Winterkill is the die-off (a bunch of things dying all at once) of fish in a pond during the winter. Winterkill is sad and frustrating for pond managers, but a winterkill event also demonstrates how hard it is to manage a pond that is a part of the larger natural world. A curious observer might wonder why one pond experienced a winterkill, but another nearby pond did not.
Several good questions might come out of a winterkill experience. Are the ponds that had and did not have a winterkill inhabited by different fish species? What are the different physical characteristics like depth, exposure to sunlight, substrate (bottom), or water chemistry in the winterkill pond? How are the plant communities different? How is the land use different around the two ponds?
First, here is a little more about what winterkill is. Winterkill is a fish die-off in a pond caused by winter conditions. An open pond has oxygen dissolved in the water (just like iced tea has sugar dissolved in it). When the pond freezes over (gets covered by ice) in the winter, it is sealed off from the atmosphere, and oxygen cannot dissolve into the pond water from the atmosphere. Fish need dissolved oxygen to breathe and will continue to use the oxygen stored in the ponds' water below the ice seal.
Winterkill happens when the water in the pond runs out of dissolved oxygen. One section of a pond can winterkill and another not, leading to a partial fish die-off. During a winterkill, most fish in a pond usually die, but that is not always the case.
It is also possible for some fish to die and others to live. For example, bullheads are likelier to live through a moderate winterkill than trout. This is because bullheads have different requirements to live than trout; trout require large quantities of oxygen in the water to live, and bullheads can live in much harsher conditions. Think about it this way: a popsicle requires extremely special conditions to remain a popsicle, but potato chips can remain as potato chips through many conditions. Trout are popsicles; bullheads are potato chips. The severity of the winterkill will determine if a few fish, many fish, most fish, or almost all fish die off.
Three common variables, or events, can work together to cause winterkill. These variables are how long the pond is iced over, how deep the pond is, and how productive the pond is (how much vegetation is growing in the pond). The longer the pond is iced over, the longer it goes without getting more oxygen dissolved into the water, and the more likely it will winterkill. Deeper ponds hold more water than shallow ponds that are the same size, which means that deep ponds have a bigger savings account of oxygen when they freeze over, compared to a similar-sized shallow pond. Since deeper water means more stored oxygen, deeper ponds can go longer without new additions of oxygen.
Lastly, large amounts of vegetation will increase a pond's chance of winterkilling. In the fall, much of the vegetation in a pond will die, like the leaves falling off a tree in the fall. Over the winter, this dead vegetation will decompose (rot, decay), and the decomposition process consumes (uses) oxygen. This means that decomposition will also use up the oxygen stored in the pond, just like the fish breathe the oxygen stored in the pond.
There is one vital but sometimes confusing detail about deep ponds and big ponds. Deep ponds are less likely to winterkill than big but shallow ponds, even if they hold the same exact amount of water. Imagine two ponds with the same volume of water, but one is big and shallow, and the other is small and deep. The small, deep pond is like a drinking glass full of water, and the large shallow pond is like the same glass of water poured into a large cooking pot. In ponds, lakes, and oceans, the shallow water area where light reaches the bottom and rooted plants grow is known as the littoral zone.
Shallow ponds might have bottoms completely covered in vegetation because light can reach the bottom of the entire pond. However, deep ponds tend to have small littoral zones and much less vegetation. Therefore, deep ponds have lower productivity than shallow ponds of the same volume because sunlight does not reach as much of the bottom of a deep pond, and plants cannot grow in the deep water of deep ponds.
The three variables related to winterkill, duration of ice over, pond depth, and productivity (amount of vegetation) work together. This can help explain why one pond may winterkill and a neighboring pond does not. Maybe one pond is deeper than the other. Local physical factors like wind, sunlight/ shade, and temperature may cause one pond to freeze later in winter than the other, shortening the duration of ice.
The land use around a pond might lead to increased productivity; fertilizer, septic system leaching, and agricultural runoff are all known to increase the plant growth (productivity) of nearby ponds.
New ponds tend to be less productive than old ponds, and ponds tend to get shallower over time; a pond may winterkill more often as it ages. A pond managed for large, sensitive game fish species may be more likely to experience a winterkill than a pond full of bullhead and catfish (ice cream and Lunchables). To make it even more complicated, it is possible that half of one species will live, and the other half will die. Possible causes of this could be variations in individual fish health or different conditions in different locations within the pond.
Nature can sometimes be very mysterious, but understanding the primary factors behind winterkill can help us understand how seemingly minor differences in a pond or a winter season can determine whether a winterkill happens.
Lastly, and most importantly, winterkill reminds us to remain humble in the face of nature.
Christine Cornwell is a horse trainer who lives in upstate New York with her husband and son. She worked for the New York State Department of Environmental Conservation as a high school science teacher. Sharing her love of a healthy, natural world is a daily passion.
Reprinted with permission from Pond Boss Magazine
