Fish Mystery: Why are they dying in Lake Isabelle?

Posted 8/4/21

By Bruce Karnick [email protected] An intrepid resident contacted us about an issue at Lake Isabelle, “Hi, I was just at Lake Isabel, there are all kinds of dead fish floating on the water. …

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Fish Mystery: Why are they dying in Lake Isabelle?


By Bruce Karnick

[email protected]

An intrepid resident contacted us about an issue at Lake Isabelle, “Hi, I was just at Lake Isabel, there are all kinds of dead fish floating on the water. Not sure what is happening.”

When we went to check it out on Friday, it took some time to see more than a few, but once we saw the few, we saw plenty. That prompted an investigation into the situation. Phone calls were made, first to Parks Director Chris Jenkins, then Parks Maintenance Supervisor Cory Likes. Both Jenkins and Likes explained they were not surprised to hear of this happening in Hastings.

“It’s been happening in other metro area lakes recently,” Jenkins explained. “I am pretty sure it is an oxygen saturation issue from what I have been hearing from other areas. The combination of high heat, no rain, and low water levels means the water is not getting replenished with oxygen.”

Jenkins was confident in his assessment based on other area lakes, but he suggested reaching out to the MN DNR. Likes had the same explanation, low oxygen saturation in the water.

Think about a fish tank. How does oxygen get put back into the water for your fish to live? The surface of the water must be moving, agitated. The bubblers help put air into the water, mostly because they cause the surface to move. Rain falling on a still lake is one of the best ways to agitate the water along with replenishing the water in the lake from rapid moving rivers. Neither are happening right now with the drought going on.

To further explore the question, a call was made to State Representative Tony Jurgens. Jurgens is an outdoor guy and well connected at the capitol, maybe he would know. He had Photo by Bruce Karnick not heard anything but made a trip down to Isabelle to check things out himself. He was surprised at the number of fish he saw floating belly up. Surprised and concerned. Jurgens made a call to the DNR Friday afternoon.

We utilized other sources with vast outdoor knowledge. One source pointed us in the direction of an issue known as algal bloom. This phenomenon can occur in all bodies of water but is more common to occur in shallow, slow moving or still bodies of water. Algal bloom is a rapid growth of microscopic algae or cyanobacteria in water. The rapid growth often results in a colored scum on and near the surface. The direct issue is not the rapid growth of the algae, it’s the fallout from the rapid death of the algae. When algae dies, it begins to decompose, that decomposition uses up a lot of oxygen quickly. Combine that decomposition with the high heat, the shallow waters, and the lack of rain agitating the surface of the water, the already low levels of oxygen drop rapidly.

According to Brian Nerbonne, Region 3 Fisheries Manager for the DNR, the DNR received reports of a fish kill at Lake Isabelle on 8/2/2021, and staff went there to investigate. They found numerous dead fish of various species, generally indicative of an environmental condition rather than a disease that would be more selective. The water was very green, indicative of a high abundance of algae. We check the dissolved oxygen level and found it to be high, which is to be expected when algae are producing oxygen during the daytime. However, it is our belief that the fish kill is due to nighttime oxygen depletion by the algae. While algae produce oxygen through photosynthesis during daylight, there is a corresponding period of respiration that they do when it is dark that depletes oxygen. When algae are especially abundant in a shallow lake, which is especially the case on Isabelle right now with the drought, that can drop the level of algae below the threshold that fish can tolerate. This can especially be an issue when the water is especially warm, which not only drives algae productivity higher but also means that water becomes saturated with oxygen at a lower level than when water is colder. We have started to see this same issue on a few other shallow lakes in southern Minnesota in the past few weeks.

Because Lake Isabelle is connected to the Mississippi River, it is able to be repopulated by fish naturally. As a result, DNR won’t take any action at this time. We also checked out Lake Rebecca, another shallow lake in Hastings, but found no dead fish. The water was not as green and the dissolved oxygen level was lower during the day as at Isabelle, indicative of less algal production. Isabelle is shallower than Rebecca and more prone to these summer kill events as a result.

Walk to the right of the Lake Isabelle boat launch and you will begin to clearly see the issue at hand where hundreds of dead fish line the shore. The further you walk, the stronger the smell and more fish you find belly up.