Authorities are tallying up the losses which could run into the millions of dollar as the worst drought in 56 years expands, devastating the corn and soybean crops and forcing ranchers to cull their herds due to scorched pasture.
"Nationwide we are talking tens of millions to hundreds of millions (of dollars in losses). It just depends upon how long it lasts and how widespread it becomes," said fisheries biologist Dan Stephenson of the Illinois Department of Natural Resources.
"If this drought persists into fall, when the duck and other waterfowl pass through on their way south, there could be a larger problem," Stephenson told Reuters.
In Iowa, losses were estimated at $10.1 million after 37,000 fish were found dead along a 42-mile stretch of the Des Moines River from the dam in Eldon to the Farmington Bridge in the northeast of the state.
"Temperatures were extremely high ... I mean 97 degrees (Fahrenheit; 36 Celsius) is essentially unheard of on this stretch of the river and it's certainly higher than anything I've ever seen," says fisheries biologist Mark Flammang of the Iowa Department of Natural Resources.
Flammang said the majority of fish killed in Iowa were shovelnose sturgeon, with a value of $116.20 per lb based on guidelines from the American Fisheries Society.
After one of the mildest winters on record, the weather turned dry and temperatures began soaring into the triple-digit degrees Fahrenheit in the Midwest, which grows 75 percent of the country's corn and soybean crops.
Excessive heat arrived early for the game fishing industry, in June rather than the usual August to September. When June's record high temperatures burned through the Midwest, it was too late for the fishing industry to fix the problem.
"We install large pond and lake aeration systems to keep the water moving and oxygenated during stressful times, however the dilemma here is that these systems need to be installed in spring to start circulating water before the heat arrives," Nate Herman, owner of Herman Brothers Lake and Land Management Company in Peoria, Illinois, said. "Cooler water at the bottom needs to be mixed slowly before summer or you kill your fish yourself."
Fish are particularly susceptible to even subtle changes in their environments. Oxygen levels usually fluctuate during the day and night depending on temperature and other factors.
But the hot weather heated some waterways to between 85 and 90 degrees, forcing oxygen levels in the water down to a degree that caused fish to suffocate.
"The warmer the water, the less oxygen it can hold," said Herman.
Five parts per million of oxygen found in water is conducive to fish, three parts per million stresses them, and at two parts per million or less they start to die, he said.
The drought hit cool-water species of fish like black crappie, northern pike and walleye disproportionately harder as they are not accustomed to warm conditions.