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Loon Lake Preservation Association
Fish Freeze Out

Freeze out is a common occurrence in Loon Lake, occurring every three or four years. Freeze out does not refer to the lake freezing to the bottom, but is actually a depletion of oxygen level in the water that results in fish and other aquatic residents suffocating, often in large numbers. When snow cover is sufficient to limit sunlight penetration, photosynthesis stops occurring. Plants die and consume oxygen as they decay, and without sunlight, oxygen isn't replenished in the system. Eventually fish die off, suffocated from lack of dissolved oxygen.

Also called winter kill, freeze out occurs during especially long, harsh winters. Shallow lakes with excess amounts of aquatic vegetation and mucky bottoms, like Loon Lake, are especially prone to this problem. The results of a winter kill are seldom noticed until spring when the ice melts. Then the dead fish, often the larger ones, are seen washing up along the edge. Because they require more oxygen, the large fish suffocate and die first. Winter kill begins with distressed fish gasping for air at holes in the ice and ends with large numbers of dead fish which bloat as the water warms in early spring. Dead fish may appear fuzzy because of secondary infection by fungus, but the fungus was not the cause of death.

Trace amounts of dissolved oxygen (measured in parts per million, PPM) are required by fish and all other forms of aquatic life. Even living plants and the bacteria that decompose organic materials on the bottom of the lake require oxygen. As a rule of thumb, the critical level of oxygen is about 2 PPM for most game fish native to warmwater lakes, and levels below 1 PPM for extended periods of time are lethal.

But species of fish vary in their tolerance of low oxygen. Trout are most sensitive; walleye, bass, and bluegill have intermediate sensitivity; and northern pike, yellow perch, and pumpkinseed are relatively tolerant. Bullheads and certain minnows are very tolerant. In Loon Lake it is not surprising that bullheads are the most abundant fish, followed by bluegills. A winter kill of bullheads is uncommon, occurring maybe once a decade, so bullhead populations are stable. On the other hand, some winter-killed bass can be found nearly every spring, and a total freeze out occurs on the average of every four to five years. There were back to back freeze outs of the bass population in the winters of 2002/2003 and 2003/2004. Currently bass are very rare in Loon Lake. Because of the periodic kill-offs, bass rarely achieve an age over five years, and smaller class sizes predominate.

In severe freeze out years, when even the bullheads and frogs die and blanket the shoreline in the spring, it is a bit of a mystery how repopulation occurs. Many believe fish eggs are transported on wading birds' legs as they move from lake to lake. It seems possible but not likely that fish might find their way from Round Lake to Loon Lake through the marsh that connects them. The most likely explanation is that some fish survive even the most severe freeze out events, and their offspring repopulate the lake.

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