Janesville77°

Even fish are feeling the heat

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Neil Johnson
July 19, 2012

— Tired of the heat? So are the fish.

Wisconsin Department of Natural Resources Fisheries Manager Doug Lubke confirmed a kill of about 200 northern pike at Lake Koshkonong. Anglers reported the kill Friday and Saturday.

DNR officials blame the extreme heat of the last few weeks, which the agency said has boosted water temperatures to unprecedented highs. The heat has stimulated algae and bacteria growth, both of which can stress fish.

The warm water led to kills of pike on several other bodies of water statewide, including the Wolf River and Lake Puckaway, DNR officials reported.

Recent temperature readings show that Lake Koshkonong's waters have jumped well into the 80s and peaked last week at 91 to 93 degrees, Lubke said.

"It's typical to get summer temps in the 80s on Lake Koshkonong, but readings in the low 90s, that's the extreme side of things," Lubke said. "It's stress inducing for the fish."

Fish kills can happen anytime water temperatures jump above 75 degrees, said Kurt Welke, a DNR fisheries biologist. Kills happen every year on Lake Koshkonong, but the recent die-off of pike is worse than in other years.

"It's above normal," Boyd Richter, a DNR conservation warden, told WCLO radio.

Though Lake Koshkonong is a relatively large, open body of water, it is shallow. It has a uniform depth between 3 and 6 feet with no steep drop offs.

Unlike in deeper bodies of water, the lake's waters are a uniform temperature from surface to bottom, Welke said. Besides areas where creeks and natural springs trickle in, fish are hard pressed to find cooler waters in shallow lakes such as Koshkonong.

"They have no retreat," he said.

Fish woes linked to warm water are numerous, Welke said. For one, warmer water tends to have lower oxygen levels, or it can support booming algae growth cycles that can cause extreme spikes and dips in water oxygen levels.

Those factors, along with the water's higher temperature, can throw off metabolism, diminish feeding and halt cellular functions in some fish species, according to the DNR.

"The fish are riding a roller coaster," Welke said. "It's a death by 1,000 cuts."

Then factor in bacteria that Welke said can proliferate in warmer water and attack already weakened fish. One common bacteria type is Aeromonas hydrophila, which can cause lesions and fatal hemorrhaging in fish.

It's too much for some species, particularly coldwater fish such as northern pike, which tend to thrive and be most active in waters between 50 and 74 degrees, according to the DNR.

Even if temperatures begin to drop off soon and return to normal, it could take some time for water temperatures to ease back enough to erase the threat of heat-related fish kills, Welke said.

Yet while DNR officials consider the pike kill on Koshkonong to be significant, Welke said it's likely a blip compared to the number of pike in the 10,600-acre lake. It's unlikely the kill will have a lasting impact on the pike population.

"This too shall pass," he said.

The lake's pike population is healthy, Lubke said. The DNR will be able to gauge what, if any, effect the kill might have had when pike begin spawning next year.

Richter, the DNR conservation warden, told WCLO radio that the DNR also has reported small kills of white bass on the Rock River near Lake Koshkonong. That species, along with some other game fish, are sensitive to water temperature and oxygen fluctuations.

The hot, dry weather has been tough on aquatic birds, too, the DNR said.

At least 50 dead birds, including great blue herons, teal, ducks and pelicans, were being collected by the DNR in an area north of Horicon Marsh National Wildlife Refuge, the Milwaukee Journal Sentinel reported.

Officials feared the birds had died from botulism, which the DNR reports can stem from warm or low water.

Meanwhile, commercial net fishers who contract with the DNR to remove carp from Lake Koshkonong are still actively working the lake, Lubke and Welke said.

In the past, net fishing has been blamed for game fish kills on the lake, but no fish kills linked to commercial fishing have been reported this year, Lubke said.



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