It’s no secret that Climate change is set to have devastating effects on a wide range of species. But, if the latest findings are to be believed, fish species will be hit harder than previously thought. Indeed, certain life stages of the water inhabitants – especially the spawn and the mating-ready adults – have a much narrower tolerance range for the water temperature. If warming continues unchecked, up to 60 percent of all fish species in the sea and freshwater could be at risk, researchers have found.
How well an animal copes with climate change also depends on how tolerant these organisms are to the rising temperatures in their habitat. When it gets warmer, the energy requirement of the metabolism and thus also the oxygen requirement increases. Living beings therefore survive a temperature increase in the long term only if they can compensate for this need. However, there are species-specific limits to this ability.
Temperature “bottle neck” in the life cycle
But there is another factor: over the course of life, the temperature tolerance changes in many animals. In phases where they need more energy for reproduction or growth, there are fewer reserves left for temperature equalization and increased breathing. “The vulnerability of species to climate change therefore depends on their most temperature-sensitive stages of life,” explains Flemming Dahlke of Germany’s Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and his colleagues. But for many animal groups, including the fish, these bottle necks are hardly known. “This limits our ability to predict whether climate change targets are sufficient to maintain healthy fish stocks, for example.”
In order to provide more clarity, Dahlke and his colleagues have now systematically evaluated all data on temperature tolerance of 694 fish species in the sea and freshwater of all climate zones in their meta-study. “Our results show that fish as embryos in the egg and as spawning adults are significantly more sensitive to heat than in the developmental stage of the larva or as a sexually mature fish outside the mating season,” says Dahlke. “In the global average, for example, fish can survive in water up to ten degrees Celsius warmer than spawning fish and fish eggs outside the mating season.”
The reason for this is that fish embryos do not yet have gills and therefore cannot actively increase their breathing. Mating-ready fish, on the other hand, must also supply their reproductive organs and germ cells with energy. This, too, makes them more sensitive.
Up to 60 per cent of fish species could have problems
These two “bottle necks” in the life cycle of the fish mean that the water temperature in the spawning areas decisively determines the survival and reproduction of the species. In these stages and areas, they are therefore particularly vulnerable to climate change – in the sea as well as in lakes, ponds and rivers. Dahlke and his team have therefore analyzed in a second part of the study how much warming could have a concrete impact on the fish species studied. To do this, they simulated future climate development and the associated increase in water temperatures in oceans and inland waters in two scenarios. One that limits warming to 1.5 degrees – that is in line with the climate target of the Paris Agreement. And one in which warming is progressing largely unchecked.
The result: “If humanity succeeds in limiting global warming to 1.5 degrees Celsius by 2100, only about ten percent of the fish species we study will have to leave their ancestral spawning areas due to too warm water,” explains Hans-Otto Pörtner of the AWI. But if global temperatures rise by five degrees, as in the unchecked climate change scenario, up to 60 percent of all fish species could experience problems. They would have to adapt either by changing their temperature tolerance. Or they leave their ancestral spawning areas and move to cooler waters. However, both involve significant risks and problems.
“When we consider that fish have adapted their reproduction to specific habitats over long periods of time and their cycles are adapted to specific food supply and ocean currents, then it can be assumed that a forced relocation of spawning areas can cause major problems,” says Dahlke. In addition, fish in rivers and lakes often cannot migrate to other areas – there are simply no paths to it. The new findings suggest that many fish species could be more vulnerable to climate change than previously thought.