Record summer heat killing more fish

This summer’s unusually high temperatures and continuing drought are killing fish across the Great Lakes region.

“There’s nothing wrong water quality-wise,” said Randy Schumacher, fisheries supervisor for the Wisconsin Department of Natural Resources. “The species simply can’t tolerate that hot of water for this extended period of time.”

There were multiple reports of fish kills in early July across Michigan, Minnesota, Wisconsin, Indiana and Illinois, according to state fisheries supervisors. There have been at least 60 separate incidents in Illinois. About a dozen have been reported in Indiana.

As many as 350 northern pike were confirmed killed in one Wisconsin report, though most of the kills are of less than 100 fish.

A few fish kills have also been reported in Ohio, New York and Pennsylvania, but they have been linked to sources other than high water temperatures by each state’s regulating agency.

In Wisconsin and Michigan, most of the reports are from the south. In other states they are less region-specific.

“We are seeing fish kills all over, from farm ponds to cooling lakes to regular lakes, in smaller watersheds, and even along some of our larger rivers like the Kankakee River, Fox River and the Mississippi River,” said Debbie Bruce, chief of the fisheries division for the Illinois Department of Natural Resources.

Pike are among the species most affected. But die-offs have also been reported among brown trout, bass, suckers, bluegill, catfish and Asian carp —though people aren’t too concerned about the latter.

Summer fish kills are not unusual, and not much can be done to prevent them, Bruce said. However, this summer’s heat and excessive drought have resulted in more of them.

As less rain falls and temperature rises, the water and oxygen drop, stressing the fish. This is because less water makes it easier to reach high water temperatures, and oxygen dissolves faster in cooler water than it does in warm water.

The amount of fish affected is hard to estimate, but each report usually ranges from 20 to 100, Schumacher said.

A few of the reports in Wisconsin, Minnesota and Illinois were of more than 100 fish.

“There was one reservoir near Rochester, Minn. where they found 200 adult northern pike,” said Jerry Johnson, fisheries supervisor for the Minnesota Department of Natural Resources. The surface water temperature at the reservoir was 90 degrees.

With this summer’s low water and air temperatures often reaching 100 degrees, many state fish experts report surface water temperatures of 90 degrees or higher, Johnson said. Some Great Lakes fish species like trout and northern pike can’t tolerate water temperatures above 75 degrees, Schumacher said.

Fishermen can help reduce fish mortality to some extent. “Fish late at night or early in the morning when it’s coolest and less stressful for the fish,” said Gary Whelan, fish production manager at the Michigan Department of Natural Resources. Also, handle them as little as possible.

“If you’re uncomfortable, the fish are probably uncomfortable too,” he said.

Most of the fish kill reports came in the first half of July and have slowed since then. Some fish experts believe it is a result of cooler weather. Unfortunately the high temperatures are back again and it will likely result in more reports. “Any place experiencing this heat wave is probably going to be losing fish,” Schumaker said.