Late Feb. Key Time for Fish Survival

The final weeks of February are a crucial time for fish under the ice across the northern plains. It's at this point in the winter when dissolved oxygen levels are typically at their lowest and the risk for winterkill on smaller bodies of water increases. With heavy snow cover on lakes throughout the eastern half of North Dakota for much of the winter, agents of the North Dakota Game & Fish Department will begin their annual assessment of dissolved oxygen content on those at-risk waters in the coming days, according to BJ Kratz, Southeast Fisheries District Supervisor.

"Winterkill is something that is very difficult, if not impossible, to predict. It's not necessarily reflective of lots of snow, or super-cold temperatures, or more cloudy days than sunny days, or low water levels, or high water levels. There's just so many variables that come into play, that it's very hard to say what normal is. The more snow that we have early on – that is, late November and then through December – the more likely on the average, that we could have winterkill issues. But we didn't really have a lot of snow until that latter part of December, which has helped a little bit," Kratz explains.

Oxygen enters a body of water through two means: atmospheric contact at the surface during the openwater season, and through the production of the element as part of photosynthesis of underwater plants year-round. The latter can even occur in winter, when ice remains clear and lacks snow cover, allowing sunlight to reach aquatic plants which continue the process. This oxygen in the water is then absorbed by fish through their gills, where tiny blood vessels in the structures take the vital element into their system, while expelling carbon dioxide. The die-off and decay of plants and other materials reduces oxygen content in the water - particularly in winter - through the process of breaking down organic items. When heavy snow cover prevents sunlight penetration through the ice and this natural decomposition occurs is when things can get dicey for certain fish species in winter as the water becomes less oxygenated, and there isn't as much for their gills to absorb.

"We have fish such as the fathead minnow that happens to be a very tolerant species of dissolved oxygen. We probably can all account for that. As fishermen we see how tough those things are when it comes to surviving in a bait bucket. Also, bullheads are right there too. So those two species are very tolerant of very low oxygen. Now you go to the other end of the spectrum, and you have largemouth bass, bluegill and walleye tend to be one of the least tolerant of low dissolved oxygen. You go down a little farther and you have pike and perch which tend to be a little more tolerant of lower oxygen, and crappies actually can be pretty good at finding adequate oxygen when the conditions are tough," Kratz explains.

It's a common misconception that lakes winterkill due to freezing solid in a harsh season, but that only occurs only on the shallowest of waters that are already unable to support fish. Rather, it's the chemical process of dissolved oxygen loss in the water which does in a population of fish. According to Kratz, ice anglers can often get environmental cues from the water during the season if it is at risk of a die-off. Typically, water low in dissolved oxygen will have an odor to it similar to natural gas or the scent emitted when wading through the shallows of a muddy lake or stagnant slough. Alternatively, if a die off is occurring under the ice, the water may have a slight fishy odor to it, suggesting some species may have already succumbed to the hypoxic conditions. While winter conditions in eastern North Dakota may have been severe this season, Kratz is optimistic that most waters will be okay.

"We did have a little break last week [Feb. 7-13], we got some good thawing weather, it cut down a lot of the snow. We'll be out checking quite a few different lakes we know that typically aren't real good through the winter and we just kind of use those as our indicator. If those lakes are still hanging on, we don't worry about checking the lakes that are typically better." Kratz advises.

NDG&F employees survey those most at-risk lakes by first checking the deeper basin, measuring snow cover and ice thickness before punching a hole and gauging water clarity using a Secchi disk. Using an electronic sensor, the agents check oxygen content in the water from the bottom to the top of the column, a meter apart in each reading, to make a determination. If the surveyed amount of oxygen is at two parts per million or higher, the lake will likely be okay and avoid a winterkill. If the reading is below two parts per million, other areas of the lake are surveyed for a complete understanding of the conditions, as some reaches may have better or worse dissolved oxygen, based on quantity of plant life, inflowing water or springs and seeps on the bottom along with other factors.

If some of the most at-risk lakes show good oxygen content over that two parts per million threshold, the department does not survey lakes they historically have found to better handle winter conditions. If a lake's dissolved oxygen content over the next two or three weeks turns up in the 1.0 to 1.9 parts per million range, however, the risk of a winterkill is increased for that water and similar ones which may require more surveying. In the spring, the agency relies on angler reports of fish die-offs to confirm conditions and to begin the re-stocking process after a full or partial winterkill takes place. After ice-out, anglers can report observed fish kills by calling 701-328-6300.