ROCKY RIVER, Ohio -- Tens of thousands of fish of all sizes and species have died in the East Branch of the Rocky River in the Cleveland Metroparks since Sunday afternoon, bringing crews from several state agencies to investigate for toxic pollutants, test river waters and document the range of the fish kill.

The dead and dying fish were first reported on Sunday afternoon by a fisherman, who called 1-800-POACHER, a Division of Wildlife hot line. Wildlife officials contacted the Cleveland Metroparks, who brought in the Ohio EPA and the Northeast Ohio Regional Sewer District.

The dead fish have sunk to the bottom of the river, said Division of Wildlife officer Randy White, making it difficult to assess the extent of the fish kill.

"The range of the fish kill was unbelievable, affecting all types of fish," said aquatic biologist Mike Durkalec of the Cleveland Metroparks, who surveyed the affected areas Rocky River to see if any species of fish survived. Durkalec's list included bass, darters, a wide variety of minnows, rainbow trout and white suckers.

"We still have no idea what caused it," said Durkalec. "We're checking out every possibility."

The park agency has been stocking large numbers of rainbow trout in the stretch of the river, but canceled a trout stocking scheduled for today. It will divert the 650 pounds of rainbow trout to Wallace Lake.

The fish kill began in an area near the Cleveland Metroparks toboggan run in the Mill Stream Run Reservation. Fish were soon dying in a three-mile stretch of river between the Bonnie Park Area and Wallace Lake in Berea. Ohio EPA officials reportedly found a few dead steelhead trout where the East Branch joins the Rocky River. The Rocky River is a noted steelhead trout stream, with the lake-run steelhead providing excellent fishing during the cold-weather months.

Stockings of yearling steelhead trout are scheduled to be released in the lower Rocky River in the coming weeks.

To reach this Plain Dealer reporter: degan@plaind.com, 216-999-5158

Below is a video of officials with the Ohio Department of Natural Resources counting dead fish, including a 19-inch trout.