Dead Fish Are Littering The Shore At Rock Run Preserve — Here's Why

Press release from the Forest Preserve District of Will County:

Rock Run Preserve – Black Road Access is experiencing a naturally occurring fish kill caused by this year’s heavy snowfall and bitterly cold winter weather.

Now that the pond ice has melted at the preserve, passersby are noticing dead fish floating on the surface of the water. Fish kills can occur following a long harsh winter especially with long periods of continued snowfall, said Andrew Hawkins, the Forest Preserve District’s director of planning and development.

“The ice and snow create a cap covering the body of water, which prevents sunlight from reaching the aquatic vegetation,” he explained. “This shuts down photosynthesis and leaves the water with much lower oxygen levels. In low oxygen situations, fish will die in large numbers.

"Once the fish die, they are essentially preserved in the cold water and ice," Hawkins said. "They begin to float to the top of the water following the first ice melt. In most situations, the population of fish will return to pre-fish kill numbers within a couple years.”

The dead fish will decompose and be scavenged by other creatures including turtles, snails, birds, mammals and other fish.

"There is no need to clean up the fish since they are not toxic," said Becky Blankenship, the Forest Preserve's wildlife biologist. "They are a natural part of the food chain and of nutrient cycling."

The Joliet preserve isn't the only spot where a fish kill occurred. Earlier this month, Block Club Chicago reported on fish kills happening in Chicago.

One of the reasons the Rock Run pond may have been affected is because it is a shallower body of water, Blankenship said.

"Deeper water has more oxygen than shallow water," she explained. "The oxygen in shallow water gets used up more quickly than in deeper waters, which is why fish kills usually occur in the smaller water bodies. Water is oxygenated by submerged plants that photosynthesize, but that cannot be done if ice and snow are preventing the sunlight from reaching them. Also, wind causes ripples or waves that can oxygenate the water, but that stops when there is ice cover."

Ponds that are less than 10 feet deep at their deepest are most likely to experience fish kills, said Seth Love, district fisheries biologist for the Illinois Department of Natural Resources.

Throughout the day and night, oxygen is being used up by decomposing plants and animals, animal respiration and nighttime plant respiration, he said.

"Ultimately, when oxygen consumption greatly outweighs production, a fish kill may ensue," he said.

Winter isn’t the only time fish kills can occur, Love added. For instance, a long cold rain on a shallow pond can cause a fish kill in midsummer.

"The change in water temperature causes the pond to turn over (cold water is denser than warm water), mixing the oxygen starved bottom water with the oxygen rich top water which may result in decreased oxygen throughout," Love said. "Summer kills may also happen if extremely dense vegetation suddenly dies, causing excessive decomposition which starves a pond of oxygen.”

And while the result of a fish kill can leave behind a temporary biological eyesore, some good could come of it, Blankenship said.

"Fish are a major predator of amphibian larvae, so an occasional fish kill from low oxygen or a drought drying out a pond is very beneficial for amphibian populations," she said.

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This press release was produced by the Forest Preserve District of Will County. The views expressed are the author's own.