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.
A few shallow, fertile waters had low late-season oxygen concentrations, but nothing indicating thorough, widespread depletions. Some waters might have had, at most, limited fish die-offs. Waters with low oxygen readings included: Carver County-- Eagle, Lucy, Susan, Virginia; Hennepin County-- Dutch, Rice Lake/Elm Creek (lake lowered in 2014 to freeze aquatic vegetation), Starring [Starting]
Very low late-winter oxygen levels and/or winterkill evidence (fish carcass sightings) were observed in some shallow waters. Those were-- Carver County: Eagle, Goose, Lotus, Susan, Swede, Turbid, and wetlands #10-0216 through 218 (Grace and Jonathan Lakes, Chaska); Hennepin County: Bass, Cornelia, Duck, Half Moon, Mitchell, Normandale, Red Rock, Rice and nearby stormwater ponds (Maple Grove), Smith/Schmidt/Mud, Spurzem/Spurzen, Starring, and wetland #27-0634 (Lake Success, Brooklyn Park); Scott County: Cynthia, Schneider.
2012-13 Low Oxygen ("Winterkill") Evidence
Staff checking lake oxygen levels in mid-March 2013 found some low readings in several shallow, fertile lakes. They were-- Carver County: Swede; Hennepin County: Duck, Snelling, and Staring (Starring). Post-ice-out corroborating evidence (fish carcass sightings) was found in Hennepin County stormwater ponds, Minnehaha Creek, Jeffers Pond (Scott County), and a bay of Fish Lake- Scott Co. (April 28-30 reports).
Low Oxygen ("Winterkill") Affected Many Waters In 2010-11
The long, top-5-snowfall-record winter increased incidences of "winterkill"-- oxygen depletion severe enough to cause fish die-offs (usually partial)-- in shallow waters 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.
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.