PondTools

Winter

Winterkill: how deep, keep a hole in the ice, where the diffuser goes

Winterkill is what happens when fish under ice run out of oxygen. Snow on the ice blocks the light that plants and algae need to make oxygen, and the water runs out of it before spring. More water volume lowers the risk (Penn State compares a 1 acre pond 6 feet deep with one 3 feet deep, and Illinois Extension wants a quarter of the pond at 8 feet or more), and snow cover decides how any given winter goes. A hole in the ice helps only when a diffuser is moving water under it. For winter, move that diffuser to shallow water near shore, about a quarter of the pond's depth, so it does not chill the whole pond.

What winterkill is

Once ice seals the surface, no oxygen enters from the air. The only new oxygen comes from photosynthesis under the ice, and Illinois Extension says "As much as 70-90% of a pond's DO is a result of plant and algae photosynthesis." Clear ice lets enough light through and snow-covered ice does not, while the muck, leaves and dead weeds on the bottom keep consuming oxygen either way. The Wisconsin DNR describes the result: "Dissolved oxygen levels in these lakes decline throughout the winter to the point that by mid too late winter fish start to die due to lack of oxygen." The bottom water sits near 39 degrees Fahrenheit all winter, warm enough for the fish, so what kills them is running out of oxygen.

What the authoritative sources say

Penn State Extension says of depth: "The more water volume there is in a pond, the less likely the pond is to experience winterkill. A 1-acre pond 6 feet deep is less likely to have winterkill than one 3 feet deep. The amount of decaying organic matter on the bottom of the pond is critical." Illinois Extension gives the only area-at-depth target: "25% of your pond should have an ideal depth of 8 feet or more." The Wisconsin DNR, on lakes it aerates, says "a lake with a maximum depth of five feet is on the edge of being too shallow."

On oxygen, the Minnesota DNR ranks the fish: "Trout require the most oxygen and start to stress at oxygen levels below 5 ppm. Bluegill and largemouth bass also are moderately sensitive to lowered oxygen levels. Walleye, yellow perch, northern pike, common carp and crappie species have intermediate tolerances down to about 2 ppm, while bullheads and fathead minnows are the most tolerant of low oxygen." Illinois Extension says "Fish within your pond become stressed at DO levels of 5 parts per million (ppm)." Wisconsin aims to keep "greater than 2.0 ppm in the upper five to ten feet of water" through late winter.

Depth and oxygen figures from the extension and agency sources
FigureValueSource
Depth comparisonA 1 acre pond 6 feet deep is less likely to winterkill than one 3 feet deepPenn State Extension
Area at depth25 percent of the pond at 8 feet or moreIllinois Extension
Lakes that aerate successfullyMaximum depth at least 10 feet, mean depth at least 6 feet; 5 feet maximum is "on the edge of being too shallow"Wisconsin DNR
Stress beginsBelow 5 ppm dissolved oxygenMinnesota DNR (trout), Illinois Extension (pond fish)
Intermediate species tolerateDown to about 2 ppmMinnesota DNR
Aeration targetGreater than 2.0 ppm in the upper 5 to 10 feet in late winterWisconsin DNR
Snow to clearAbout 30 percent of the icePenn State, Illinois, Connecticut DEEP

On snow, three sources give the same number. Penn State says to "keep about 30% of the ice free of extended snow cover to maintain enough light to allow sufficient photosynthesis. Be sure that the ice is safe for the proposed method of snow removal."

On aeration and the hole, Penn State says "Consider installing an aeration system that helps keep a small pond area ice-free," and it "should be turned on when ice forms on the pond." Illinois gives the placement rule: "A bottom diffuser system can improve resistance to winter kill. However, this system should be moved to the surface or a shallow area during winter." Iowa State gives the warning: "Winter aeration may be dangerous due to the resulting hazardous opening in the ice or management practices during inclement weather." The Wisconsin DNR puts a barricade around every opening its aerators make. No extension or agency source gives a figure for a de-icer, and none says cutting or drilling holes by itself prevents winterkill.

On signs and recovery, Connecticut DEEP says "Fish typically die during the winter and are only observed following ice-out." Minnesota says "In lakes that support gamefish, the fish population can rebound quite dramatically in years following winterkill," while Connecticut says complete kills "are more likely to occur in very small, privately owned ponds. In this case, it may be necessary to restock your pond."

