GRAND FORKS (AP) - Two bacterial infections likely were the cause of a large catfish die-off in a stretch of the Red River upstream from this city, preliminary test results show.

A near-dead catfish captured for testing at a Minnesota Department of Natural Resources pathology lab in St. Paul tested positive for both columnaris and aeromonas bacteria, staff said. Additional tests are planned to confirm the diagnosis.

More than 1,600 channel catfish are believed to have died earlier this month. They ranged in size from 5 inches to about 30 inches.

Lab staff said water temperatures and low river flows in the Grand Forks area might have created the right conditions to weaken the catfish and enable the bacterial infections to strike.

Ling Shen, a DNR fisheries health specialist in St. Paul, said the likely scenario is that columnaris affected the gills and weakened the immune system of the fish that was tested, and aeromonas occurred as secondary infection and caused the rest of the damage.

Officials also are testing for two viruses, as a precaution. Shen said results will not be available for some time.

Lynn Schlueter, Red River fisheries biologist for the North Dakota Game and Fish Department, said he was not surprised by the preliminary finding.

"Both of those are fairly common diseases, both in hatchery situations and in the wild," he said. "And with any disease like this, you've got to have all of the factors in place and just right for the stuff to really blossom. Typically, a lot of this is temperature dependent. It's out there and doesn't show up until you get that first blast-off of cold."

Biologists will be working to determine exactly what made the catfish susceptible to infection.

"While we might not be able to prevent it, at least we can start being aware of what triggered it," Schlueter said.