Current marine heat waves could increase short-term resistance of fish towards warm waters, but this capacity might be damaged by long-term temperature increase with climate change in Baltic Sea (Dissertation defense: MSc Giovanna Mottola, 30.9.2022, animal physiology)

After conducting a research on fish populations in the Baltic sea,  Doctoral Candidate MSc Giovanna Mottola found that some fish can still survive from current summer heat waves. But according to the research the general increase of temperature due to climate change all the year around can threaten this capacity.

Marine heat waves are hitting hard fish populations in the Baltic Sea. It is therefore important to investigate what the effects of heat waves on fish are.

– I am studying the effects of environmental stresses, such as temperature changes and pollutants on the physiology of fish, for example on the heart and respiratory system. With increasing occurrence of summer heat waves, I started to be interested in the effects that such temperature increase would cause to fish.What happens to fish body when a heat wave occurs?Will they be able to survive heat waves in future when Baltic Sea waters temperatures will be higher compared to the present times?, Mottola says.

To answer these questions, Mottola needed to find a place where fish were living already in “future” warm waters, andasked help from Olkiluoto and Loviisa nuclear power plants. “

– One would wonder why ask for help from nuclear power plant companies for studying fish? The reason is simple! When producing energy, nuclear power plants warm up the surrounding waters by 2-5 ºC, because they are using ambient water as coolant. Thus, fish living in these areas have already been exposed to predicted future climate conditions for many generations,about 50 years since the construction of plants. Therefore, nuclear power plants areas became my time machines to study whether fish from a future climate scenario will be more resistant to warm waters and will be able to survive heat waves, Mottola says.

Even a slight temporary rise in the sea temperature can cause massive problems for the fish in the future

Tutkijat pyydystivät kaloja Pyhärannan kontrollialueella.
Researchers catching three-spined sticklebacks from one of the control areas in Pyharanta.

In order to understand if fish in the vicinity of nuclear power plants could resist heat waves better than fish in other areas, the researchers caught three-spined sticklebacks from the vicinity of Olkiluoto and Loviisa plants, and from control areas in the Baltic Sea, Pori, Pyhäranta, Kotka and Porvoo. The researchers then measured the thermal tolerance of the fish, that is the maximum temperature that they can tolerate acutely.

The thermal tolerance of fish from the vicinity of nuclear power plant was similar to that of the ones collected from the other areas. This suggests that the high temperatures in the vicinity of the nuclear power plant did not increase the resistance of fish to warm waters.

–  What happened when we exposed them to a heat wave? After one-week heat wave, the thermal tolerance of the fish was measured again. The acute heat wave increased the fish resistance to warm water. However, this increased temperature tolerance was similar in all fish, suggesting that fish experiencing future climate scenario for several generations could not tolerate high temperatures better than the other fish. I concluded that fish possess a small capacity to resist current heat waves, but such capacity has strong limits in future scenarios as the heat waves will become stronger and fish will be already living in their upper limits even without heat waves, Mottola emphasizes.

Future projection for climate change predicts that the average temperature, as well as the occurrence of heat waves will increase.

–  My research suggests that fish cannot increase their resistance to high temperatures easily. This would cause many populations to suffer because of heat waves, as has already been seen by many fish die-offs in recent years, says Mottola.


MSc Giovanna Mottola defends her dissertation in animal physiology titled ”The Phenotypic Plasticity of Thermal Tolerance and Its Modulation in a Complex Environmental Scenario” at the University of Turku on 30 September, 2022 at 12.00 (Tauno Nurmela lecture halI, Agora, Yliopistonmäki).

Opponent: Professor Gudrun De Boeck, University of Antwerp, Belgium
Custos: Associate Professor Katja Anttila, University of Turku

The audience can also participate in the defense by remote access (Zoom)

The University of Turku actively follows the coronavirus situation and the authorities’ instructions. The University s its instructions according to the situation.

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Created 23.09.2022 | Updated 27.09.2022