Temperature drop could ease fish killsAs the Dakotas’ temperatures move away from the extreme heat of July, the cooling could help ease the trend of localized fish kills on the states’ lakes and rivers.
By: By Dirk Lammers, Associated Press, The Jamestown Sun
SIOUX FALLS, S.D. — As the Dakotas’ temperatures move away from the extreme heat of July, the cooling could help ease the trend of localized fish kills on the states’ lakes and rivers.
Warm August days are typically the ones biologists worry about, but a milder forecast could mean that fish simply experienced summer stressors earlier than normal this year, said Scott Gangl, a fisheries management director for the North Dakota Game and Fish Department.
“They came really early and that got us really scared,” Gangl said. “If we had another week of hundred-degree temperatures in July and August, we could start experiencing it even more.”
Nearly a dozen North Dakota water bodies had some level of fish kills this summer, affecting northern pike, trout, panfish and yellow perch.
In South Dakota, low flows, high water temperatures and elevated ammonia levels late last month contributed to hundreds of fish that died and washed up along the Big Sioux River in Sioux Falls. Some kills have also been reported on the James River in the Sand Lake area and south of Huron, said Todd St. Sauver, a South Dakota Game, Fish and Parks regional fisheries manager.
St. Sauver said fish are more susceptible to bacteria, viral infections and high ammonia levels when river water is warm. And as water levels decline, fish are pushed into more crowded pools where they compete for oxygen.
Many of South Dakota’s lakes turn green with algae blooms each summer, which often leads to isolated fish kills that can claim up to a few hundred fish. Plants produce oxygen in the day but use it at night, so sundown on an algae-ridden section of lake could choke some fish from the air they need to survive, St. Sauver said.
“They look kind of bad in that one area, but there are always lots of other fish that survive,” he said.
People often wonder why fish on larger lakes don’t simply migrate to deeper water, but it’s not that simple, Gangl said.
“Often times the deeper, cooler water in some of our shallow prairie lakes doesn’t have any oxygen in it,” he said.
And Gangl said when it gets hot, fish just grow lethargic, stop moving and succumb to the elements, he said.
St. Sauver said some relief from the heat should help, but fish need water to survive, so persisting dryness will continue to stress the state’s fishery.