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Why do fish suffer heat stroke?


Ongoing global warming will result in elevated average water temperatures, as well as more frequent and intense heat waves. But what determines the upper temperature limit for fish? And more importantly, can fish adjust to increasing water temperatures? Andreas Ekström, a researcher at Gothenburg University, strives to answer these questions in his recently published PhD thesis.

Andreas Ekström

Tolerance to increasing temperature is a crucial factor for where fish can survive. It has been shown that when sea temperatures increase, some fish populations migrate north searching for cooler waters. Unfortunately, for other species that are less mobile, they have to stay and endure the heat. For these species, an extreme heat wave may be enough to locally eradicate populations of fish. Regardless if the fish migrate or die, increasing water temperatures may have fatal consequences for aquatic ecosystems and serious implications for the fishing industry.

- We currently do not know exactly what determines the temperature tolerance of fish, but evidence suggests that their heart plays a central role, says Andreas.

The fish heart, just like ours, pumps oxygen-rich blood to all parts of the body. But if the water temperature gets too high, heart function ceases completely, but why?

- My results show that the heart's ability to generate energy is reduced when it gets too hot, which may be partly due to a limitation of the heart's own oxygen supply. This in turn result in the heart being incapable to pump enough oxygen-rich blood to the body, says Andreas.

Fish may adapt to warming

A large part of Andreas Ekström's research has been conducted at the 'Biotest Biotestsjönenclosure', which is located in the archipelago outside the Forsmark nuclear power plant. This artificial lake constitutes a unique research site as it receives cooling water from the nuclear reactors, which elevates the water temperature by 5-10°C when compared to the surrounding Baltic Sea.
For a couple of summers, Andreas has collaborated with an international team of researchers and used the Biotest enclosure as a 'window' to a warmer future. They have studied how a population of perch, which is one of the species present in the heated lake, differ from the perch that live in the surrounding colder archipelago.

- Our findings show that perch living in the heated Biotest enclosure are not nearly as temperature sensitive as perch living in the colder archipelago, says Andreas. The differences in temperature tolerance likely reflect the improved function of the heart in perch from the Biotest enclosure at high temperatures. These fish have also developed a more efficient way to convert the energy the heart needs.

How will the fish survive a warmer future?

- Physiologically speaking, the perch that we have worked with should get along relatively well, as these fish are rather heat-tolerant, says Andreas. However, many other heat-sensitive species may find it harder to survive extreme conditions such as heat waves. Yet, there does seem to be an upper limit to the temperatures that fish are able to adjust their physiology to. If the temperature rise above this limit, entire fish stocks may be eliminated, which could have major consequences for aquatic ecosystems, concludes Andreas Ekström.

Andreas Ekström´s Ph.D thesis entitled “Thermal tolerance in teleost fish –importance of cardiac oxygen supply, ATP production and autonomic control” is available at

Andreas Ekström
Phonenumber: +4631-786-3547
Mobile 073 – 680 24 91,

Photo of the Biotest enclosure: Göran Hansson
Picture of Andreas Ekström: Erik Sandblom