Wednesday, March 18, 2009

Biologists predict sizeable fish kill on Big Eau Pleine Reservoir

WAUSAU – Low levels of dissolved oxygen in the Big Eau Pleine Reservoir are responsible for recent fish die-offs, and fisheries biologists now expect the ongoing fish kill to continue and perhaps worsen in the reservoir’s lower reaches.

Historically, fish kills have occurred during the winter. During winter months the reservoir typically suffers from low dissolved oxygen levels as organic materials decay and consume oxygen. At the same time the water’s frozen surface limits or prevents surface air mixing and light penetration.

Since 1981, the Wisconsin Valley Improvement Company, the operator of the dam, and the Marathon County parks department have operated an aeration system on the flowage. The aeration system has provided a refuge for fish when oxygen levels are low. However, the efficiency of the 28-year-old system has been declining in recent years, and this made its operation questionable for 2009.

Additionally, there was not enough rain this past fall to refill the reservoir so water levels were low going into the winter.

In October, a WVIC crew and county park workers assessed the aeration system and completed repairs and modifications, said Dave Coon, director of environmental affairs for WVIC. Still, the system was operating at less than optimum efficiency with one pump and one aeration manifold. WVIC contributed $5,000 to cover energy costs.

Since that time WVIC and the DNR have partnered to operate the system.

DNR wildlife supervisor Tom Meier said the cooperation of the Halder Sportsman’s Club in early January helped the DNR fence what would become an open water area. The aeration system began operating January 14.

Dissolved oxygen levels remained stable until the end of January when WVIC monitoring started picking up low levels above and below the aeration system. By early February WVIC and DNR agreed that a second aeration pump and manifold would be started to further aid fish in the refuge area.

“The aeration system has provided some refuge to the fish, although sporadic fish kills have occurred,” said Tom Meronek, DNR fisheries biologist. “Icy runoff from the early February thaw added more highly oxygenated water to the reservoir and provided some relief, especially in the upper portions.”

That same runoff, however, carried manure from farm fields and other organic materials from land surfaces into the reservoir, increasing the biological oxygen demand.

The latest monitoring shows very low dissolved oxygen levels in the lower portion of the reservoir below the county park. The highest readings in this portion of the reservoir were near the aeration system, but they did not appear high enough to sustain the fish.

The ongoing fish kill is not expected to affect the entire fishery. Higher oxygen readings were found upstream of County S where a larger fish refuge now exists. Many fish were migrating for spring spawning and were able to seek refuge during late February and early March when favorable conditions existed upstream of the aeration system.

Individuals with questions about these processes can contact Tom Meronek at 715-359-7582.

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