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Hundreds of northern pike have died in southern Wisconsin waters over the last several days, apparently due to heat stress.

Dead northern pike have been reported on Big Muskego Lake, Lake Puckaway, Grand River Marsh, Lake Poygan, Lake Winneconne, Partridge Lake and the Rock River above Lake Koshkonong.

Responding to reports from anglers, Department of Natural Resources personnel counted 276 dead northern pike and four dead common carp on a boat tour last Friday through Boom Bay, Lake Poygan, the Wolf River and Partridge Lake.

The northern pike were adult fish ranging from 22 to 40 inches in length.

Although the DNR continues to look into causes for the die-off, the reason appears to be the top news story over the last week in Wisconsin and much of the nation: Heat.

"Nothern pike start to die when water temperatures reach 88 to 90 degrees," said Randy Schumacher, DNR district fisheries manager. "They are very tolerant of cold water and even low oxygen, but not heat."

Schumacher said the die-offs were not likely related to VHS fish disease, which is present in the Winnebago system, but is most active in cool and cold water temperatures. And the disease is not present in all the waters that have had recent northern pike kills.

Low dissolved oxygen also does not appear to be the culprit. Preliminary tests showed the waters with fair to good levels of oxygen.

But last week's 100-plus degree air temperatures pushed water temperatures to extremely high levels.

The DNR reported surface water temperatures of as high as 91.5 degrees last Friday in the Winnebago system. The temperature 3 feet below the surface was 88 degrees.

Northern pike are found in northern latitudes, including Alaska, Canada, Russia and Scandinavia. Wisconsin is near the southern end of the specie's range.

All of the recent northern pike kills were on relatively shallow, fertile waters, Schumacher said.

Walleye, arguably the most popular game fish in the state and present in good number in the Winnebago system, have not been part of the recent die-offs. Schumacher said walleye and smallmouth bass are more heat tolerant than northern pike.

Schumacher said the agency is attempting to find dead fish in "decent" shape to submit for necropsies.

It's not known what percentage of the northern pike population has been affected, although no small pike were reported dead.

Schumacher said future fisheries assessments will help quantify the extent of the fish kills and related changes in northern pike populations.