​​​​​​​What Happens To Fish When A Lake Freezes?

Ice is annoying for anglers but it can be a matter of life and death for fish. Here’s what’s going on when water stiffens.

THE maritime climate of the British Isles mean that we experience milder winters than our latitude would suggest. Even so, every few years we suffer a really cold winter, normally when the Atlantic jet stream moves further west out to sea.

The jet stream blows across the Atlantic and brings with it warm air from the tropics. When it moves offshore it enables colder air from more northerly climes to cover the British Isles, bringing with it colder winters and sustained periods of sub-zero weather.

Fortunately, very cold winters are a rare event. But when they do occur they can have a profound effect on fisheries, particularly stillwaters. Smaller bodies of water will see their surface water temperature fall below zero and ice will begin to form in the margins. Over time, the rest of the surface will freeze.

An unusual property of water is that it reaches its maximum density at 4oC. Below this temperature water begins to expand again. By the time ice forms the density is considerably less than that of liquid water, so the ice floats. Without this feature of water, life below zero would be almost impossible for aquatic organisms, because it means that even though the surface can be covered with a thick layer of ice the water underneath will remain liquid.