(KUOO) — Extremely high water temperatures across the Iowa Great Lakes may be contributing to fish kills and could cause delayed mortality among several popular game-fish species.
Mike Hawkins, Northwest Regional Fisheries Management Supervisor for the Iowa Department of Natural Resources, told KUOO News late last week that surface temperatures exceeded 80 degrees this summer. Temperatures reached the mid-80s at times, including one reading above 87 degrees on Big Spirit Lake.
Fish Kill Reported in Templar Lagoon
One fish kill is taking place in Templar Lagoon on the west side of Big Spirit Lake. Hawkins said the DNR has been seeing gizzard shad in greater numbers.
“Most of the fish in Templar Lagoon that are dead are those gizzard shad; they are very small,” Hawkins said. “…But there’s also a very heavy blue-green die-off going on in the lagoon itself, and that can take oxygen out of the water.”
Disease following a large gizzard shad spawn could be another factor. Hawkins said the deaths may also represent a natural correction following the increase in the shad population.
A significant blue-green algae die-off can reduce oxygen levels in the water and, in extreme cases, cause fish to die.
Effects Could Continue as Water Cools
Hawkins said water temperatures above 80 degrees place significant strain on fish. The effects are not always immediate and may continue even after temperatures begin falling.
“We had surface water temperatures well over 80 degrees,” Hawkins said. “That puts a lot of stress on fish, and you may not get a fish kill occurring right then, but as that water temperature slowly cools down those fish continue to have stress and eventually can succumb to that.”
Yellow perch, northern pike and walleye are among the cool-water species most susceptible to high temperatures. Hawkins said delayed mortality among those species remains possible through the rest of August.
It is unclear whether this summer’s temperatures set a record because historical water-temperature information was not readily available. Hawkins said, however, that he does not recall seeing readings this high.
“In my recollection I don’t remember seeing surface water temperatures as warm as they were this summer,” Hawkins said. “We had water temperatures getting into the mid-80s. We had one day with one of our work boats on the water where the surface water temperature on Big Spirit was over 87 degrees. So, that’s incredibly warm.”
Cool-water fish can begin experiencing severe stress when temperatures reach approximately 80 degrees. Hawkins said cooler air and water temperatures would help relieve some of that pressure.
