Fish in high-flow zone of rivers may be the first victims of climate change

Maanantai 25.9.2023 klo 18.56 - Mikko Nikinmaa

Fish living in high-flow areas of rivers have chosen (or evolved in) that habitat, because they need a lot of oxygen for their high activity. Flowing water becomes well oxygenated, as it is continuously exposed to air. However, the amount of oxygen taken up by water decreases with increasing temperature, simultaneously as poikilothermic fish need more of it, as temperature increase speeds up their metabolism.

If temperature increases enough, fish heart cannot pump enough oxygenated blood to sustain metabolism. This we have clearly shown in our article Anttila et al, Comp Physiol Biochem A 275, 111340 (2023). Although the study was done on seabass, the principle holds also for river fish. The amount of oxygen initially reaching the gills, where it is taken up, further decreases the maximal temperature the fish can tolerate.

Zhi et al (Nature Climate Change 2023;  https://doi.org/10.1038/s41558-023-01793-3) have shown that the oxygen level in rivers throughout Europe and United States has decreased for the past forty years. While increased temperature was the main driver, the oxygen level decreased more than expected because of temperature increase. It is possible that the general eutrophication with increased oxygen consumption.

Regardless, associated with the physiological response to increased temperature of fish, the temperature-caused decrease in maximal amount of oxygen the water holds, can cause extinction of salmonids living in the fast-flowing head streams.