Winter takes bite out of fish population

One of the aesthetically pleasing things about living in the Gingerwood section of Loves Park is the small lake and charming pond adjacent to Pebble Creek. These bodies of water offer an excellent opportunity to study wildlife of many types, and afford fair fishing for sunfish and bullhead catfish. Though I have never caught one, rumors circulate that bass are also present in the pond and lake.

These small, shallow bodies of water are now frozen over and covered with a layer of snow, and I can envision what is happening beneath the ice. The supply of dissolved oxygen that is necessary to all forms of life is gradually being depleted, and when the 2 parts per million level is reached, fish will start suffocating.

Oxygen is primarily recharged in an impoundment in three ways: the inflow of oxygenated water from a tributary or rain; oxygen absorbed from the atmosphere; and the oxygen produced by aquatic plants via the process of photosynthesis. This last process, using energy from the sun, converts carbon dioxide into plant tissue and liberates free oxygen. When the pond is covered with ice and snow, the sunlight that is essential for photosynthesis is denied the aquatic plants, and they eventually die. This adds to the problem, as the process of decay requires oxygen, and the supply of the dissolved, vital gas is further depleted.

Winter fish kill is the most common type of fish kill, and when it is severe, it has devastating effects on fish populations. Shallow impoundments with abundant vegetation and murky bottoms are especially prone to winterkill. We seldom notice the effects of winterkill until spring when the ice melts. Then the dead fish, especially the larger ones, .pile up along the shoreline. Larger fish require more oxygen, so they are the first to suffocate and die. Most of these dead fish will have a white, fuzzy appearance caused by secondary infections of fungi attacking after death. But the fungi did not kill the fish.

Different species of fish vary in their tolerance for low dissolved oxygen content of the water. Trout are the most susceptible, while bass, walleye, and bluegills have intermediate sensitivity. Members of the pike family, yellow perch, and pumpkinseeds are relatively tolerant, while bull head catfish and some minnows are very tolerant.

Often it is possible to judge whether a body of water is subject to severe winterkill by its fish population. In such impoundment, low oxygen-tolerant species will far outnumber the less tolerant types. Fortunately, sufficient numbers usually survive in a deep part of the impoundment or in connecting waters to repopulate in a year or two. Fish stocking is only required if the winterkill is abnormally severe.

Some aquatic vertebrates have evolved ways of overcoming the lack of oxygen in a frozen body of water. The diamondback terrapin is a prime example. When winter arrives, they burrow into the bottom and shut down almost all physiological functions.

A hormone is produced that acts to constrict the blood vessels supplying the organs of the body, and as a result, the metabolism of the animal is reduced to practically zero. The physiological process known as glycolysis provides what little energy is required to keep the heart beating a few times a minute. In this operation, no oxygen is required to metabolize small amounts of stored foodstuffs, and no poisonous carbon dioxide is produced, as is the case when oxygen is used to burn the fuel of the body.

The only long-term solution for fish winterkill is to reverse the natural process of filling and enrichment (eutrophication) of bodies of water subject to kill. Dredging or sucking bottom sediments will increase the volume of water. Such projects are expensive, however, and may require special permits from governmental agencies, and a site on which to deposit the dredged material must be found.

Another way to assist a body of water fight winterkill that is far less complicated than the above, and that is to keep down excessive plant growth and subsequent eutrophication by doing everything possible to keep chemical fertilizers out of the water.

Natural winter fish kills are disturbing, and may affect fishing and predator-prey balance for years. They are, however, often not serious in the long run and may be considered as nature’s way of thinning out fish populations in crowded environments.

I hope this winter’s fish kill will not be severe, and next spring I can take my fly rod down to the pond or lake near my residence and entice a hand-sized bluegill to accept an artificial fly or popping bug (and, there is always the slim chance a lunker bass will surprise me).

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