AUSABLE WATER WISE: Where do trout go in the winter?

Trout and other stream fish prefer to spend winter in habitats with large, deep pools and boulders and survive well under blankets of ice and snow. (Photo provided — Kelley Tucker)

Trout and other fish have many strategies to survive a northern winter in a frozen stream or lake.

In rivers and streams, having access to a mix of habitats including pools, areas where ice conditions are stable, and areas of slow current will lead to the best winter survival rates for trout and other fish. In the fall, after spawning, trout stop defending feeding positions and move short or long distances to seek winter refuge. Most often, they move downstream to quiet, slow waters.

As water temperatures cool, fish metabolism slows. Since food availability is greatly reduced in winter, this is an effective survival mechanism. The main goal is for a fish to expend minimal energy to catch its food. As a result, many trout, for example, shift to eating insects that are drifting in downstream currents as opposed to hunting for them. Occasional warm days might bring hatches of midges and small mayflies, which could mean easier picking. A slower metabolism also means food takes longer to digest, and so, food lasts longer. This means trout can survive longer without food. In winter, growth almost completely ceases as fish enter survival mode. If a fish managed to grow a lot last summer and increased their fat stores, they are in better shape to survive. But when the food is gone or fully digested, fish tap into fat stores. Mortality from starvation is more likely for smaller and younger fish because they have less time to build energy stores. Fall spawning expends a lot of energy leading into winter and can also decrease survivability in adults.

Not all winter mortality is caused by starvation; fish can also die from stress. Because they are slower moving, there are fewer chances of escaping from predators or quick-changing ice and flow conditions. Unfortunately, starvation and stress lead to increased winter mortality and many fish will die. When spring finally arrives and when ice and snow melt away, the trout that remain will start to move upstream to feed as quickly as possible.

Beyond slowing their metabolism, changing feeding strategies, and seeking out winter habitat, there are other strategies that fish use to survive. Large fish might school or aggregate in winter. Smaller fish may become nocturnal and move around a lot. Or they might simply find and occupy a single rock crevice for the entire winter. Some fish move upstream into snow-blanketed tributary streams. Stable fish habitat can exist where surface ice blankets the river in early winter and stays until spring. Such conditions allow fish to be more active during the daytime. Beaver ponds also offer excellent winter habitat for trout, and groundwater inputs, springs or seeps, can support aggregations of larger fish because they provide access to warm water. Backwaters and side channels are also important for escaping from winter flooding and ice breakups.

Both historical influences and the increased frequency and intensity of storm events have led to variable ice patterns in the Ausable. Habitats for aquatic animals change constantly through the winter season. When ice breaks up and refreezes, for example, fish can quickly be cut off from accessing necessary habitats. The goal for trout and other fish is to be able to consistently access a mix of habitat features. Then, their chances of survival are greatly increased. For stewards of freshwater resources, preserving and restoring habitat complexity and diversity is critical for species like our native brook trout. At the Ausable River Association we build better winter habitats for fish as we build stream health and flood resilience.

(Carrianne Pershyn is a research associate at the Ausable River Association.)