Climate change could force many of the world’s fish species to leave their traditional spawning areas and even affect their mating patterns, researchers suggest.

In a new meta-study, scientists from Germany’s Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) report that fish that are ready to mate and their subsequent young are particularly sensitive to changes in water temperature.

A team led by marine biologist Flemming Dahlke compiled published observational and experimental data to assess the life-stage-specific thermal tolerances for 694 marine and freshwater fish from waters worldwide.

“Our findings show that, both as embryos in eggs and as adults ready to mate, fish are far more sensitive to heat than in their larval stage or sexually mature adults outside the mating season,” Dahlke says.

“On the global average, for example, adults outside the mating season can survive in water that’s up to 10 degrees Celsius warmer than adults ready to mate or fish eggs can.”

The findings apply to all fish species, the researchers write in a paper in the journal Science.

It’s due in part to the anatomy of fish. All organisms must breathe for their bodies to produce energy, but fish embryos don’t have the gills that would allow them to take in more oxygen.

Adult fish do, but those that are ready to mate need extra oxygen to supply the egg and sperm cells they produce. Even at lower temperatures, the researchers say, their cardiovascular systems are under enormous strain.

In a second stage of the study, Dahlke and colleagues analysed to what extent water temperatures in the spawning areas of the species investigated would likely rise due to climate change.

They conclude that only 10% of the investigated species would be forced to leave traditional spawning areas if climate warming is limited to 1.5 degrees Celsius by the year 2100, but this could rise to as much as 60% if high greenhouse-gas emissions produce average warming of five degrees Celsius or more.

Affected species would then be forced to either adapt through biological evolution or to mate at another time of year or in some other place.

“Some species might successfully manage this change,” Dahlke says, “but if you consider the fact that fish have adapted their mating patterns to specific habitats over extremely long timeframes, and have tailored their mating cycles to specific ocean currents and available food sources, it has to be assumed that being forced to abandon their normal spawning areas will mean major problems for them.”

Fish living in rivers and lakes have the added problem of physical restraints: migrating to deeper waters or to cooler regions is nearly impossible.