Local Winterkills & Other Fish Die-Offs
Dead Fish After Ice-Out Likely Result of "Winterkill"
The retreat of lake and stream ice can sometimes leave a grim reminder of winter's effects: dead fish.
In most cases the dead fish result from a normal process known as "winterkill." When snow and ice cover a lake, they limit the sunlight reaching aquatic plants. The plants then cut back on the amount of oxygen they produce. If vegetation dies from lack of sunlight, the plants start to decompose, which uses oxygen dissolved in the water. If oxygen depletion becomes severe enough, fish die.
Winterkill potential is worse in winters with abundant or early snowfall. Lower autumn water levels increase the probability and severity of winterkill. Early ice-on and late ice-out dates also increase the winterkill potential. Wetlands and shallow, soft-bottom lakes are more winterkill-prone than deeper, hard-bottomed lakes.
Some species of fish are more vulnerable to winterkill than others. Trout require the most oxygen. Bluegill and largemouth bass also are moderately sensitive to lowered oxygen levels. Walleye, northern pike, carp and crappie species have intermediate tolerances, while bullheads and fathead minnows are the most tolerant of low oxygen. Winterkills seldom result in the death of all fish in a lake. Lakes with regular winterkill events are usually dominated by bullhead species.
Winterkill can have some beneficial effects. In lakes with overabundant panfish, winterkill can result in increased growth rates and less competition for survivors. It also can greatly reduce carp abundance, allowing for improved water quality and increased success of subsequent fish stocking efforts.
Notes: 1) a lake's winterkill potential and status based on oxygen-level readings in a few locations and/or sensory evidence; majorities of fish populations might not necessarily be affected 2) these lists are informational and should not be considered complete or "official," as only a portion of all local waters are tested or reliably reported.
Late Oxygen Drop ("Winterkill" Potential) Seen On Several Late In 2012-13
The prolonged winter caused late-season drops in shallow lakes and lake bays. Some die-offs might occur in waters with previous winterkill events. Lakes with measured low oxygen in mid-March 2013 included-- Carver County: Swede; Hennepin County: Duck, Staring, and Snelling.
Low Oxygen ("Winterkill") Affected Many Lakes In 2010-11
The long, top-5-snowfall-record winter increased incidences of full or partial "winterkill"-- oxygen depletion severe enough to cause fish die-off-- in shallow waters or lake basins with insufficient oxygen.
Local lakes and [streams] with low oxygen and/or fish kills in 2010-11 included-- Carver County: Auburn (east basin), Eagle, Firemen's Clayhole, Goose, Hydes, Lotus, Lucy, Parley, Reitz, Rice Marsh, Rutz, St. Joe, Stone, Swede, and Virginia; Hennepin County: Cedar-- Minnneapolis, Cornelia, Diamond, Duck, Eagle & Pike, Gleason, Haften, Harriet, Lake of the Isles, Loring Park Pond, [Minnehaha Creek], Mitchell, Mooney, Normandale (Mt. Normandale), Pomerleau, Powderhorn, Red Rock, Ryan, Schmidt (Smith, Mud), Smetana, Spurzem, Staring (Starring), Twin, Webber, Whaletail, Winterhalter, and Wirth; Scott County: Cleary, Cynthia, Dan Patch, O'Dowd, St. Catherine, and Thole.
Lakes Affected By Low Oxygen ("Winterkill") In 2009-10
The winter of 2009-10 was similar to more historic local wintertime conditions. The longer, more snow-covered conditions increased incidences of "winterkill"-- oxygen depletion severe enough to cause fish die-offs. An explanation of winterkill can be found below the list of waters.
Lakes (and [streams] in our Area boundaries with witnessed or reported winterkill in 2009-10 include-- Carver County: Rice Marsh, Hazeltine, and Rutz; Hennepin County: Duck, Smetana, Diamond, Snyder/Kraft, Whaletail, Ryan, Penn, Lamplighter Pond, Holy Name, Victoria, Galpin, Neill, Shady Oak and Starring; and Scott County: Cynthia, Legends golf course pond, and the unnamed pond in Prior Lake's Lakefront Park.