Loss of breeding grounds in a warmer world – ScienceDaily

Some types of fish prefer extremely cold water – and can only reproduce in cold water. Atlantic cod, a well-known and appreciated consumer fish, is one of them. Polar cod, which winters in the Arctic in large swarms under sea ice, is even better adapted to the cold. Polar cod spawns at water temperatures between 0 and 1.5 degrees Celsius, as fertilized eggs / embryos can develop best at this temperature. In contrast, Atlantic cod spawns between 3 and 7 degrees, which, from a human point of view, is still extremely cold. Researchers AWI, Flemming Dahlke and Daniela Storch, are convinced that this dependence on cold water could prove fatal for both species. As a result of climate change, the waters of the North Atlantic and the Arctic will warm considerably unless humans can find a way to mbadively reduce carbon dioxide emissions. Added to this is the problem of acidification: the more carbon dioxide is released into the atmosphere, the more carbon dioxide dissolves in the ocean. Carbon dioxide binds to water to form carbonic acid, which acidifies the ocean as it decomposes. "This means that Atlantic cod and polar cod will be doubly stressed in the future: their habitat will simultaneously become warmer and more acidic," says Flemming Dahlke, marine ecologist.

As the first researchers in the world, Dr. Daniela Storch, project director, has now used careful experiments to study the adverse effects of simultaneous acidification and warming on the eggs of both species. In this context, the two AWI experts have particularly focused on the development of embryos until they hatch as larvae of only a few millimeters. During this phase, they are particularly sensitive to the changing environmental conditions that climate change could realistically produce. The researchers' conclusions are sobering: in both species, even a slight rise in temperature can cause egg death or deformity in larvae. "As we can see, embryos are very sensitive, especially at the beginning of their development," says Flemming Dahlke. As the experiments clearly show, the situation worsens when the water is acidic: the number of embryos that do not survive increases by 20 to 30% at a pH of 7.7, even at optimal temperatures.

In addition, the work of the two AWI researchers is unique in that they combine laboratory results and established climate models. The models predict how climate change will affect temperatures in various waters and their acidification. Through their experiments, the two researchers can now accurately determine in which areas Atlantic cod and polar cod will not be able to reproduce in the future. It is also becoming clear that we may be seeing changes in fish populations as adults will have to look for new spawning areas where their eggs or embryos can still find viable conditions for normal development. In this regard, Dahlke and Storch focused on three climate scenarios: the status quo scenario, in which there is no significant reduction in carbon dioxide emissions by the end of the 21st century; a climate scenario with moderate warming and a scenario in which the IPCC target of 1.5 ° C – according to which the Earth's temperature can not rise more than 1.5 ° C in order to d & # 39; avoid the worst effects of climate change – is accomplished. In collaboration with AWI's climate moderator Martin Butzin, they came to interesting conclusions. According to Flemming Dahlke, "they show that, in the status quo scenario, the situation of the young cod will particularly deteriorate in the North Atlantic at the end of the century." In the regions surrounding Iceland and the Norway, percent fewer cod larvae will hatch from their eggs. " In general, cod populations in the northeastern Atlantic Atlantic are likely to move into the Arctic, where spawning grounds still provide adequate conditions. This could be particularly problematic for the fisheries sector as the Icelandic and Norwegian coasts are home to the largest cod population in the world: about 800 000 tonnes of cod worth 2 billion annually. euros are harvested. If these populations decline, as the findings of the AWI experts indicate, the losses could be enormous.

In addition, the status quo scenario also appears to be gloomy for polar cod. If the waters warm up, they will move north, not only for the status quo scenario, but also in the moderate warming scenario. Since polar cod depends on sea ice for its wintering phase, it remains to be seen how populations will be affected if the extent of sea ice continues to decline. It is also unclear to what extent the Atlantic cod will encroach on the territory of the polar cod. Since Atlantic cod is considerably larger and more aggressive than its polar cousin, the latter may have to fight for food. Whether this happens or not, a decline in the polar cod population would be catastrophic, as it is a staple food for many Arctic organisms – including seals, seabirds and even whales.

The boundaries of the distribution of fish species also depend on where the prevailing temperatures are optimal for spawning. The Dahlke and Storch experiments confirmed for the first time that acidification makes fish embryos more sensitive not only to higher temperatures, but also to lower ones. "We have observed that young Atlantic cod react not only negatively at warmer temperatures, but also at particularly cold temperatures," said Daniela Storch. "Acidification amplifies this effect." In other words: the additional burden of acidification reduces the temperature range suitable for laying Atlantic cod and polar cod. As Flemming Dahlke reports: "Fish is becoming more sensitive to extreme temperatures and therefore to anticipated warming." This would ultimately mean that the potential spawning grounds for both species are shrinking and that they may have less available habitat.

Flemming Dahlke points out that although the experiments have yielded very clear results, it is extremely difficult to predict the development of fish populations. "For example, the survival of embryos and larvae also depends on ocean currents and available food." Atlantic cod are now spawning near Lofoten, an archipelago in northwestern Norway. The current takes the eggs that float in the water, then the larvae, further north, where ideal living conditions await them. "If the Atlantic cod populations and their spawning grounds move north-east in the future, the fish will likely spawn in completely different stream systems," Dahlke says. "If that happens, we can not yet begin to evaluate the effects."

According to Daniela Storch, there is also good news: "Achieving 1.5 ° C weather targets can prevent the worst, preserve important spawning grounds and minimize risks for both species."