It is winter in the Northern Hemisphere. The vicious cold has transformed the scattered blue lakes of the North Woods into white disks — barren wastelands of ice. The harsh winds rushing across the icy plains combined with average air temperatures that are just above freezing seem to offer a less-than-hospitable refuge for wildlife.
But a keen sportsman knows better. Cutting a hole in the ice and dropping a colorful lure down into the depths of the lake, a patient ice fisher knows that luck is on her side. Obscured from human eyes underneath the ice lies a healthy stock of fish, tantamount to populations in the warmest months of the year.
“They survive just fine under the ice,” says Jake Vander Zanden, Director of the University of Wisconsin–Madison Center for Limnology. “They are adapted to survive in these low temperatures; it’s not that big of a deal.”
Fish survive quite well in the winter because they evolved experiencing the annual changes that take place in the Northern latitudes, which include big changes in temperature and the availability of oxygen throughout the seasons.
In the summer months, the water at the surface of a freshwater lake is heated by the sun, while the water at the bottom of the lake remains colder. Because cold water is more dense, it gets “locked in,” stuck underneath the warmer, less dense water.
As the months move by and the weather gets colder, the lake slowly moves toward an even temperature. Once the temperatures match between layers, the density differential dissipates and the water column flips over, in a process called fall mixing. The same mixing process happens again in the spring once the ice melts and the winds can churn the waters once again.
Following the fall water cycle, water temperatures across the lake reduce and the lake surface freezes. Because fresh water is maximally dense at 4 C, or 39.2 F, the water at temperatures below 4 C actually rise to the top of the water column, making the bottom layer the warmest, and the most attractive habitat for certain fish species to survive in during the winter.
Freshwater fish are “poikilotherms” that cannot regulate their body temperature except by their own actions, like swimming or basking. They are divided into two categories, warmwater and coldwater species.
“An example of a warmwater fish is a bass, they have their optimal temperature conditions on the warmer side,” Vander Zanden says. “They might be just found at around 25 C (77 F) and above, whereas coldwater fish may have their optimal conditions at 10 C (50 F).”
Outside of their optimal temperature range, fish must make adjustments to survive. One of the most common ways that fish adjust to the winter temperatures is by decreasing movement, thereby slowing down their metabolism to conserve energy, and diminishing their need to hunt or forage. And certain fish, like some species of catfish, will actually burrow into the soft silt down on the lake bed to stay warm.
“Many fish species are low energy during the winter, they’re sitting there not moving around very much, and not feeding at all,” says Vander Zanden. “But, if you’re a fish swimming around, you still might get eaten by another fish.”
“The same predator prey interactions are happening under the ice,” he adds.
The consistency in the food chain under the ice assures that ice fishermen can secure a catch, knowing that hungry fish will be attracted to their lures. But food is one side of the survival coin for fish. On the other side are their oxygen needs.
“From the perspective of a fish or any organism that needs oxygen, the aquatic environment is not a great place to be, because oxygen is in really low abundance in aquatic systems versus air,” Vander Zanden says.
When the water is not covered by ice, oxygen from the air is readily cycled into the water. But once that icy lid is placed over the top of the lake, that process largely stops. Some amount of oxygen is replenished through the photosynthesizing plants that survive under the ice, although light cannot get through the ice when heavy snow is packed on top. Underneath the ice, fish consume an ever-decreasing supply of oxygen.
According to Vander Zanden, Lake Mendota presents some additional challenges for fish looking for oxygen.
Due to farming runoff and pollution, algal blooms form and sink to the bottom of the lake. In the winter as the blooms decompose under the ice, the process sucks precious oxygen out of the water. An area where a larger mass of the blooms is decomposing can become anoxic, or oxygen-starved.
“We never have [anoxic events] in the atmosphere, we never say, ‘Oh yea, my backyard went anoxic today,’ that doesn’t happen,” laughs Vander Zanden. “But it is an issue for fish, so fish have a lot of adaptations for extracting oxygen from their environment.”
According to Vander Zanden, fish can extract oxygen through a variety of adaptations, not only through their gills. Different fish species do this either by absorbing oxygen into their skin, into the blood vessels in the walls of their swim bladders, stomach and gut, and some even inhale the air bubbles that form underneath the ice through their mouth.
However, sometimes anoxic events become too widespread for the fish populations to escape. When an entire lake becomes oxygen starved, winter-kill events take place. As the anoxic zone creeps upwards into the water column, fish cling to the under-surface of the ice as the oxygen is depleted, until they suffocate to death. This can lead to some alarming sights, like this photograph captured after a winter-kill event in South Dakota.
Winter-kill events are more common in lakes much smaller than Mendota, says Vander Zanden, where the volume of water makes those events unlikely.
“The fish are there in the fall and they are there again in the spring,” Vander Zanden says. “The whole food web is alive and kicking in the winter.”