Naturally yours: Fish still active even in deep freeze
Looking out across a frozen lake, it may appear that all is quiet. Under the ice, however, fish remain active, swimming, feeding and even spawning.
Since ice floats, all but the shallowest ponds have water all winter. The limit for fish is whether there is enough oxygen to survive in that water. Small lakes with little circulation cannot support big fish but large lakes and moving water have the oxygen that fish need. More than anything, it is winter oxygen that determines what fish live in what lakes.
One fish that can survive with very little oxygen is the stickleback. This 5 cm (2.5 in) fish, with spines on its back, is often the only fish to be found in shallow lakes and beaver ponds across the region.
Even unfrozen water gets much colder in winter and that also affects fish activity. In winter, the warmest water is at the bottom but it is only 4 degrees C. Fish are cold blooded so they cool down with the water, moving more slowly and eating less.
Most of the aquatic invertebrates that fish eat are also active through the winter. If you look down an ice fishing hole, you may spot fresh water shrimp, diving beetles or even a damselfly larva. Living in cold water often extends the life cycles of these creatures but it doesn’t slow them down completely.
Another feature of life under the ice is the darkness. Low light doesn’t affect fish too much since many are active both day and night anyway. Algae and other plants, however, die without light. As the plants decompose precious oxygen is used and that means that conditions by late winter usually determine if a fish is going to make it through the winter or not.
For some fish, winter is spawning time but those fish have to prepare ahead. Since fish grow very slowly in the winter, reproduction isn’t possible unless eggs and sperm are ready and waiting in the body by November.
Our most common winter spawning fish is the burbot, or ling. This bottom feeder lives in larger lakes and rivers and spawns under the ice in February and March. Arctic grayling also begin to travel to spawning areas at this time of year and northern pike spawn at the ice edge in early spring.
Fish are hard to get to know since they live their lives out of our sight. Just like land creatures, however, they lead fascinating lives that are greatly affected by the changing seasons.