Warmer Seas a Threat to Fishes


Fish can adjust to warmer ocean temperatures, but heat waves may be more likely to kill them, a team of researchers from the University of Gothenburg, Norway and Australia reports in an article published this week in Nature Communications.


A warmer climate makes it more difficult for fish to survive. Photo: Timothy Clark.

"In a future with a warmer climate and more extreme weather, it will be more difficult for fish to survive," said Erik Sandblom, researcher at the Department of Biological and Environmental Sciences.

Sandblom and his colleagues studied European perch that live in a unique enclosed basin of warm water off the Swedish Baltic Sea coast. The man-made basin, called the Forsmark Biotest Enclosure, was created three decades ago by piping warm water from the nearby Forsmark nuclear power plant into an enclosed basin.

Photo of Biotestsjön The water in the enclosure is between five and ten degrees warmer than the surrounding Baltic Sea depending on the season, but otherwise experiences natural daily and seasonal fluctuations. It offers researchers a kind of crystal ball on what can happen to fish in a warmer world.

"It allows us to study how fish adapt to a warmer environment over time, which is crucial to the understanding of the impact of the ongoing climate change," said Sandblom.


Compared body functions of perch

He and his colleagues compared body functions of perch living in the enclosure with perch living in the normally tempered waters of the archipelago surrounding the enclosure. The perch living in the normally tempered water was then warmed up for one day in tanks with water from the Biotest Enclosure to reach the same body temperature as the perch living in the enclosure.

"In this way we can compare how the adjustments to a prolonged rise in temperature that corresponds to the situation with climate change, differs from short-term heating in the lab,” says Erik Sandblom.

In a warm environment, the body requires more oxygen to maintain its functions. The research shows that perch living in the Biotest Enclosure adjust their oxygen uptake to the warmer environment.

Portrait of Erik Sandblom

A particularly important factor in this context is the span between the maximum oxygen uptake and oxygen requirement at rest, the aerobic scope. Previous research has indicated that this range decreases when the oxygen demand at rest increases with heating. The new research shows that perch living in the Biotest Enclosure down-regulate the oxygen needed at rest, while they maintain their maximal oxygen uptake. In this way, they have an unchanged oxygen uptake capacity, and can handle the warmer environment.

"Other body functions have also changed, such as the number of heart beats per minute. The resting heart rate of the fish in the Biotest Enclosure is lower than for the perch outside the lake when tested at the same temperature, but again, the maximum heart rate is unchanged,” said Sandblom.

A ceiling of the adaption

But even if the fish adapt to a warmer environment, there is a ceiling to how much they are able to adapt. At a certain temperature the fish dies. Research shows that perch living in the Biotest Enclosure only has shifted this limit upward slightly in the warmer environment. This makes the difference between the body temperature of the perch and its upper lethal temperature limit several degrees less for fish in the Biotest Enclosure than the fish living in normal temperature water.

"Perch does not completely compensate its lethal limit, and as the difference between their body temperature and their lethal limit becomes smaller, they also become much more sensitive to transient extreme environmental events, such as heat waves," said Sandblom.

In the future the weather is expected to get warmer, and extreme weather is expected to be more frequent.

"These fish would have difficulties surviving in such an environment,” said Sandblom.

Contact information

Erik Sandblom, Associate Professor, +46 (0)703-286358


Top: Timothy Clark.
Middle: Göran Hansson.

Link to the article in Nature Communications:

"Physiological constraints to climate warming in fish follow principles of plastic floors and concrete ceilings"