Study links warmer waters to fish die-offs |

Study links warmer waters to fish die-offs

Fish die-offs in Wisconsin lakes are forecast by scientists to double by mid-century and quadruple by the end of the century due to warmer water temperatures, according to a study published today in the journal Nature Climate Change.
Sierra Sun file photo

Climate change may be a contributing factor in freshwater fish die-offs, according to a recent study published in the journal Nature Climate Change.

Researchers from the University of California, Davis, and Reed College in Portland, Oregon, analyzed a database of freshwater fish die-offs in Wisconsin combined with lake-specific temperature profiles simulated for north temperate lake ecosystems, and found that more than 100 of 502 mass mortality events in fresh water lakes, recorded between 2004 and 2014 in the state, were linked to warmer water in summer months and periods of abnormally high temperatures.

“This research takes a substantial step forward in connecting the dots between the occurrence of rare ecological catastrophes and climate warming,” said lead author Samuel Fey, a mass mortality specialist and assistant professor at Reed College in a news release from UC Davis. “Historically, the causes of animal die-offs have been difficult to study because these events tend to be rare and unpredictable.”

Die-offs driven by extreme summer conditions occurred disproportionately in lakes with warmer average surface temperatures, according to the study, and during periods of extreme heat.

“This study is unveiling another reality of what climate change will look like for north-temperate lakes across the world.”— Andrew RypelAssociate professor at UC Davis

During the years in which die-offs were measured in Wisconsin lakes, a median of four observable summertime die-offs occurred annually in the study region. In future decades, scientists have forecast that fish die-offs driven by summer environmental conditions could double by mid-century in north temperate lakes, and could quadruple by the end of the century, particularly at southern latitudes of the state.

“This study is unveiling another reality of what climate change will look like for north-temperate lakes across the world,” said co-author Andrew Rypel, an associate professor at UC Davis and the Peter B. Moyle and California Trout Chair in Coldwater Fish Ecology, in a news release. “Analyses provide an up-close view of how fish populations will die, and how species will die due to climate change. It’s particularly problematic for freshwater fishes in landlocked lakes, as they don’t have the ability to adapt to changing climates by migrating. Effects have to be dealt with by managers right where they are.”

Temperature, according to researchers, is fundamental to the study of cold-blooded animals because it impacts vital ecological rates, including reproduction, growth, susceptibility, to disease, and migratory ability. Fish species in the study, such as bass and walleye, have critical thermal maximum values.

Researches say the anticipated increase in die-offs will likely accelerate biological changes for north temperate lakes in terms of fish community compositions and traits of individual fish. One potential outcome is a decrease in fish body size because size is a primary factor limiting the ability of fish to fully oxygenate, though the validity of this mechanism is still under debate.

Additionally, high surface temperatures often accompany an increase in the duration and intensity of lake stratification, according to the study, resulting in less oxygen at lower depths thus decreasing the suitable habitat for fish. Warm water also holds less dissolved oxygen, which may not be enough for the survival of certain aquatic species.

In Lake Tahoe, the process of deep-water mixing, according to University of California, Davis Tahoe Environmental Research Center’s 2017 State of the Lake Report, hasn’t occurred in six years. Surface water temperatures reached record highs during that year, according to the report, which stated the average surface water temperature in 2017 was 53.0 degrees compared to 50.3 degrees in 1968.

“As it gets warmer, some (native species) get stressed, and so it makes them more prone to disease,” said Geoffrey Schladow, director of the Tahoe Environmental Research Center in a statement made in 2018. “It also creates new openings for species that couldn’t have existed in Tahoe 20 years ago, but now it’s started opening up a niche for them. It’s making the possibility of new invasive species surviving higher than it would have been in the past.”

Future mass mortality events will likely dampen the nation’s sport fishing industry as well. According to a 2018 report by the American Sportfishing Association, freshwater fishing contributed $41.9 billion to the nation’s gross domestic product. In California, sport fishing brings in more than $60 million annually, according to the California Department of Fish and Wildlife.

Justin Scacco is a reporter for the Sierra Sun. Contact him at

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