leave their traditional spawning areas due to rising temperatures. But if greenhouse gas emissions remain at a very high or very high level, this can lead to an average warming of 5 ° C or more, which can endanger up to 60% of all fish species.

Then, these affected species will either be forced to adapt through biological evolution — a process that is likely to take too long — or mate at another time of the year or elsewhere. Some species can successfully handle this change. But given the fact that fish adapted their mating patterns to specific habitats for very long periods of time and mating cycles to specific ocean currents and available food sources, it should be assumed that they will be forced to abandon their normal spawning area. This will mean serious problems for them.

In addition, fish living in rivers and lakes have one more problem – their habitat is limited by the size and geographical location of the waters in which they live: migration to deeper waters or to colder areas is almost impossible.

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