CLEVELAND, Ohio--Thousands of dead fish were found Sunday floating in the Cuyahoga River and lining the river banks between Heritage Park and Merwin Avenue in the Flats.

Red bruise-like marks appeared on some of the fish, which varied in size.

"I've seen large clumps of them floating down the river for the last three days," said T.J. Towner, who works in the area.

The cause of the fish kill was not immediately known, but a spokeswoman for the Northeast Ohio Regional Sewer District noted that fish kills of Gizzard Shad are common this time of year.

The species of fish go into thermal shock when they suddenly go from cold to warm water -- extreme temperature fluctuations that can be caused by changes in the weather, or warm effluent being released into the river or both.

"It's naturally occurring at this time of year," said the sewer district's Jean Chapman. "But we won't know for sure until someone looks at it."

Officials with the state Environmental Protection Agency and the Department of Natural Resources were notified of the fish kill but could not be reached for comment Sunday.