(CNN) -- Arkansas officials are investigating the death of an estimated 100,000 fish in the state's northwest, but suspect disease was to blame, a state spokesman said Sunday.

Dead drum fish floated in the water and lined the banks of a 20-mile stretch of the Arkansas River near Ozark, about 125 miles northwest of Little Rock, said Keith Stephens of the Arkansas Game and Fish Commission. A tugboat operator discovered the fish kill Thursday night, and fisheries officials collected some of the dying animals to conduct tests.

Stephens said fish kills occur every year, but the size of the latest one is unusual, and suggested some sort of disease was to blame.

"The fish kill only affected one species of fish," he said. "If it was from a pollutant, it would have affected all of the fish, not just drum fish."

Ozark is about 125 miles west of the town of Beebe, where game wardens are trying to find out why up to 5,000 blackbirds fell from the sky just before midnight New Year's Eve.

Dead birds a mystery in Arkansas



Biologists believe the bird deaths were stress-related from either fireworks or weather and are unrelated to the fish kill near Ozark, Stephens said.

CNN's Tina Burnside contributed to this report.