The die-off of 50,000 white perch in the lower Potomac River last weekend appears to have been a natural event in which severe cold killed the fish when winds and tides trapped them in shallow water, a Maryland official said.

Charles Poukish, an environmental program manager with the Maryland Department of the Environment, said investigators had found no evidence that the fish were killed by pollution or a disease. Instead, he said, the fish seem to have died of "thermal shock" while trapped in water colder than they could stand.

"We're pretty convinced that it was the conditions, the winter conditions," Poukish said yesterday.

The fish were discovered this week in two spots along the Potomac bank in Southern Maryland. Most were found near Swan Point, in lower Charles County, but 1,500 to 2,000 washed up farther downstream, in the Tall Timbers area of St. Mary's County, Poukish said.

Poukish said some of the fish showed signs that they had been under stress before they died, including blisters on their scales. But he said the primary cause of the fish kill seemed to be an unusual combination of strong winds and tides, which kept the fish from returning to warmer, deeper waters.

"We can rule out that water quality was an issue," Poukish said, after tests showed no evidence of abnormal pollution.

He said fish kills of that size are not unusual in Maryland. But some activists concerned with the river said they were surprised at the scale of the die-off, which left beaches littered with dead perch from four to eight inches long.

"Could it just be what they said, just bad luck for 50,000 fish? Yeah, could be," said Ed Merrifield, an environmentalist whose title is Potomac Riverkeeper. But, he said, "it would be good to follow up on that, as best we can."