The thermal tolerance of fish is not increased by 40-45 years of exposure to increased temperature

The waters in the vicinity of nuclear power plants give a possibility to investigate how increased temperature affects fish populations. Nuclear power plants use ambient water to cool down the units where energy is produced. The cooling water is returned to the environment. As a consequence the water in the vicinity of the power plant is 2-5^oC higher than in the environment generally. In Finland, nuclear power has been produced 40-45 years, so organisms have experienced the increased temperature for that period of time. Both the temperature increase and the duration of exposure are similar to what can be expected to occur as a result of climate change.

In her Ph.D. thesis, which is defended on September 30, 2022, Giovanna Mottola used this natural exposure to evaluate, if the 40-45-year exposure was able to improve the thermal tolerance of stickleback. The results indicate first, that regardless of the previous exposure history, a short heat wave increases the highest temperature tolerated acutely, and second, that previous history of living in the high temperature of the vicinity of nuclear power plant does not affect the temperature tolerance as compared to non-exposed fish. This means that fish living at an increased temperature are closer to the tolerance limit than fish living in cooler temperatures, and if a heat wave occurs, are consequently more likely to succumb. Although the studies are only on one species, similar results have been observed with the couple of other species so far studied. This suggests that the upper thermal tolerance of a given species is fixed and cannot be evolutionary increased within the time window available in climate change scenarios.

If one then considers on the basis of the natural laboratory data the ecological status of fish populations in the climate change scenarios, it appears likely that marked disappearance of fish occurs, before the migration of more temperature-tolerant, southern fish occurs. This will be true because of the time constraints of movement. Also, coastal fish are not likely to cross open water areas, so in their case the northward migration of southern populations is further slowed down.