Do you ever wonder what happens to the fish in a frozen lake?

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. (Read Article)