Climate change increases frequency of mass fish kills

As global temperatures rise, so does the prevalence of mass fish kills, according to a new study led by the University of Arkansas. Such catastrophic events can have severe impacts on the function of a variety of ecosystems, endangering fish stocks and reducing the world's food supply. Experts argue that the frequency of these events is constantly increasing, with potentially dire consequences for the entire world if greenhouse gas emissions are not controlled.

That's the conclusion of a recent paper co-authored by two members of the University of Arkansas Department of Biological Sciences: PhD student Simon Tye and Associate Professor Adam Siepielski, as well as several of their colleagues.

The study, "Climate Warming Amplifies Frequency of Mass Fish Mortality Events in Northern Temperate Lakes," compiled 526 documented cases of fish kills in Minnesota and Wisconsin lakes between 2003 and 2013. The researchers discovered three main causes of these events: infectious diseases, summer deaths and winter deaths.

The researchers then narrowed their focus to summer kills fish kills associated with warm temperatures. They found a strong relationship between local air and water temperatures and the occurrence of these events, meaning that they increased in frequency as temperature increased. Also, their models using either air or water temperature provided similar results, which is important because air temperature data is more widely available than water temperature data around the world.

Finally, with a historical baseline established, the team used models based on air and water temperature to predict the frequencies of future summer kills.

Based on local water temperature projections, the models predicted an approximately six-fold increase in the frequency of fish kill events by 2100, while local air temperature projections predicted a 34-fold increase. Importantly, these predictions were based on temperature projections from the most severe climate change scenario, which was the only scenario with the data needed for these analyses.

As Tye explained, "If there are now eight summer deaths per year, the models suggest we could have around 41 per year based on water temperature estimates or around 182 per year based on air temperature estimates."

"We believe that the water temperature model predictions are more realistic, while the air temperature model predictions indicate that we need to better understand how and why regional air and water temperature estimates differ over time to predict how many mortality events can occur".

However, their models reveal strong associations between rising temperatures and the frequency of ecological catastrophes.

Although the study used data related to northern temperate lakes, Tye said the study is relevant to Arkansas. "One of the paper's findings is that similar deviations in temperature affect all types of fish, so a regional heat wave could lead to both hot and cold water fish mortality," he said.

The results were sobering. Based on local water temperature projections, the models predicted an approximately six-fold increase in the frequency of fish kill events by 2100, while local air temperature projections predicted a 34-fold increase. Importantly, these predictions were based on temperature projections from the most severe climate change scenario, which was the only scenario with the data needed for these analyses.

As Tye explained, "If there are now eight summer deaths per year, the models suggest we could have around 41 per year based on water temperature estimates or around 182 per year based on air temperature estimates."

“We believe that the water temperature model predictions are more realistic, while the air temperature model predictions indicate that we need to better understand how and why regional air and water temperature estimates differ over time to predict how many mortality events can occur”.

However, her models reveal strong associations between rising temperatures and the frequency of ecological catastrophes.

Although the study used data related to northern temperate lakes, Tye said the study is relevant to Arkansas. "One of the paper's findings is that similar deviations in temperature affect all types of fish, so a regional heatwave could lead to both hot and cold water fish mortality," he said.

"Specifically, climate change is more than a gradual increase in temperatures because it also increases temperature variation, such as we experienced much of this summer," he explained.

Siepielski added: “This work is important because it demonstrates the feasibility of using readily available data to anticipate fish kills.

"As with many examples of how a warming climate is negatively affecting wild animal populations, this work reveals that extreme temperatures can be particularly damaging."

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