It’s Winter Time:
Do You Know Where Your Fish Are?
Winter is trying for us warm-blooded creatures. But think for a minute about fish that live in cold water all winter long? They can’t throw another piece of wood into the stove so where do they go, what do they do and how well do they survive North Country winters?
Winter is a time of stress for fish. Our fish species have experienced major physiological adaptations in order to survive this yearly ordeal. Each of the two major habitats for fish, lakes and rivers provide particular challenges for fish to survive their struggle with cold temperatures.
Unfrozen lake water receives oxygen from two sources, the mixing of the water with air at the surface and from oxygen released into the water as a by-product of the photosynthesis of aquatic plants. In winter most of the plants that produced oxygen are dead and not producing oxygen. Oxygen levels are further reduced because the armies of microbes are using oxygen while working away to digest the dead plants, a double whammy for fish.During most winters, lakes freeze over so no mixing of air into water can take place at the surface. If there is snow cover then the water below the ice is in darkness so no photosynthesis is possible at all not even by phytoplankton. Deeper lakes become stratified with the oxygen depleted water at the bottom. As oxygen becomes depleted fish migrate up the water column toward the surface or toward inlet streams where oxygen levels are higher.
Over the winter in order to balance the low dissolved oxygen levels, fish being cold blooded experience a drop in their heart and metabolic rate. They need less oxygen and their blood is better able to absorb any oxygen available as they just sit out the cold. A reduced metabolic rate reduces the energy required to maintain the body. The downside of a slow metabolism is that it limits what fish can do. For instance, they are much slower to escape danger.
In the worst winters the stage is set for a winterkill. If low oxygen conditions persist and fish are unable to move to other areas, the situation can result in a die-off due to asphyxiation. Winterkill is most common in shallow lakes because of longer periods of no photosynthesis in the water. Lakes with rich, dense aquatic plant communities in summer are also susceptible to winterkill during harsh winters because more plants mean more microbes and more oxygen removed from the water. Game fish such as trout, pike and perch are especially sensitive to winterkill because of their need for high oxygen levels.
Now what happens in rivers? Many of the winter conditions in rivers are the same as in lakes: plants are producing little oxygen and there is a drawdown on available oxygen by decomposition of the plants. The three identified types of river ice: frazil, anchor and surface ice each create their own problems for river fish.
Rivers and streams seldom freeze over completely so the air into water mixing at the surface replenishes oxygen in the river most of the winter. Fish kills seldom occur in the winter from oxygen depletion in riverine habitats unless there are unusual ice jamming events or surface ice influences flow conditions that strand fish in areas that have no water circulation.
As water temperatures drop fish retreat to deeper pools in search of less current. This is in response to their slower metabolism and less energy to fight the current. Studies show that larger fish prefer the deeper pools. As winter progresses smaller fish are pushed out of the deeper quiet water to shallow or riffle areas where they must search out any substrate formation like a bolder that offers them protection from the current.
There is a relationship between good winter habitat and the size of the fish in it, the better the habitat the bigger the fish. Yet river ice is a major challenge to all fish. Even ideal holding areas can deteriorate if ice reduces the total area available for flowing water. That can force fish together. Wild fish are stressed by close proximity to other fish.
Anchor ice freezes from the bottom up in shallower areas of a stream. When it does, eggs of all aquatic species and immobile macroinvertebrate life die of asphyxiation because the anchor ice stops water and thereby oxygen from reaching any life form living on the bottom of a stream.
Surface ice can reduce the available flow area of a stream side to side and in the vertical dimension as well. Currents will increase even in the deeper pools as the size of the available channel is reduced but must still move the same amount of water. The increased velocity of the water through deep water pools can rob fish of that sanctuary.
Frazil ice is uncongealed ice crystals floating just below the surface ice. Studies of the effects of frazil ice have shown that fish can die because of the ice crystals getting lodged in their mouths and on or between their gill plates.
One last ice threat can occur as winter ends. During high water ice out events the moving ice can scour the bottom of the rivers sweeping away all life including fish eggs and macroinvertebrates leaving a sterile river bottom until it is repopulated over time through naturally occurring drifting of species into that reach of stream.
Fish have evolved their own survival responses to winter and because of that are mostly successful during this challenging time. The evolutionary adaptations that lower their need for food and oxygen and their instinctive selection of holding areas that have higher oxygen concentrations and slower flows help fish overcome a North Country winter, but it isn’t easy!
David L. Deen is the upper valley River Steward for the Connecticut River Watershed Council. CRWC has been an articulate voice for the Connecticut River for more than half a century. David is a member of the Community Advisory Committee of the Nature Museum at Grafton.
