We already knew that warming waters will create unlivable conditions for many adult fish. The authors of a new study now show that the effect will be even more pronounced for embryos and reproducing individuals. As a result, even if humanity reaches its current climate targets, more than a third of the fish species will no longer be adapted, at the end of the century, to the environment they occupy today.
The German-based team found that there is a “thermal funnel effect” in the life cycle of the majority of fish species. While larvae and non-brooding adults are relatively resistant to temperature increases, embryos and adults in the reproductive period are much more sensitive to it: the latter can tolerate, on average, a temperature window narrower by 20 ° C .
Researchers came to this conclusion by analyzing existing studies of 694 species of marine and freshwater fish. In order to fill in some missing data, they extrapolated the values of other genetically related species. Review Science publish their results in its edition this Friday.
“The greater sensitivity of eggs and breeding adults means that marine and freshwater fish live much closer to their thermal limits than previously thought,” said Jennifer Sunday, a biologist from the. McGill University, which signs a analysis accompanying publication in the journal.
In the scenario where the CO concentration2 In the atmosphere reached 1000 parts per million in 2100, 60% of species can no longer complete their entire life cycle in their current environment. In the scenario where the CO concentration2 returns to its level from 2005 to the end of the century, only 10% of species would experience this drama.
“Clearly, several species of fish, as well as people whose diet is based on healthy fish, would benefit from intensified efforts to stabilize global warming at 1.5 ° C or less,” write Flemming. Dahlke and colleagues at the Alfred Wegener Institute for Polar and Marine Research.
All fish can survive within a limited range of temperatures. Above or below these thresholds, they have trouble controlling the level of oxygen in their body – or, in other words, they have trouble breathing. When it’s too hot, their metabolism consumes too much oxygen; when it is too cold, their bodies cannot transport enough oxygen to their tissues.
Since the embryos have a cardio-respiratory system still in development, they resist smaller temperature differences. Adults who produce eggs or sperm must feed a more massive body, without having a better oxygen supply.
In practice, climate change means that fish will have to move to colder waters, or even try to adapt to their local environment.
For populations that remain in place, certain mechanisms will allow them to compensate – at least temporarily – for the death of the overheated embryos, explains Jennifer Sunday. “Environmentalists know full well that, even under normal circumstances, not every egg in a fish becomes an adult,” she said in an interview. We may not see a population decline right away, but one day the effect may suddenly turn out to be dramatic. “
As for the populations who will be forced to move, there is no indication that they will find suitable new habitat. “Is their food there? Do they choose their spawning grounds according to the temperature, or because they find good food there? Will a new predator eat their eggs? It’s very difficult to predict, “says Mme Sunday.
The authors of the article published on Friday warn that their forecasts are probably conservative, since they do not consider the deoxygenation, acidification, and the increased frequency of episodes of extreme temperatures that climatologists predict for the next decades.
In addition, fish may find it difficult to eat. Last year, an international collaboration of researchers, including several Canadians, felt that each degree of warming would cut ocean animal biomass by 5%. They associated this decline with warming of surface waters limiting the supply of nutrients from deeper layers.
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