FOR IMMEDIATE RELEASE: April 1, 2003
Media Contact: Tom Jones 802-241-3700, Shawn Good 802-483-2744

FISH “WINTERKILLS” ARE NATURAL EVENTS

WATERBURY, VT – Some natural fish mortalities will likely be discovered as Vermont’s ice and snow pack melt from lakes and ponds this spring, according to Vermont fisheries biologists.

Fish deaths due to “Winterkill” is relatively common, but Vermont Fish & Wildlife Department fisheries biologists are expecting to see more fish losses this year because of the early and persistent ice and snow conditions.

“This type of fish kill is most common in shallow ponds and lake areas with large quantities of weed growth and high accumulations of organic matter,” said Fish Health Biologist Thomas Jones.

Many shallow lakes and ponds normally receive the majority of their dissolved oxygen through plant photosynthesis. The remainder of the oxygen found in these lakes and ponds is supplied by wind and wave action.

“Moderate to heavy snow packs on top of the ice will greatly reduce sunlight penetration and photosynthesis,” said Jones. “As aquatic plants die, the decomposition process uses up oxygen. The levels of oxygen can become depleted in just a few days or over the course of the winter, depending upon the severity of the conditions. When oxygen is being consumed at a higher rate then it is being produced, fish will suffocate.”

Winter-killed fish die under the ice and may not be noticed until ice-out. Severity of the fish losses depends on how severely the oxygen levels were depleted. In most cases, winterkill does not result in total loss of a fish population. Usually, enough fish survive that the population will recover.

Winterkill is just one type of natural fish mortality.

Jones says biologists annually document spring and summer fish kills caused by a combination of stress factors that overwhelm the immune systems of fishes. Rapidly fluctuating water temperatures, extremely warm water temperatures, spawning activity, unbalanced fish populations, low oxygen levels, poor nutrition, and fish diseases are common natural causes of fish dieoffs.

“Natural fish kills can seem alarming to people who witness them,” added Jones. They may temporarily affect fishing and the predator-prey balance. Natural fish kills may reduce over-populated, unhealthy fish populations. They can be thought of as Mother Nature’s way of keeping a good balance between fish populations and available habitat.”
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For Further Information please contact: Tom Jones at 802-241-3700 or email to tomj@fwd.anr.state.vt.us

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