How do fish survive under a frozen lake?

How it works

Most things shrink when they turn solid, but water is unusual. As it freezes, its tiny molecules link up in a pattern full of gaps, so ice takes up more space than the same water did. That makes ice lighter for its size, which is why an ice cube floats in your drink.

In winter the top of a lake cools first and freezes. The ice then acts like a lid, keeping the water below away from the freezing air, so the lake is much slower to freeze any further. At the bottom of a deep lake, the water often stays at about 4 °C, much warmer than the air above the ice.

Fish are cold-blooded, so when the water cools their bodies slow down. They eat less, move less and need less oxygen. Cold water can also hold quite a lot of oxygen, so fish can swim slowly under the ice and wait quietly for spring.

Where to see it in New Zealand

New Zealand winters are fairly mild, so big lakes do not usually freeze. In the coldest inland parts of the South Island, such as Central Otago and the Mackenzie Basin, small lakes and ponds can ice over. That ice can look solid but be dangerously thin, so enjoy it from the shore and never walk on it.

Try it together

Drop an ice cube into a glass of water. Does it float or sink, and how much of it pokes above the surface? Then half-fill a plastic cup with water, mark the level, and freeze it. Is the ice higher than your line?