RATHBUN LAKE — While approximately 4,500 fish have died at Rathbun Lake in recent days, that isn’t nearly enough to raise alarm about conditions in the south-central Iowa lake.
Mark Flammang, the fisheries management biologist for Rathbun Lake, began investigating a fish kill — mainly crappies and freshwater drum — in early March.
Flammang said while the number of dead fish seems high, it doesn’t compare to some of the lake’s more notable fish kills.
“It represents only a very small part of the total population and thus does not hurt the fishing in the lake,” he said.
In the 1990s, he said, the lake lost an estimated 80,000 fish.
The deaths occur when lake water is released through the spillway. While the release was at minimal levels, fish can still get swept up, Flammang said.
If the outflows are combined with colder temperatures and schooling fish, it can be fatal.
Crappie, he said, are known to stick together in “schools.” While that’s good for their social lives, it can put them in risky situations.
“These fish are sitting in 50 feet of water in the lake, and in the matter of a second or two, are released at the surface of the tailrace,” into the Chariton River below the lake, he said.
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“This results in a large pressure change and some fish, especially in cold temperatures, are not able to withstand this large pressure change and die,” he added. “Crappie and freshwater drum are two of these species.”
The fish kill was first reported by a fisherman.