What do fish do in the winter?

What happens to all the fish in the rivers during winter? Where do they go? How do they survive the cold temperatures and ice over the rivers? Turns out that all fish really need are some deep, slow-moving waters where the winter ice remains intact, and a few bugs to tie them over. Fish are cold-blooded, which means they don’t have much control over their body temperature. This makes them especially sensitive to changes in water temperature, which has become an increasingly large issue as our summers, and therefore water temperatures, are warming due to climate change. In the winter, as the water temperature decreases, their metabolism also slows down, making them sluggish while taking longer to digest their food. As food availability also decreases in the winter, needing less food and expending less energy is a pretty effective strategy.  

Since they aren’t moving around much, a lot of northern fish species go into a sort of torpor state. They might find a nook under a rock in a deep pool to hang out in for most of the winter. Fish might gather in areas with varied habitat features and slow currents. When temperatures get really cold, as they do here in Vermont, the fish want to hang out in an area where there is a stable and thick layer of ice to act as insulation. Fish are less likely to survive if the river has patchy snow and shifting sections of ice that keep refreezing, as the water can experience severe temperature swings. 

Beaver ponds can provide excellent, stable winter homes for our trout species, while areas where warmer groundwater enters the body of water also help the fish make it through the cold. Side channels of rivers, or areas with more debris, tend to experience less ice formation, ice dams, or anchor ice, which forms on the bottom of rivers where the flow is quick. Moving and shifting ice, or ice dams, can prove harmful to fish during the winter, as they can trap fish in one area without food, or force them to leave their winter “hang-out space”. If the fish are forced to move around, this stresses them and can force them to spend too much energy, increasing their risk of mortality.

What do they eat though? This question really interested me, as I thought all the bugs and plant matter would be gone from the rivers during the winter. Turns out that not all bugs die out during the winter, and since the fish have such a slow metabolism in the cold, they can usually survive on the few invertebrates that drift downstream. They can also tap into the fat and protein stores they built up over the spring, summer, and fall. Much like bears and other hibernators try to build up as much fat reserve as they can in the warmer months, fish also depend on their ability to store this energy for winter when food is scarce. Still, sometimes the fish are in need of more fat reserves and starve, with smaller and younger fish being less likely to make it through the winter.  

These days, with a shifting climate we are also experiencing shifting ice patterns, increasing the chances for fish mortality during the winter. According to an article I read from the Ausable River Foundation, if ice patterns change more rapidly during the winter due to increased melting and freezing, it could cause more stress for fish. With this in mind, it is important to maintain and encourage diversity along and in our rivers, providing a range of debris, plant matter, river bottom types, and general habitat complexity to ensure our fish have a safe winter habitat to sit out the cold weather.