Fish do freeze in a frozen lake, sometimes. But few lakes freeze to the bottom.
Most lakes freeze only on the surface, allowing fish to survive in the water beneath the ice.
Interestingly, even when fish do find themselves trapped in ice, they may be able to survive the experience.
Let’s look first at how lakes freeze.
Ice forms on the surface of lakes because ice is less dense than water. This is the opposite of most substances, which are more dense as solids than they are as liquids.
Once a cap of ice forms on the surface of a lake, it serves to insulate the lake from the atmosphere, which greatly slows the freezing process.
Even when it has been twenty degrees below zero for a month, chances are that fish are doing fine.
Small, shallow ponds are different. In hard winters, they may freeze enough to kill fish. That doesn’t mean that the fish freeze to death. They more likely suffocate as the oxygen in the tiny supply of water around them diminishes.
It is interesting to ponder what would happen if ice were to form on the bottom of lakes.
In the short term, fish would not have access to insects and other invertebrates on the lake bottom and would starve.
In the long term, bottom ice would build up and completely fill many lakes. The depth of the water would initially protect the bottom ice from seasonal melting. When the ice had built up to the surface of the lake, its tremendous mass would prevent any significant seasonal melting.
It is lucky for us, and lucky for fish, that ice forms on the top of lakes.
Why does this happen?
If you have ever swum in a natural lake in summer, you remember the warm surface temperature of the water and the cooler water around your toes as you trod water. If you dove down, you found the water to be cooler still.
Shouldn’t ice, then, form on the bottom of the lake, where the coolest water of all should be?
The answer is no, due to a curious property of water.
Above 40.1 degrees Celsius, water behaves like most substances, i.e., the warmer it is, the less dense it is. But below 40.1 degrees Celsius, water behaves differently than most substances and becomes less dense the cooler it is. Once the water temperature of an entire lake is below 40.1 degrees Celsius, the coolest water is found on the surface of the lake. And this is where ice forms.
What about the fish that do find themselves trapped in ice, for example in the pool of a small stream during a cold snap?
Fish blood contains salts which make it freeze at a lower temperature than fresh water. A fish caught in a mixture of water and ice is experiencing a temperature of 0 degrees Celsius, which is not cold enough to freeze it. Only if the ice freezes completely around the fish, and then cold air lowers the temperature of the ice below 0 degrees Celsius, will the fish freeze.
Loren Eiseley, the famous natural science writer, once found a fish frozen in a block of ice on the Platte River in Nebraska. He took the fish, a catfish, home and placed it in a pail, thinking to study it the next day. Overnight, the fish revived.
Eiseley put the catfish in a tank and kept it alive for some time afterward, until one night the catfish leaped onto Eiseley’s living room floor. The catfish gambled, as Eiseley saw it, on being able to flop over to the main river channel from the small pool of water in which it found itself.
There is one last way in which fish avoid freezing to death, particularly in the ocean.
Ocean water contains enough salt that it freezes at a lower temperature than fish blood. As a result, some species of saltwater fish experience freezing temperatures in a liquid ocean.
These fish manufacture special proteins, known as antifreeze glycoproteins, which either prevent their blood from freezing altogether, or allow the fish to survive a certain degree of freezing.