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Lost Breeding in a Warmer World – ScienceDaily

The latest research done by AWI experts that survival chances for offspring of important fish species will deteriorate significantly unless the 1.5 ° C goal of the Paris Climate Agreement is reached. Under conditions of further warming and acidification of the ocean, Atlantic cod and its Arctic relative polar cod would be forced to search for new habitats in the far north. Their population could decrease. If so, it could be disastrous, as polar cod is the most important source for Arctic seals and seabirds. In addition, fishermen could lose the world's most productive area of ​​cod fishing in northern Norway. The results of the study also show that a strict climate policy could prevent the worst consequences for animals and humans.

There are some fish that prefer extremely cold water – and can only be spawned in cold water. Atlantic cod, a well-known and popular food, is one of them. Even better to adapt to the cold of the polar cod, which is wintering in the Arctic in large swarms under the sea ice. Polar cod is stretched at water temperatures between 0 and 1.5 ° C because the fertilized eggs / embryos can best develop at this temperature. On the other hand, cod is decomposed at a level of 3 to 7 degrees, which is extremely cold in human terms. Researchers AWI Flemming Dahlke and Dr. Daniela Storchová are convinced that this dependence on cold water can be fatal to both species; due to climate change, especially North Atlantic and Arctic waters, are warming up considerably if human beings do not find a way to massively reduce greenhouse gas emissions. In addition, there is an acidification problem: the more carbon dioxide gets into the atmosphere, the more carbon dioxide in the ocean dissolves. Carbon dioxide is combined with water to produce carbon dioxide that acidifies the ocean as it decomposes. "This means that cod and polar cod will be doubled in the future: their habitat will be warmer and more acidic," explains marine ecologist Flemming Dahlke.

He and Project Director Dr. Daniela Storchová, as the world's first researchers, is now using careful experiments to find out how current acidification and warming affect eggs of both species. In this context, both AWI experts focused primarily on the development of embryos until they occur as larvae, only a few millimeters long. During this phase, they are particularly sensitive to the changing environmental conditions that climate change could bring realistically. Researchers' knowledge is clear: for both species, even a small rise in temperature may cause eggs to die or produce deformations in the larvae. "As we can see, embryos are very sensitive, especially at an early stage of their development," says Flemming Dahlke. As experiments clearly show, the situation is even worse when water is acidic: the number of non-surviving embryos increases by 20 to 30 percent at pH 7.7, even at optimal temperatures.

In addition, two AWI researchers are unique in combining laboratory findings with established climatic models. The models predict the extent to which climate change will affect temperatures in different waters and how much they will acidify. On the other hand, thanks to the experiments, both researchers can precisely determine the areas in which cod and polar cod will no longer be successful in the future. It is also obvious that we could see shifts in fish stocks because adults will have to look for new friction sites where their eggs or embryos can still find viable conditions for normal development. In this regard, Dahlke and Storch considered three climatic scenarios in particular: the "usual" scenario in which there is no meaningful reduction in carbon dioxide emissions by the end of the 21st century; a moderate-climatic scenario and a scenario in which the IPCC's 1.5-degree target, according to which Earth's temperature can not be increased by more than 1.5 degrees in order to avoid the worst effects of climate change – is achieved. Together with climate modeling engineer Martin Butzin of AWI, they came up with interesting conclusions. According to Flemming Dahlke, "for the" business as-usual "scenario, conditions for young Atlantic cod will particularly deteriorate in the North Atlantic near the end of this century. In Iceland and Norway, under 60, fewer egg hatches out of their eggs. "In general, cod stocks in the Northeast Atlantic are likely to move to the Arctic, where friction sites still offer the right conditions. This could in particular pose problems for the fisheries sector, as the coastal regions of Iceland and Norway are currently home to the largest cod stocks in the world. Approximately 800,000 tonnes of cod are harvested each year worth 2 billion euros. If the population were to decline, as the AWI experts show, the losses could be enormous.

What's more, the scenario "business as-usual" also looks like a kiss for polar cod. If the water becomes warmer, it will retreat north not only for the normal scenario but also for the mild warm-up scenario. Since polar cod depends on sea ice for its overwintering phase, it remains to be seen how populations will be affected if the extent of sea ice is further reduced. It is also unclear to what extent cod will interfere with the polar cod. Considering that cod is much larger and more aggressive than its polar cousin, this cod may need to fight for its food. Whatever happens or not, the decline in the polar population of cod would be catastrophic, as it is a basic food for many organisms in the Arctic – including seals, seabirds and even whales.

The distribution limits of fish species also depend on where the predominant temperatures are optimal for friction. Dahlke and Storch's experiments have first confirmed that acidification makes fish embryos more sensitive not only to higher temperatures but also to lower temperatures. "We have noticed that young Atlantic cod not only reacts unfavorably to warmer temperatures but also to cold," says Daniela Storchová. "Acidification enhances this effect." In other words, the increased acidification load reduces the appropriate temperature range for cod and polar cod to be propagated. As Flemming Dahlke says: "Fish are more sensitive to extreme temperatures and consequently to expected warming." This would eventually mean that potential species non-species could be reduced and that less habitats could be available.

Flemming Dahlke points out that although attempts have made very clear findings, predicting the development of fish stocks is extremely difficult. "For example, whether embryos and larvae survive also depend on ocean currents and available food." Atlantic cod is now breeding at Lofoten, an archipelago in the northwest of Norway. The stream takes over the eggs floating in the water and later the larvae, further to the north, where they are waiting for the ideal living conditions. "If cod cod stocks and their friction sites move in the future to the northeast, fish will probably spawn in completely different current systems," explains Dahlke. "If that happens, we can not begin to evaluate the effects."

There is also good news, says Daniela Storchová: "Achieving the climate targets by 1.5 ° C can prevent the worst, keep important areas of breeding and minimize the risks of both species."

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