Two popular fisheries in northeastern Utah have suffered major winter fish kills.

Biologists with the Utah Division of Wildlife Resources have confirmed major fish die-offs in Calder Reservoir and Crouse Reservoir.

"It's likely a complete winterkill in Crouse Reservoir," said Ed Johnson, fisheries biologist. "Calder also had a major die-off, and it may also be a complete kill. We will know more after we can set a few nets. But that won't be for a little while. We need the water to warm up so the fish are more active."

Johnson said biologists do not expect to find a winter fish kill at Matt Warner Reservoir, another popular fishery in northeastern Utah.

"Matt Warner is much larger and deeper, so there is more water to hold oxygen," Johnson said. "Unlike the other reservoirs, it has not had a winterkill since its dam was remodeled."

Biologists tried to reach Matt Warner recently to survey it, but a large snowdrift blocked the road leading to the reservoir.

Matt Warner is the third and highest reservoir on the Pot Creek drainage.

"Crouse Reservoir wasn't much of a surprise," said Roger Schneidervin, regional aquatics manager. "The water level in Crouse was quite low because Diamond Mountain has been in a severe drought for years. Because Crouse is the lowest reservoir in the drainage, it doesn't get the water.

"Calder is much more of a surprise. There has been only one other time in the last 18 years (that fish died in the winter), and the reservoir was lower that year than it was this time."

The kill at Calder might be an incomplete kill. Biologists who checked the reservoir said they saw or heard something they thought might be a fish jumping.

Winterkill happens when fish die from either a lack of oxygen or a buildup of lethal chemicals in the fish. Both can happen when ice on the surface of the water doesn't allow oxygen to be exchanged between the water and the air.

Fish and other aquatic animal life use oxygen. When ice covers the surface of the water, most of the plants die. The plant decomposition process uses oxygen. The decomposing plant material can also give off toxic chemicals.

Sometimes other toxic chemicals are released from the mud at the bottom of the water. Other natural or human-made sources can also cause toxic chemicals to be in the water.

"Some lakes and reservoirs survive long winters because streams bring in a supply of dissolved oxygen," Johnson said, "but in the case of Calder and Crouse, no waters were flowing in.

"Water depth and total water volume also play a role in winterkill," he said. "Small, shallow lakes cool down quicker. That causes them to freeze earlier, which makes them more likely to winterkill. Larger, deep lakes also have the advantage of holding more dissolved oxygen to begin with."

DWR aquatic managers are checking other reservoirs in Utah to see if they may have suffered a similar fate. The managers will likely readjust fish-stocking schedules to provide catchable-size fish to restart or enhance these fisheries. That should make these waters good places to fish in 2008.