DNR to treat Mary Lake with Rotenone to restore walleye rearing

(2006-10-12)

The Minnesota Department of Natural Resources will treat Mary Lake with Rotenone on October 25, 2006 to reduce the bullhead population. Mary Lake has been used as a walleye rearing pond for the past 20 or more years, but an over population of bullheads has limited walleye rearing.

Mary Lake is located about five miles east of Parkers Prairie, in Southeast Ottertail County. The lake is roughly 211 acres in size and has a maximum depth of 7.5 feet. The Rotenone treatment will kill all the fish in the lake (which are mostly bullheads) and allow DNR to continue using the lake as a rearing pond. Before the bullheads became a problem in recent years, the lake was able to produce over 10,000 pounds of walleye fingerlings in a summer. Without the treatment, the lake will continue to be unusable as a rearing pond. Rotenone is a naturally occurring pesticide that is extremely toxic to fish. It is also used as an insecticide. It occurs naturally in plants that grow in Central and South America. It can be applied as a powder (made from ground up plants) or as a liquid (combined with petroleum-based “carriers”). Both forms will be used in the DNR’s treatment of Mary Lake.

Treatments are done in the fall, while lakes are cooling down. This allows the chemical to mix throughout the water column, creating toxic conditions for fish throughout the entire volume of lake water.

The DNR has treated many shallow lakes in Minnesota to remove unwelcome numbers of carp and bullheads. After treatment, DNR restocks the lake to provide fishing or fish rearing opportunities.

Bullhead removal is essential to restoring many shallow lakes. Removing the rough fish allows sediments to settle and sunlight to reach the aquatic vegetation. The lakes the DNR proposes for Rotenone treatment are so degraded that few species of fish remain other than carp and bullheads.

Rotenone treatments present no short-term or long-term risks to either the environment or humans living nearby. DNR biologists conduct careful environmental analysis before a treatment is planned. They monitor lake water temperature and chemistry, review records for rare or endangered species, and determine the prevalence of non-target wildlife species.

The timing of the chemical application, which is carried out by trained professionals, is done to reduce harm to any animals other than the targeted fish. Although toxic to fish living in the treated lake, Rotenone breaks down to carbon dioxide and water within a few weeks, rendering it harmless.

In the concentrations the DNR uses to treat lakes, Rotenone is not toxic to humans or any wildlife that eats fish killed by the substance. The Minnesota Department of Health has also determined that the products, when used properly, present no long-term health risks. However, because the US Environmental Protection Agency has not established guidelines for consuming fish killed with Rotenone, the DNR urges people not to eat the fish killed by Rotenone treatment.

DNR biologists said that dead fish that end up on land as a result of wave or wind action are no more of a threat to public health than fish that die of natural causes.

For more information, contact: David Bohlander, DNR Fisheries Specialist, Brainerd, 218-833-8635