What pond owners on the Pond Boss forum do

The forum asks for more depth than the extension figures do, because most posters are in the snow belt. Bill Cody (Moderator) wants northern Indiana ponds at 15 to 18 feet over 20 to 25 percent of the surface. esshup (Moderator), an installer there, wants 40 percent of the surface at 12 feet in lake-effect snow. In the thread Winter 100% fish kill - aeration?, where drought had left 3 feet of water before ice, esshup read it as a pond with all its fish biomass crowded into less than 30 percent of its area, which likely caused the kill.

On snow, Bill Cody keeps 20 to 40 percent of the ice clear and never lets snow cover the whole pond longer than two to three weeks. On holes, he is blunt: drilling holes in the ice will not save fish, because bottom water does not rise to the opening on its own. A hole works when a diffuser is under it: esshup keeps a 30 foot hole open at one end of his 1 acre pond and has had no kill.

On where to put the diffuser, the posters nearly all agree. esshup runs a shallow diffuser all winter at about a quarter of the total pond depth, close enough to shore that the ice melts to the bank. Bill Cody pulls the diffuser into 3 feet of water at one end, away from skating. teehjaeh57 (Moderator) gives the reason: a deep diffuser under ice supercools the water, a different kind of fish kill. Theo Gallus (Moderator) explains how, since the diffuser lifts the warm 39 degree bottom water to the surface where it cools. The dissent is over how much it matters: Bill Cody says supercooling is not all that troublesome unless you have redear or coppernose bluegill. The worst outcome posted is Aaron Klusmire's 3 acre, 16 foot Wyoming lake, where two diffusers at 15 to 16 feet held a 50 by 50 foot hole open all winter and killed every trout.

On safety, esshup almost lost two Springer Spaniels through his hole and now keeps it where anything that falls in can walk out. On which fish die, esshup found most floaters in 4 feet or less, mouths open and gills flared, and in another kill 98 percent of hybrid stripers died against 70 percent of trout. ewest (Moderator) cited Iowa farm pond records where winterkill took both bass and bluegill in 18 ponds, bass alone in 15, bluegill alone in 2. The loss is measured in years of growth: mnfish (Hall of Fame) lost "5 years of fish growth" under ice. Before restocking, ewest says assess what survived. esshup has seen client ponds kill under a windmill during two calm, cloudy weeks while grid-powered ponds next door came through.

Where the tiers differ and why it matters

The extension depth figures (6 feet beats 3, a quarter of the pond at 8 feet) come from Pennsylvania and Illinois; the forum's 12 to 18 feet comes from the lake-effect snow belt and is practitioner judgment with no published trial behind it. The right number depends on how many weeks of snow-covered ice you get. The quarter-of-depth rule for the winter diffuser is esshup's own practice; Illinois gives the same instruction in words without a figure. Two ideas circulate without an authoritative basis: that a drilled hole or a floating de-icer alone protects fish (no source says so, and Bill Cody says it does not), and that winter aeration is a safe default (Iowa State and Wisconsin both treat the open water as a hazard to fence). The forum adds a risk the extension pages leave out: deep winter aeration can kill by cold.

What to do

  1. Measure the maximum depth and the share of the pond over 8 feet. If less than a quarter is that deep, or you are in the snow belt with nothing over 12 feet, plan on aeration or snow removal every winter.
  2. If you aerate, size it with the PondTools pond-aeration calculator (also standalone). It takes the pond area in acres, the maximum depth and whether the shape is irregular, and returns the diffuser count, compressor flow and the staged start-up. For summer sizing see how big an aerator a one acre pond needs.
  3. Move the diffuser for winter to about a quarter of maximum depth, 2 to 4 feet in most ponds, near shore and away from where anyone skates. Start it when ice forms and leave the deep diffuser off until spring.
  4. Fence or mark the open water; thin ice around a hole hides under snow.
  5. If you do not aerate, clear about 30 percent of the ice of snow and keep it clear. Check ice thickness first; Iowa State wants a minimum of four inches of clear ice.
  6. Do not rely on drilled holes or a stock tank de-icer alone, and do not make a windmill your only winter protection.
  7. At ice-out, walk the shore for two weeks. If you find floaters, net the pond before restocking; the tolerant species (bullheads, fatheads) are usually what is left.

Sources