Fish in Winter

While Lake Champlain is not as cold or icy this winter as it has been in winters of yore, it is still not a place for a casual swim at this time of year. This inaccessibility gives the depths of the lake a certain mystery. What is going on underneath the dark surface of Lake Champlain as winter unfolds? How do fish hunker down and survive what seems to be a hostile environment of icy cold water all season long? 

To understand what fish do in the winter, we first need to know what changes the lake undergoes this time of year. Generally, cold water is denser than warm water, so in the summer the coldest of water is at the bottom of the lake in the layer called the “hypolimnion.” Water is actually most dense at 4° C (about 39° F) and becomes less dense at colder temperatures than that point, so when temperatures drop to near freezing in the winter, the hypolimnion becomes the warmest layer at around 4° C. This unique property of water is the reason ice floats.

The stratification of water temperatures in the winter sets the stage for Lake Champlain fish, and their roles vary depending on what kind of species they are. Almost all freshwater fish are cold-blooded: they are about the same temperature as their environment and must thermoregulate through their behaviors (e.g. moving to warmer water) rather than through physiologic processes like shivering as humans and other warm-blooded animals do. Essentially, fish do not protect themselves from the cold at all—they just become one with it. Fish can sense even slight changes in temperature, and each species of fish have a different range of optimal water temperatures and are generally categorized as warm-water, cool-water, and cold-water species based on that range. For example, a warm-water fish like the common carp ranges from 20-25° C (68-77° F), a cool-water fish like yellow perch ranges between 17 to 25° C (63 to 77° F), and cold-water fish like lake trout prefer between 4-10° C (40-50° F). Temperatures above or below a fish species’ optimal range affects their metabolism and slows their growth.

In winter, fish take something like a season-long slow-down. At near-freezing temperatures, the lipids (fats) in a fish’s body start to stiffen (think of putting olive oil in the refrigerator), making their muscles less able to move freely. From this physiological slow-down, fish activity drops—because of a slowed metabolism, fish are not eating or moving as much as they are in the summer. Though not quite in a state of hibernation or torpor, fish are hardly doing anything at all but surviving off stored energy until the water warms closer to optimal temperature in spring.

That is, at least, the popular thinking of what fish do in the winter. Dr. Ellen Marsden, UVM professor in the University of Vermont Rubenstein School of Environment and Natural Resources and fisheries biologist, points to new research that indicate a more complicated reality of what fish are really up to under the surface--“The idea that ‘everything just slows down in the winter,’ may be a misconception.” she noted.

The split between warm, cool, and cold-water fish species turns out to be more predictive of fish winter behavior than previously thought. For warm-water species like carp, the popular thinking of fish behavior in winter is largely true. They tend to settle in deeper waters near or in the hypolimnion layer, as this is where water is warmest, and will feed only opportunistically. For cold-water species like lake trout, colder winter waters can actually open up a wider range of habitat than they have in the summer (when they are more relegated to colder, deeper waters). The whole water column can become their oyster, and while their metabolisms and activity levels do slow, it is to a much lesser extent than their warm-water counterparts.

Cold-water fish take full advantage of the wider habitat and slowed competition (and prey). Lake trout will forage (or eat) all year round, consuming smaller warm-water fish in the winter who have less energy to escape them. Lake whitefish, another cold-water species, were found to consume 34% of their annual energy in the winter, and actually increase lipid content during the cold season rather than deplete fat stores for survival. Similarly, burbot were found to be more active in winter than in summer. This cold-loving species require temperatures of between 2° to 4°C (35° to 39° F) to incubate their eggs. For this reason, they wait until winter to spawn in shallow waters, and spend much of the warmer parts of the year in the hypolimnion. Read more about burbot in our Nature Note here.

For the in-between cool-water species like yellow perch, timing is everything. Their unique position allows them to be active in early winter and early spring when warm-water species are still in their frozen slumber. From October to early December, lake trout take the opportunity that the fall temperature mixing of the lake presents to spawn. Lake trout eggs, and indeed most freshwater fish eggs, are high in fat and make an excellent source of energy for a cool-water fish like the yellow perch, so right around now yellow perches in Lake Champlain are likely sporting bellies full of lake trout eggs. As the season gets colder, it becomes the cold-water fishes’ turn to feast on the eggs of a host of fish species.

In Lake Champlain, the impacts of climate change on fish are more well understood for summer. Warmer water temperatures will make the top layer of the lake—the epilimnion—deeper and warmer. This will drive fish, especially cold-water species, further down into the depths to avoid temperatures that are too high. The problem here is that deeper waters are oxygen depleted. This puts cold-water species between a rock and a hard place—if they go further down, they lack oxygen, and if they go further up, waters are too warm. While this is a greater problem in smaller lakes with narrower hypolimnions, Lake Champlain will become a more stressful place for cold-water fish as temperatures rise. It is likely that warmer winter water and altered ice dynamics will also bode poorly for cold water fish, who may now face greater competition from cool and warm- water species.

Freshwater fish activity in the winter is not an easy subject to study—there are challenges in the field when temperatures drop and the lake freezes, such as malfunctioning equipment, fewer sampling methods available, and of course the danger of sub-zero temperatures. With new technology like radio telemetry, researchers are discovering that fish are much busier in the winter than we may think. Dr. Marsden urges us to “[not] forget about winter!” What we know of fish in the winter may just be the tip of the iceberg, and understanding winter activity will help us understand Lake Champlain fish better as a whole. There are still mysteries yet to be unlocked under a winter lake.