Last week I went for a walk in the Rocky Mountains on a snowy trail that wandered through some evergreen trees and ended up at an ice covered lake.  The ice was thick enough for me to run on, although it was hard because every time I tried to slow down or turn I slipped and fell down.

The funny thing is, I remember hiking to that same lake in the Rockies during the summer and watching my owner catch fish.  He must have caught ten trout in that lake and a couple more in the stream that leaves the lake on the way up the valley.  But, now I see nothing except ice and snow.

It makes me wonder where are all of the fish?  Here comes another fun nature fact.  I know they are still there somewhere because after the snow and ice melt in the spring the fish are there again when we go fishing.

What happens to the fish in winter?

Lakes and ponds freeze during the winter, but they do not freeze completely like an ice cube.  Only the top layer of the water freezes.   The water that is below this top layer of frozen ice is not frozen.  There are a couple of reasons for this.  The first reason is that frozen water is less dense than cold water, which means the ice will float.  The second reason is that the layer of ice on top of a lake serves as a layer of insulation to keep the water warmer than the air above the ice.  So, in lakes and ponds that are slightly deep, the fish are still alive under the frozen surface of the water.

Small streams and rivers may freeze over on the top of the surface of the water.  If the water is deep enough or if there is enough water flowing, then similar to lakes and ponds the water under the top layer will not be frozen.  The fish can still be active and living under the frozen layer in the water towards the bottom.  In smaller steams the fish may all gather in deeper pools of water in order to give them selves the most unfrozen water to swim in during winter.

However, if the stream is very small, shallow, or the water is flowing very slowly then the entire steam may freeze from the surface of the water all the way down to the bottom of the water.  If this happened, then any fish that were in the water at the time would not be able to survive and would die.

Any ponds that are very shallow also could freeze completely from the top to the bottom of it is cold enough, killing any fish that were living in them.  Oftentimes on hikes during the warm months you may come across a beautiful pond high up in a mountain valley and wonder if there are any fish in it.  If the pond is shallow, then most likely there are no fish as they would not survive the winter freezing.

Fish adapt during winter – The fish are not as active as they are during the summer simply because the water is colder.  Colder water means their body temperature is colder since they are ectotherms (they can not regulate their own body temperature, but use their surroundings to make themselves warmer or colder).  In order to survive the colder water in winter, fish slow down their metabolisms and are much less active.

To answer my question – the fish are still in the ponds, lakes, streams, and rivers in winter.  They are simply down towards the bottom underneath the ice on the top and they are less active than they are during the warmer months.