For Fish, Warmer Waters Are Okay, But Not Extreme Heat

The Biotest enclosure. Göran Hansson
Janet Fang 25 May 2016, 15:52

Researchers working at a site in the Baltic Sea near a nuclear power plant have revealed that fish can adjust to warmer ocean temperatures, but extreme heat will kill them. The fish are constrained by what’s described as “plastic floors and concrete ceilings,” according to findings published in Nature Communications this week.

Some aquatic species are able to tolerate warmer environments by making adjustments and acclimatizing. But there are upper limits to their flexibility, or plasticity. To explore the underlying physiological traits that determine thermal tolerance, a team led by University of Gothenburg’s Erik Sandblom studied European perch (Perca fluviatilis) from the Biotest enclosure – a 1-square-kilometer man-made coastal ecosystem near a nuclear power plant in Forsmark, Sweden. While the site maintains natural temperature fluctuations, it’s been warmed over the last three decades by heated cooling water from the nuclear reactors. The water there is 5 to 10°C (9 to 18°F) warmer than water in the surrounding archipelago throughout the year. And the migration of fish in or out of the enclosure has been limited.

"It's a fantastic model for studying climate change effects," study author Fredrik Jutfelt of Norwegian University of Science and Technology said in a statement. "It's a whole natural ecosystem experiencing long-term warming." During August and September of 2012 and 2013, adult “heated” perch were collected from the inlet into the Biotext enclosure and reference fish were caught from waters with temperatures that were the same as the surrounding archipelago. 

When the team warmed up the fish, the resting metabolic rate of the reference perch went up dramatically. The resting metabolic rate of the Biotest fish was much lower at the same temperature, and their resting cardiorespiratory functions appeared to be thermally compensated. However, their heat tolerance has an upper limit, and they’re already living very close to it. The “heated” fish weren’t able to survive temperatures that are 4.6°C (8.3°F) warmer than the waters they’re used to. 

"A species might adapt and grow well (in warmer waters) but once you get strong heat spells, the water temperature might reach lethal temperatures and kill them," Jutfelt explained. "They have a very small safety margin to their lethal temperature.”

While their base-level energy requirements and resting cardiorespiratory functions (floors) are thermally plastic, their maximum cardiorespiratory capacities and upper critical heat limits (ceilings) are far less flexible. 

Image in the text: Researchers catching European perch. Fredrik Jutfelt/NTNU

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