Fish winterkill not an easy problem to see

Dead deer and pheasants are easier to spot with a white blanket of snow, but under the ice, a fish kill can be less noticeable.

Some obscured sloughs that suffer winterkill may go unnoticed. Depending on the severity, dead and decaying fish can be cleaned up by the natural cycle of gulls, coyotes and scavengers scavenging after the ice melts.

Aquatic vegetation produces dissolved oxygen through the respiration process of photosynthesis, just as land-based plants release oxygen into the air. The fish then process the oxygen using their gills.

Of course, sunlight is needed to start the process of photosynthesis, and in some winters, the combination of thick ice and snow on the surface prevents sunlight from reaching aquatic plants. At some point, the lack of sunlight kills the aquatic plants, so they no longer release oxygen into the water.

To compound matters, when plants begin to die, not only do they no longer produce oxygen, but they also actually draw oxygen from the water as they decay. In addition, the length of the day in December and January is so short that it naturally reduces the overall photosynthesis.

If the oxygen content of the water drops enough, the fish will begin to die. And when that happens, their decomposition also uses oxygen.

In any given year, several dozen reports of varying degrees of winterkill are noted. While our fisheries division manages more than 450 waters, consider last winter when 100 were on the official “watch list” as department staff rounded up lakes with low dissolved oxygen levels .

While some of these lakes may not experience any winterkill, many may suffer some degree of winterkill. Winterkill severity can range from a partial kill, where many fish die, to a total kill, where all fish die.

Some suggest combating winterkill by removing snow from the ice. While this sounds good, it’s easier said than done. Last winter, or any recent snow, you realize that it’s hard enough to keep your own driveway clear of snow, let alone enough acres of ice to make a difference for even a lake where winterkill is approaching. It is easy to imagine that a few hours after clearing part of a lake, the wind will quickly blow snow into the open space.

With hundreds of lakes monitored, fisheries managers appreciate reports from ice fishermen who observe dead minnows floating in holes. Once winterkill is suspected, all biologists can do is go in with nets as soon as the ice breaks out to see what’s left and possibly restock.

So, while driving through a stunning winter landscape of snow and ice reflecting the bright sun, it’s easy to see how the deer and pheasants are doing, but in terms of fish, the story unfolds beneath the snow and ice.