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Winterkill Update: Spring 2023

Winterkill – yeah, it happens.

If you’re an angler keen to get out on the lake in the spring, chances are you may have observed a fish winterkill, especially where the lake is relatively small and was completely frozen over all winter. The usual evidence is a few or many fish carcasses floating on the surface or along the shoreline. Depending on the severity of the winterkill, fishing may also prove challenging.

Which lakes were affected this past winter?

There is no comprehensive list of all lakes that have had winterkills each year in B.C. Only a subset of lakes that are known to have potential winterkills are monitored annually for oxygen levels. In 2023, most of these lakes exhibited very low dissolved oxygen levels.

Specifically, lakes in the Thompson-Nicola (Region 3) and Cariboo (Region 5) that are confirmed to have experienced winterkill this past winter, or that have observed low-oxygen levels and are likely to have suffered at least a partial winterkill, include:

This list will be updated as the ice comes off more lakes and reports come in. You can also contact the Fish and Wildlife offices in Regions 5 and 3 (where lakes are monitored for winterkill) to get the most up-to-date information.  

So what is winterkill?

Winterkill is a term used to describe a fish die-off due to depletion of oxygen in the water while the lake is covered with ice. An adequate level of dissolved oxygen in a lake is usually maintained during much of the year in various ways. These can include interaction of the lake’s surface with air and wind; the production of oxygen by aquatic plants through photosynthesis; and inlet streams that tend to bring in water well-oxygenated from flowing and cascading over rocks.  

Once frozen over, a lake is cut off from most or all sources of oxygen. The lake’s waters will gradually become oxygen-depleted, sometimes to critical levels depending on the specific bathymetry (lake shape and size) and biological oxygen demand (from plants respiring, animals breathing, and detritus decomposing as bacteria feed on decaying material).

The length of time a lake is covered with ice also has a major influence – if winter lasts longer than usual, you may see more fish mortalities that year. Snow cover can exacerbate the situation, because it will reduce or eliminate the light that aquatic plants need for photosynthesis.

Small, shallow lakes will experience more frequent and severe winterkills, as will lakes with an elevated load of detritus from abundant nutrients and summertime algal growth. Large, deep lakes with lots of water volume tend to retain higher levels of dissolved oxygen. While they may experience some degree of winterkill, it is rarely at a level significant enough to impact the fish population.


Image: Dairy Lake, May 1, 2023. | Mike Phelps, Ministry of Forests

The level of oxygen that is considered critically low for fish depends on the species. Bullheads, carp, goldfish, and some minnows are the most tolerant of low oxygen, while pike, perch, crappie, and walleye tend to have an intermediate tolerance. Trout species are the most sensitive, and are usually the first to suffer winterkill.

Depending on how severely the dissolved oxygen level was depleted, you may see small numbers of the more sensitive species, or you could see a larger number of dead fish of many species. Once the ice breaks up, dead fish may be seen floating on the surface in various states of decay that can indicate when they died.

Is winterkill normal, and should I be concerned?

Winterkill is a normal and natural process that occurs for lakes in regions that see ice cover and snow. While it may be perceived as a negative occurrence (and it certainly isn’t attractive to the eye), it is not always detrimental to a lake’s ecosystem, and can sometimes provide benefits to fisheries and fish populations depending on severity.

For example, a partial winterkill can reduce fish numbers and, with less competition for food, the remaining fish can grow larger. A severe winterkill of invasive species like yellow perch could be beneficial in creating a fishery for native species like trout that can be re-stocked after good numbers of invasive fish have been depleted.

Managing for lakes with winterkill

Avoiding winterkill requires preventing ice from fully covering a lake, and circulating the most oxygen-deprived water near the bottom to the top layers. Several lakes around the province have been outfitted with aerators – small electric propellers on the surface attached to a tube that extends deep into the lake to draw up the oxygen-depleted water and aerate it. However, since aerators require access to power and routine maintenance, they are generally only utilized in highly accessible and high-use fisheries.

The effects of winterkill can also be addressed by alternate approaches to stocking. The Freshwater Fisheries Society of BC annually produces a limited number of larger trout that are catchable-sized, and immediately available to the fishery during the open-water fishing season. Provincial biologists can request stocking these “catchables” into known winterkill lakes. However, large catchables are limited in number due to their space and food requirements in the hatchery; it is not always possible to meet all requests, especially after a severe winter has resulted in large numbers of winterkill lakes.


Image: Duffy Lake, May 1, 2023.  | Mike Phelps, Ministry of Forests

Should I report a fish kill?

Absolutely. While winterkills are normal, the Provincial Fisheries Biologist of your region is still very interested in receiving reports. This information helps with their management planning. Sometimes, a fish die-off may be for other reasons that do require an immediate response (e.g., a chemical spill or other water contamination). Either way, documenting where and when fish kills are observed ANY TIME OF THE YEAR in ANY WATERBODY and reporting these to your local Fish and Wildlife office is helpful to managing and protecting aquatic ecosystems and fisheries. If you’re able, also note the fish species, and try to get an idea of the approximate number of dead fish. 
 

Authors: Sue Pollard and Paul Askey (Freshwater Fisheries Society of BC), with contributions from Marcus Boucher (Freshwater Fisheries Society of BC), and Mike Phelps (Ministry of Forests). 
Images: Brent Gill, Mike Phelps, Ministry of Forests