Fish Kill in Upper Pond Could Have Multiple Causes

Rainy Conditions Could be Factor

Marshfield, WI (OnFocus) Walkers along Wildwood Park’s Upper Pond have noticed a large number of dead fish along the shore.

“We are aware of the fish kill but aren’t sure exactly what caused it,” said Steve Burns, Zoo Manager. “All the aerators are working, but heavy rains and cloudy days can still decrease oxygen levels in the pond.”

There are a number of factors that may have created a poor environment for the fish. In cloudy or rainy conditions, plankton growth increases. The decomposition of these short-lived plankton then lowers oxygen levels in the water.

“Heavy rains can also cause ‘turnover’ where the oxygen rich water towards the top of the water table mixes with the oxygen poor water at the bottom. This mixing can reduce oxygen throughout the entire pond and cause a kill,” said Burns.

Another possibility is herbicide runoff from surrounding agricultural fields. However, the fields around the zoo are planted with winter wheat and are not likely the source, since this crop does not receive herbicide.

Even without a recent fish kill, it’s not recommended to consume the fish from Wildwood’s ponds.

“If you want my honest opinion, I wouldn’t eat the fish from the upper or lower ponds regardless of whether there has been a recent kill or not,” said Burns.