Mississippi River Pool 7-8 Fish Kills

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Biologists with the departments of natural resources in Minnesota and Wisconsin have determined that several fish kills reported in and around the Mississippi River near La Crosse are due to hypoxia, which occurs when there are unusually low levels of dissolved oxygen in the water. These kills can occur during periods of hot summer weather and heavy rains causing rapid fluctuation in river levels.

The two state DNRs have been working together and with the U.S. Fish and Wildlife Service to investigate reports of dead fish in Blue Lake and Lawrence Lake in Pool 8 of the Mississippi River, in Round Lake in Pool 7, and in other isolated locations in backwaters of the Mississippi River. Pool 7 runs from roughly Trempealeau to La Crosse, while Pool 8 includes that portion of the river approximately between La Crosse and Genoa.

Hypoxia can occur during periods of high summer temperatures because hot water holds less oxygen and it speeds up decay of vegetation and other organic matter, a process that consumes oxygen. Oxygen levels near zero have been recently measured in some backwater areas near La Crosse. With recent rains, that hypoxic water has been flushed out into other parts of the river, causing the death of those fish that were unable to escape quickly.

Fish species noted in the kills have been primarily bluegill, yellow perch, largemouth bass, gizzard shad, common carp, and northern pike. The number of dead fish observed in these areas usually represents a small portion of the overall fish population, as most fish are able to move out of hypoxic waters into areas with higher dissolved oxygen levels.

However, there is potential for higher mortality rates in areas that do not have good connection to areas of better water quality. Such fish kills generally have little to no noticeable impact on the fishery.

While low oxygen levels should soon improve, it is possible that other areas of the river will be affected.

People who see a large number of dead fish in an area are encouraged to contact either their local DNR office or, in Minnesota, the state duty officer, who is available 24/7 at 800-422-0798. In Wisconsin, people can call 800-TIP-WDNR to report fish kills. The sooner a fish kill can be investigated, the more likely it is that the cause can be determined.

source: Minnesota Department of Natural Resources