Ken Semmens
Aquaculture Specialist
WVU Extension Service
11/2000
This article was published in the November 2000 issue of the West Virginia Farm Bureau News.
Winterkill, like summerkill, is a form of oxygen depletion that sometimes occurs during severe winter weather. Both conditions occur when oxygen concentrations become too low for the fish to survive. Before you become concerned for the fish in your pond, it is important to understand that winterkill is far less likely to occur than summerkill in West Virginia. It is most likely to occur during a severe winter in a shallow pond with abundant plant life or organic matter.
The winterkill process is relatively simple (see figure). Ice covers the pond and
prevents oxygen exchange at the water surface. This is not a problem as long as
light shines through the ice to plants in the water. More oxygen probably enters the
water through photosynthesis than at the water surface, anyway. Heavy snowfall
covering the pond may block sunlight and prevent plants from generating oxygen through
photosynthesis. Still there is no problem unless the demand for oxygen exceeds the
supply.
Without light, the plants may die and create an abundance of organic matter. The term "organic" refers to material derived from living organisms. It could be leaves, manure, hay, or-in this case-dead aquatic plants. Bacteria will take advantage of this food source and consume oxygen in the process. Oxygen levels may then become too low for the fish to survive, and there is a fish kill. Winterkill is more sinister than summerkill because the loss of fish may go unnoticed for several months. It may not be discovered until the first fishing in the spring.
The risk of winterkill can be decreased through proper pond design and management. A shoreline slope of 3:1 (3-foot run for every foot of drop) and minimum depth of 3 feet will help prevent rooting aquatic vegetation from proliferating. Some people recommend an average pond depth of at least 8 feet. The presence of color in the water caused by microscopic algae (phytoplankton) will help shade out submerged rooted vegetation while generating oxygen and providing a basis for the food chain upon which the fish rely. Reducing the nutrient flow into the pond may also be helpful. Manure, fertilizer, and other nutrients could be washed into the pond during a heavy rain.
If you have experienced winterkill and wish to prevent its recurrence, there are two basic strategies; you can keep ice from covering the pond, or you can remove snow from the pond surface so light will reach plants in the water. Continuous aeration keeps the water moving and keeps a portion of the pond ice free. It will also add oxygen directly and may reduce the buildup of organic matter. It is effective against all other forms of oxygen depletion.
Many models of electric aerators are available from aquaculture supply companies. You will find links to these sites at the West Virginia Aquaculture Association web site.
Snow removal may be a more practical approach because it costs less. Snow accumulations of under 2 inches soon melt and do not warrant removal. Heavy snowfall accumulations should be removed as soon as possible. Removing 30 percent of the snow, especially along the shoreline areas where submerged aquatic plants are most abundant, should be adequate.
Another option is to live with winterkill. Some fish farmers in midwestern states grow bait minnows in ponds that experience winterkill. In early spring, broodstock fathead minnows are released into the ponds. Winterkill may eliminate predatory fish, and the fertility of the system provides abundant food for the broodstock and their offspring. The minnows spawn and grow throughout the summer and can be harvested in fall or early winter.
The figure accompanying this article appears on page 36 of Ohio Pond Management Handbook.
