Surviving Winter By Heidi Poca
Winter is now upon us in Canada, meaning colder weather, snow and ice; but what does this mean for our fish and other aquatic species? Fish are confined to the frigid waters of rivers and lakes, yet they survive year after year! This blog will discuss the many challenges fish face in their aquatic environments and how they have adapted and evolved to survive such harsh conditions.
For starters river and lake systems experience winter differently. Lakes undergo a turnover event where all the water in the lake mixes as surface water cools and the temperature drops to a similar temperature as the cooler deeper water. During summer, lakes are stratified into different zones due the difference in temperature (and densities) preventing mixing. When turnover occurs in the fall the entire lake is 4oC, the temperature at which water is its densest. This turnover event is extremely important for all organisms in the lake as it mixes and replenishes nutrients and oxygen throughout the water body. Lakes often freeze over in the winter, and this adds an additional layer of complications to life under the surface. The benefit of ice is that it insulates the lake, however it blocks the lake from atmospheric oxygen and sunlight which aquatic plants need to photosynthesize to produce oxygen. This means that long winters can result is low dissolved oxygen levels and can lead to die-offs.
Diagram of lake stratification and turnover throughout the seasons.
To combat winter, fish have evolved traits to help them survive the harsh conditions. For starters most fish are poikilothermic, meaning they are cold-blooded and their body temperature changes with their environment. This means that they do not have to spend energy to maintain a certain body temperature. Fish also change their behaviour based on the season. Come winter, food becomes scarce, and oxygen is not being replenished; to counteract this many warm water species retreat to deep water and search for safe refuges where they can hide from predators. They then enter a dormant state where their metabolism slows, which in turn reduces respiration, stops growth, and reduces the need to feed. They will also rely on fat stores that they have built up over the summer and fall. Younger fish are more susceptible to starvation during winter because they had less time to store fat, compared to older, and larger fish.
Lake in Algonquin Provincial Park in the winter. Photo credit: Heidi Poca
Rivers on the other hand, do not freeze over due to their constant currents. Although river fish do not have to worry about low dissolved oxygen like the lake fish, they do have other challenges to deal with. The strong currents force fish to expend energy while swimming, however conserving energy is crucial for survival. To avoid the currents fish retreat to slow deep pools which also provide shelter from predators. In addition to deep pools river species will also converge around groundwater upwellings. These areas provide constant input of warm water which can support larger groupings of fish. Food is also scarce in river environments come winter, so like lake fish, river fish rely of fat stores. Finally, ice formations can cause various difficulties for fish. One example is ice jams. Ice jams are formed when stationary ice melts and freezes at frequent intervals. This results in physical barriers to fish, preventing them from accessing their refuges and potentially causing die-offs.
Marine fish have their own adaptations to survive frigid temperatures. Fish that live in polar regions have evolved to produce special glycoproteins which act as anti-freeze. These proteins work by lowing the freezing point of bodily fluids below the freezing point of sea water. This, in addition to binding to ice crystals in the body and preventing them from growing, prevents polar fish from freezing to death.
Fish are not the only organisms facing the cold; birds and mammals also have unique evolutionary traits that help them survive Canadian winters. One particular trait is countercurrent heat exchange (CCHE). Ever wonder how Gulls, Geese, and Ducks can happily walk and stand on ice for hours on end, seemingly unbothered? Countercurrent heat exchange is what prevents their feet and legs from getting frostbite. Without CCHE birds would have to expend large amounts of energy to keep extremities warm, as well as to heat cold blood before it returns to the heart. CCHE is an efficient solution to this problem. It works by having veins and arteries close together with semi-permeable membranes. Warm blood travels down to the leg via arteries and the heat then transfers across the semipermeable membrane to the vein which is carrying the cold blood back to the heart. This allows enough heat to be transferred to the extremities to prevent frostbite, while the rest of the heat warms the returning blood. CCHE is also present in beavers, otters and muskrats.
Diagram of countercurrent heat exchange in birds
Winter is a difficult time for many species, but they have evolved special traits that allows them to survive their climates. Evolution of these special traits doesn’t happen overnight, and rapid climate change is altering environments faster than species can evolve and adapt to them. Hotter summers and colder winters, as well as more intense storms are just some of the challenges wildlife and humans must face as a result of global warming and climate change.
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