“We were mainly curious about how many mass fish deaths might occur under future climate change,” said Simon Tay, a doctoral student in the Department of Biological Sciences at the University of Arkansas and lead author of the study.
The study used a decade of documented fish kills across Minnesota and Wisconsin and more than a million water and air temperature profiles from 8,891 boreal lakes.
Projections of future temperature increases as a result of climate change were also used.
Assuming the current pace of climate change does not slow down, computer models predict a 600 percent increase in fish mortality by 2100.
“It’s so dangerous, it’s so amazing,” Tay said. “It’s worrying for the future of the fish across those lakes.”
“I was surprised by the magnitude of the effect,” said Nick Phelps, an assistant professor at the University of Minnesota who helped compile the fish-kill database. “It’s a scary future if that’s the way things are.”
Phelps explained, “Currently, we estimate that there are about 500 fish deaths annually here in Minnesota. Conservative estimates from this research are that by the end of the century, there will be several thousand fish deaths each year.”
The change will be gradual over decades with a marked increase in the next 30 years.
While temperatures are expected to rise steadily, Phelps said more extreme weather events are also likely, with heat waves rapidly raising water temperatures and causing fish to die.
The study also predicts that there will be less Fish are killed during the winter. Fish deaths often occur in winter when lakes are covered with heavy snow that blocks sunlight, reducing the oxygen produced by aquatic plants. Future climate forecasts will reduce snow and ice in Minnesota lakes, which is expected to reduce winter fish mortality, Tay said.
According to researchers, the main causes of fish kills in the upper Midwest are infectious diseases and low oxygen levels in the water. High water temperature exacerbates both of these risks.
Warmer water contains less oxygen, and warmer water stresses fish making them more susceptible to disease.
Recorded fish kills document large numbers of adult fish that die in a short period and usually wash shore. There are ripple effects on the lake’s complex ecosystems that are not well understood, Tay said.
“There are many adverse effects on juveniles, fetuses and eggs, which we will not record in our datasets, because no one is recording that,” he said.
Threat to cold and warm water species
Water temperature rise is a well-documented threat to cold-water fish species such as salmon, trout, or cisco.
But the researchers say there is also a significant threat to warm-water fish species.
“Most of the documented deaths were from gill fish and sunfish, which are major fishing fish in the area,” Tay said. “These deaths are primarily due to low concentrations of dissolved oxygen.”
With fish deaths occurring so often, Phelps said, it will become more difficult for fish populations in the lake to recover. If this prediction is proven over the remainder of this century, Minnesota’s fisheries will change dramatically.
He said, “There will be a shift from a system dominated by perch, to a system dominated by bass, with the temperature of the water rising, and that is due to the effect on production and availability of food and everything else.” We’ll see that change happens over time probably through mortality events, fish kills. ”
The researchers believe that these effects could be mitigated somewhat if temperatures did not rise as much as expected. There may be management techniques in place to maintain the health of water bodies as the water temperature rises.
“We know that healthy ecosystems are more resilient,” Phelps said. “So the things that can keep our lakes healthy, like limiting coastal development, preventing invasive species from entering, and reducing chemical runoff, are all good things for the lake.”
But climate change could also mean managing a new normal where fish populations vary or are much less. Tay suggests that such a significant increase in fish deaths would be difficult to manage.
“It will probably increase the resources of the NRM, because if there are thousands of lakes that are getting warmer, there is a lot more that we can devote our time to rehabilitating,” he said. Too much mess for us to clean up in theory.”
Based on computer models that ran for this project, Tay believes the expectations in this study are conservative and not a worst-case scenario.