Ecology

Winter wonders

Anne Gageby
Strawberry Hill Nature Preserve

(1/2025 As a Naturalist at Strawberry Hill, I frequently get questions about nature from family and friends. Questions such as "are the trees in my backyard pines or spruces?" and "have you seen any lantern flies at your work?". Recently, a family member and I were chatting about winter weather and the impact it has on a stream’s ecosystem. This led to a discussion on fish and whether fish freeze during the winter. After all, frogs freeze during the winter and revive after thawing in the spring. So, if a creek or pond becomes crusted over with ice, the fish might freeze as well, right? Fortunately, the answer is no, they do not turn into popsicles for the duration of winter. The reasons why are as fascinating as you can imagine.

The first is fish antifreeze. Fish cells are full of omega-3s, those polyunsaturated fatty acids that doctors encourage humans to consume as part of a healthy diet. It turns out that they really are powerful nutrients for fish as well as people. Omega-3s contribute to cell elasticity and make cells resistant to freezing. This allows them to survive frigid waters. Fish have another neat adaptation that helps them survive even the harshest winters. They enter a state called torpor in which they’re able to slow their metabolisms to the bare minimum. In doing so they ensure their bodies require little to no food.

The second reason fish don’t freeze is water density. General knowledge tells us that water, like air, becomes denser, and therefore heavier, as it cools. So, we can make the educated assumption that a pond’s surface water will always be warmer than the water below it as the warmer water has risen to the top and cold has sunk to the bottom. Unfortunately, this isn’t entirely accurate. Yes, water density changes as its temperature changes but water is no ordinary substance and likes to break its own rules. Water’s density hits a maximum around 39 degrees. As the temperature creeps below 39 degrees, water density doubles back and gets lighter as water reaches the freezing point. In other words, the colder water gets, the lighter it becomes, which is why ice floats.

What does this mean for our pond? It means the pond is warmer at the bottom since it’s being warmed by the earth and because the colder, freezing water has risen to the top. This creates a stable, albeit cold, environment for our aquatic friends. Most fish species will gather in schools at the deepest part of a pond or lake and take a winter rest during the coldest parts of the year. Others burrow under sediment the way some frogs and other amphibians do. Gobis, for example, will often burrow under a pond’s silt and go dormant.

It’s quite interesting to take a walk around Strawberry Hill’s pond this time of year and marvel at the seemingly-lifeless landscape. The water is dark and usually capped with an icy sheet. It’s hard to imagine the thousands of creatures that are nestled snug in their wintery beds, hearts barely beating or not beating at all. The days of flitting dragonflies and basking painted turtles are a distant memory.

And yet, life endures. There are signs of it everywhere even in the deepest parts of winter. Foxes are using their keen sense of smell to detect mice under heavy blankets of snow. They use their impressive jumping skills to dive-bomb their prey, leaving behind easily identifiable tracks and the occasional blood splatter from a successful hunt.

Great horned owls’ mating season runs from late fall to early winter, providing plenty of opportunities to hear the familiar hooting calls of mating pairs. Females will be sitting on eggs by February and early March though sometimes eggs are ready by late January. If you’re looking to go owling this winter, the best time to go is an hour before sunset though you’ll almost certainly be able to hear some calls any time at night.

Then there’s the ermine, one of the three weasel species native to Pennsylvania. Their winter routine is as fashionable as it is practical. They change colors to match the snow. From spring through fall their fur is brown with white underparts. As the days shorten their coats turn white, camouflaging them with their surroundings. Though obviously harder to spot without a keen eye for such things, they will often leave evidence of their adventures in the form of long tracks through the snow.

And we can find evidence of life in our aquatic environments though we may have to look a little closer. Trout may huddle in deeper pools but they’re still active. Lower numbers of stream invertebrates mean trout may shift to eating drifting insects rather than chasing down a meal. Letting food come to them allows them to conserve precious energy. When the temps rise trout may feast on midges and mayflies if they’ve hatched. As the ice melts and spring makes its debut, trout start to move upstream to feed during runoff.

Bass, on the other hand, are quite active this time of year. They must be to survive as a species. Much of the calories consumed this time of year go toward reproduction as female bass develop their eggs primarily in winter. When food is scarce, their bodies will use stored energy to fuel egg development, ensuring next season’s spawn will happen.

The winter solstice was only two weeks ago and while the days have slowly begun lengthening, we still have many weeks of long, dark nights ahead of us. And yet, neither the forest nor the pond is truly quiet this time of year. Fish might be huddled in their schools while insects are nestled under leaf litter and other forest debris. But life is still there, waiting patiently for the return of warmer temperatures.

Anne Gageby is the Environmental Education Manager of the Strawberry Hill Foundation. Strawberry Hill inspires stewardship of our natural world by
connecting the community with educational opportunities.
 Learn more by visiting StrawberryHill.org.