By: Dana Sackett

News sources are reporting that the record temperature for continental Europe (48oC or 118.4oF) may be broken this week;, Portugal has already set a new record for all time high temperature. Source.

In my recent adventures as a military spouse, I found myself and my family uprooted once again and headed to a new home; this time overseas in Germany.  We made this move just a few short weeks ago during the intense heatwave that wracked the eastern portion of the United States.  In fact, the day we flew away from the states it was ~100 degrees as far north as New York (see here for a report on this heat wave).  In the chaos of our move, I had hope that we would arrive in a relatively cooler Germany.  Germany is much further north than where we started in the US and relatively well known for its milder summer climate (for instance, there are little to no central air conditioning units in Germany).  While it was very slightly cooler when we first arrived, just a few days later and even Germany was seeing temperatures high enough to shock the locals; and I don’t just mean the people.  News reports quickly began to surface of the numerous locations across Germany where thousands of dead fish were being pulled from the water.

But why is it that there are such large fish kills across all different types of water bodies and species during heatwaves?  You may assume that it is the heat itself and that the fish are intolerant of high temperatures; and in some cases you would be right.  Different species of fish do have different levels of tolerance to high temperatures (thermal tolerance) with high temperatures or a quick rise in temperatures being lethal.

The current heat wave in Europe is causing fish kills. In Hamburg, Germany firefighters are pumping freshwater into waterways in an attempt to raise oxygen levels and save the aquatic life. Source.

However, another culprit to such widespread fish death during heat waves is oxygen. Temperature and oxygen have a very strong predictable relationship because of a physical characteristic of water: hot water cannot hold as much dissolved oxygen as cold water.  So the hotter the water the less dissolved oxygen it can hold for the fish to breath.  Also, while some fish are or can become more accustomed to low oxygen (hypoxic) waters or find ways around the low oxygen areas, it is difficult for any fish to survive when the water has no oxygen at all (anoxic) or the change in temperature and oxygen levels are too rapid to allow the fish to acclimate. Thus, the culprit to the widespread death of fish during heatwaves is often suffocation.  See here for another article on how these widespread fish kills can also be caused by high rainfall, nutrients and algae blooms, which can be exacerbated by high temperatures.


The amount of dissolved oxygen water can hold is determined by temperature and atmospheric pressure with warmer waters holding less dissolved oxygen.  Source.

The ability of fishes to cope with rising temperatures and lower dissolved oxygen levels will depend on their ability to shift to more favorable environments, physiologically acclimate and/or adapt through natural selection (when those individuals with the traits needed to survive these harsher low oxygen conditions survive to pass on those traits).  Fishes in landlocked freshwater systems or fishes that are prevented from leaving smaller regions due to an intolerance of other surrounding conditions (saltwater, elevation, barriers such as dams) will have fewer options to move to more survivable conditions.  With climate change continuing to cause more frequent and intense heatwaves across the world, understanding how fish, an important natural resource to millions, will survive or adapt is vital.

Satellites that monitor vegetation, plant growth and forests captured the consequences of the current and prolonged heat wave on Germany, which is having adverse impacts on agriculture, wildlife, and have led to forest fires. Photo Credit: Copernicus Sentinel/ESA/dpa   Source.

References and other reading material:





Chrétien E, Chapman LJ. 2016. Tropical fish in a warming world: thermal tolerance of Nile perch Lates niloticus (L.) in Lake Nabugabo, Uganda. Conservation Physiology 4 https://doi.org/10.1093/conphys/cow062

Elshout PM, Dionisio Pires LM, Leuven RS, Wendelaar Bonga SE, Hendriks AJ. 2013. Low oxygen tolerance of different life stages of temperate freshwater fish species. Journal of Fish Biology 83: 190-206.

McBryan TL, Anttila K, Healy TM, Schulte PM. 2013. Responses to temperature and hypoxia as interacting stressors in fish: implications for adaptation to environmental change. Integr Comp Biol 53: 648–659.

Portner H.-O. 2010. Oxygen- and capacity-limitation thermal tolerance: a matrix for integrating climate-related stressor effects in marine ecosystems. Journal of Experimental Biology 213: 881-893.

Rajaguru S. Ramachandran S. 2001. Temperature tolerance of some estuarine fishes. Journal of Thermal Biology 26:41-45.

Rogers NJ, Urbina MA, Reardon EE, McKenzie DJ, Wilson RW. A new analysis of hypoxia tolerance in fishes using a database of critical oxygen level (Pcrit). Conservation Physiology: https://doi.org/10.1093/conphys/cow012

Speers-Roesch B, Norin T. 2016. Ecological significance of thermal tolerance and performance in fishes: new insights from integrating field and laboratory approaches. Functional Ecology, 30: 842-844.


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