Department of Natural Resources officials today warned ice fishing and snowmobile enthusiasts that ice on lakes and rivers should be presumed to be unsafe and urged extreme caution when traveling across lakes given the mild weather that the state is currently experiencing.

"Just because a lake or stream is frozen doesn't mean the ice is safe," said Lt. Andrew Turner, marine safety and education supervisor for the DNR Law Enforcement Division. "Ice fishing has its own set of safety rules that if not followed, can cause a day of fishing to end in tragedy."

According to Turner, you can't always tell the strength of ice simply by its look, its thickness, the temperature or whether or not it is covered with snow. New ice, he said, generally is much stronger than old ice; a couple of inches of new clear ice may be strong enough to support you, though a foot of old, air-bubbled ice will not.

"Clear ice that has a bluish tint is the strongest," Turner said. "Ice formed by melted and refrozen snow appears milky, is very porous and weak."

Ice covered by snow always should be presumed unsafe. Snow acts like an insulating blanket and slows the freezing process. Ice under the snow will be thinner and weaker. A snowfall also can warm up and melt existing ice.

If there is slush on the ice, stay off. Slush ice is only about half as strong as clear ice and indicates the ice is no longer freezing from the bottom.

Turner said anglers should be especially cautious in areas where air temperatures have fluctuated in recent days. Any newly formed ice that is created after a cold front moves through should be regarded with caution. A cold snap with very cold temperatures quickly weakens ice and can cause large cracks within half a day. A warm spell may take several days to weaken the ice.

When temperatures vary widely, causing the ice to thaw during the day and refreeze at night, the result is a weak, "spongy" or honeycombed ice that is unsafe, he said.

Anglers also should bear in mind that ice weakens with age, and late in the season, when it turns dark and gets honeycombed, it's time to quit for the season. A cold snap sometimes halts the deterioration, but honeycombed ice never will refreeze to its original strength.

The DNR does not recommend the standard "inch-thickness" guide used by many anglers and snowmobilers to determine ice safety because ice seldom forms at a uniform rate.

Three or four inches of ice on a shallow farm pond with no inlets or outlets, for example, cannot be compared to the same amount of ice formed over a river with strong current, or to ice covering the bays of the Great Lakes, where ice cover always will be more fragile, Turner said.

Deep inland lakes take longer to freeze than shallow lakes. Ice cover on lakes with strong currents or chain-of-lakes systems also is more unpredictable.

"Always presume that ice is unsafe," Turner said. "Do not venture out onto the ice unless you test the thickness and quality with a spud or needle bar or an auger. Ice that is six or seven inches thick in one spot can be only two inches thick close by."

On the big lakes, ice cover in some spots may be thick enough to safely hold a car while other areas of ice are little more than an inch thick. Conditions can change within just a few feet because of currents under the ice. Be especially careful around pressure cracks. When the currents are stronger, the ice gives way to open water.

Ice near shore tends to be much weaker because of shifting, expansion and heat from sunlight reflecting off the bottom. If there's ice on the lake but water around the shoreline -- proceed with caution.

Avoid areas with protruding logs, brush, plants and docks. These structures can absorb heat from the sun, thus weakening the surrounding ice. Also avoid aeration devices, such as warmwater bubblers used near marinas.

"I personally would never recommend that you take a car or truck onto the ice," Turner said. "But those are personal decisions. I would urge that anyone wear a life jacket, wear bright colors and take a cell phone when walking onto a frozen lake or river."