Manure running off a frozen farm field has killed more than 100 brown trout in the Sugar River's West Branch, a stream that was just removed in October from the federal government's list of impaired waters.

More than $900,000 in federal and state grants and thousands of hours of volunteer labor had turned the shallow and muddy stream into a prized trout fishery.

But a farmer's decision last week to spread liquid cow manure on an icy field has caused considerable damage to that effort, officials said Monday.

"One landowner making a poor decision may have undone 30 years of work in this watershed," said Frank Fetter, executive director of the Upper Sugar River Watershed Association.

The Department of Natural Resources is considering legal action against the farmer and refused to release his name because of its investigation. David Wood, a DNR conservation warden, said the farmer may have violated a state law that forbids introducing a pollutant to waterways. The farmer, Wood said, would be subject to a fine of about $430 and restitution costs of about $26 per trout.

By late Monday, DNR workers had recovered more than 100 dead trout, some of them as long as 19 inches and perhaps 7- or 8-years-old, from the cold stream. But Kurt Welke, a DNR fish biologist, said that number is expected to increase dramatically. He said many dead fish remain on the bottom of the stream or stuck beneath banks and won't surface until they begin to decompose.

The steep field on which the manure was spread is near Highway 78 and Blue Valley Road in the town of Blue Mounds, south of Mount Horeb. Welke and Wood both said the farmer is cooperating in the cleanup effort and is among the landowners who worked on efforts to restore the Sugar.

The manure, Welke said, was spread on the field late last week by a contract manure spreader hired by the farmer. But the spreader did not work the manure into the ground because it was too frozen. A thaw Friday caused the manure to run into the stream, according to Michael Sorge, a DNR water resource specialist who was on the scene at the time.

The fish were killed not by the lack of oxygen, as happens during the warm summer months, but by ammonia from the manure, Sorge said.

A major worry, Welke said, is that much of the manure remains on the frozen ground and another thaw may send the rest of it running downhill into the stream.

"If there is a gradual warm- up in temperature," Welke said, "then there may be a chronic, but lesser amount of manure discharging to the river. A quick jump in temperature may spell trouble in that a major slug could impact the river."

Workers spent the weekend constructing earthen berms in the field to hold back the flow of manure in the event of a thaw. And today, Sorge said, heavy construction equipment is being used to break through the frozen surface of the field to allow the manure to work into the soil.

Such runoff pollution, also called nonpoint pollution, is a vexing problem, even though an ambitious law was passed two years ago to help control the problem. But a lack of money has crippled that law. Plus, critics point out that farmers are required to install erosion-control practices only if they are provided cost- sharing dollars - money that isn't available at the levels expected.

Welke said this spill highlights another more immediate problem - that of farmers spreading manure on frozen ground. It's a common and legal practice, Welke said, but remains an enormous environmental problem. He said the agency has received a number of complaints about the practice this winter.

"It seems to me," Welke said, "that it's time to have a frank discussion between the agricultural community and the regulatory community about certain practices that have long been viewed as normal. We cannot winter spread manure."

Welke added that alternatives do exist. Perhaps, he said, there should be a regional digester where farmers can haul their manure in the winter.

"I would certainly hope that the high profile of this raises the eyebrows of my administrators and also of the legislative community," Welke added.

Contact Ron Seely at rseely@madison.com or 252-6131.