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

One of the critical points in the life cycle of fish is their low tolerance to heat during

mating.In other words, the temperature of the water in the spawning grounds largely determines how successfully they reproduce. This makes fish especially vulnerable to the impacts of climate change—not just in the ocean, but also in lakes, ponds and rivers. According to scientific research, if this process is not controlled, climate change and rising water temperatures will negatively affect the reproduction of up to 60% of all fish species.

Organisms must breathe so their bodiesproduced 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 fromThe 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 do this, 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 howBoth embryos in eggs and adults ready to mate, fish are much more sensitive to heat than at the larval stage or as adults outside the mating season. On average worldwide, for example, adults outside the mating season can survive in water that is 10°C warmer than adults ready to mate or lay eggs.

Flemming Dahlke, marine biologist AWI

The reason for this temperature deviation lies infish anatomy: 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.

Research results apply to all species.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 temperature of the water can increase due to climate change in the spawning areas of the studied species. For this purpose, they used new climate scenarios.

Phylogenetic and ontogeneticpatterns in thermal tolerance of fish (A-C). Circular chronograms show (A) the upper temperature limits (Tmax), (B) the lower temperature limits (Tmin), and (C) the thermal tolerance ranges (Trange) of species and their life stages from the inside out: offspring, embryos, larvae, and adults. Prominent taxonomic groups are recognized, particularly the warm, warm (Fundulidae and Cyprinodontidae) and cold stenothermal groups (Notothenioidei). Credit: Dr. Dalke et al., Science(2020)

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

If people can successfully limit warmingclimate up 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 be forced eitheradapt through biological evolution - a process that is likely to take too much time - or mate at another time of the year or in some other place. 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, in fish living in rivers and lakes,there is another 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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