Winterkill/issue for fish

[Lee Borgersen]

          Winterkill an issue for fish

When we talk about winter wildlife mortality in North Dakota, most people think of pheasants or deer, as these species are often visible in the countryside along ditches, shelterbelts or harvested agricultural fields. While snow and cold create challenges for these animals on land, they can also make life difficult for fish under water, a situation that is much less obvious than pheasants or deer highlighted against a white background.



When we talk about winter wildlife mortality in North Dakota, most people think of pheasants or deer, as these species are often visible in the countryside along ditches, shelterbelts or harvested agricultural fields.

While snow and cold create challenges for these animals on land, they can also make life difficult for fish under water, a situation that is much less obvious than pheasants or deer highlighted against a white background.

Winterkill is also a word used to describe a situation when water no longer contains enough dissolved oxygen for fish to survive. Without oxygen in the water, fish suffocate, just as they do when you take them out of water.

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

Of course, sunlight is needed to start the photosynthesis process, and during some winters, the combination of thick ice and snow on top of it prevents sunlight from reaching aquatic plants. At some point, lack of sunlight kills aquatic plants, so they no longer release oxygen into the water. To compound matters, once plants begins to die-off, not only do they no longer produce oxygen, they actually take oxygen out of the water as they decompose. In addition, the day length in December and January is very short thus naturally reducing overall photosynthesis.

If the water’s oxygen content falls low enough, fish start to die. And when that happens, their decomposition also uses oxygen.

The extensive wet cycle from 1993-2011 kept many lakes, sloughs and reservoirs flush with fresh water. The extensive snow cover during each winter from 2009-11 did create some issues, but on the other hand, good snowpack means good runoff into lakes, which in the long run is a good thing overall.

Winterkill has been a limiting factor on many North Dakota waters for decades.

It’s a word that draws winces of pain from anglers who were banking on another year of successful fishing in a lake that maybe wasn’t big enough or deep enough, or didn’t have the right aquatic habitat, to sustain fish through a winter that produces heavy snow, especially when it comes early and stays.

Lakes that suffer a die-off can be restocked and again become productive fishing waters. But every so often, maybe once every five-to-15 years, the right combination of circumstances come together to threaten that fishery. It should be noted that some lakes are more prone to winterkill than others. This is usually due to shallow waters and excessive nutrients.

The Department manages about 400 fisheries and even in the worst years maybe only 10-15 percent will suffer any winterkill. But at the start of each winter, it’s impossible to tell which ones, if any, will succumb to low dissolved oxygen.

In snowy winters, some suggest battling winterkill by clearing snow from the ice. While this sounds good, it’s much easier said than done. One look outside and I realize it’s hard enough to my keep my driveway clear of snow, let alone enough acres of ice to make a difference for even one lake where winterkill is looming. It’s easy to imagine that hours after clearing part of a lake, winds would quickly blow snow into the open space.

With hundreds of lakes to monitor, fisheries managers appreciate reports from anglers who observe dead minnows floating up in holes. Once winterkill is suspected, all biologists can really do is go in with nets once the ice goes out, see what’s left and possibly restock.

So, while driving by a picturesque winter scene of snow and ice reflecting the bright sun, it’s easy to see how the deer and pheasants are faring, but in terms of fish the story is unfolding beneath the snow and ice.

Doug Leier is a biologist for the Game and Fish Department