GRAND FORKS, N.D. - Fisheries crews from North Dakota and Minnesota are trying to determine why more than 1,600 channel catfish have died in the Red River south of here.

"The fact they're distributed over a wide area and just channel catfish kind of points at some kind of disease, bacterial infection or something," said Henry Drewes, a regional fisheries supervisor for Minnesota's Department of Natural Resources.

Oxygen levels on the affected stretch of river are fine and the water temperature is normal for this time of year, he said. That tends to rule out pollution, such as the discharge of storm sewer water that killed fish of all species near Fargo after heavy rain in the summer of 2006, Drewes said.

Officials were able to catch one catfish that was just about dead. The fish was sent to a lab in St. Paul, Minn. Test results were expected within a few days.

"We'll know a lot more when we get the analysis back from the lab," Drewes said.

A Grand Forks angler who spotted dead catfish over the weekend notified wildlife officials. The dead fish have ranged in size from 5 inches to 30 inches.

Some appear to have died recently, while others have been dead for several days, Drewes said. Some of those that died recently had lesions on their skin.

Lynn Schlueter, Red River fisheries biologist with the North Dakota Game and Fish Department, said he traveled about 15 miles of river by boat south of Grand Forks and encountered dead catfish the entire way.

He said the nature of the die-off suggests a disease such as columnaris, a highly contagious bacterial infection that includes skin lesions.

Karry Kyllo, the angler and fishing guide who notified officials, said the die-off coincides with a pronounced decline in fishing success.

Normally, catfish anglers encounter some of the best fishing of the year in September, when the fish feed aggressively to bulk up for winter. That has not happened this year, Kyllo said.

"I don't know if it's my imagination or not, but the fish I've been catching lately seem sluggish," Kyllo said. "We pulled in a 24-pounder here a few weeks back, and a bigger fish like that usually heads to the bottom. This one didn't even fight. It just laid on the surface."

Drewes said that if a disease is to blame for the die-off, typically such incidents do not have a long-ranging impact on fish populations.