As we settle in for the heart of the winter most of us don’t think too much about what is happening under the ice of our local lakes and rivers. While we can turn up the thermostat, or throw another log on the fire, fish and other aquatic animals are at the mercy of the elements. As coldblooded animals trout, and salmon, slow down when the water cools and, although warm water temperature in the summer are much more critical to fish survival, the winter months also bring their own special challenges.
While we take ice for granted, for the organisms which live in the water there are actually several forms of ice that they must contend with. On still water, such as lakes, surface ice forms fairly quickly when the water temperature drops below freezing. A cover of ice, especially if it is covered with snow, acts as an insulating blanket and ensures the water temperature below the ice remains about 4̊C. One problem which may occur with ice cover is that no oxygen exchange takes place and, in small shallow ponds, prolonged period can result in winter kill where fish and amphibians, such as frogs die from lack of oxygen.
In running water the turbulence means that the entire mass of water must reach 0 before it freezes. Running water forms two types of underwater ice, anchor, and frazil ice. Frazil, or slush ice results from crystals forming in very cold water and the slush can become quite thick. Anchor ice forms on the bottoms of streams in fast water areas. Anchor ice freezes from the bottom up and on very cold days the white blanket you see covering the bottom of streams is anchor ice. One problem that can happen with anchor ice is when the temperature rises, the anchor ice lifts off the bottom and drifts downstream. This can take a lot of the gravel with it and, if there are fish eggs buried in the gravel they will be exposed, and die.
Trout and salmon have evolved to survive winter conditions by finding habitats where they can survive with a minimum of energy. Young salmon, known as parr, find shelter down in the gravel, often behind large rocks which protect them from the current. Brook trout, as well as slink salmon are more likely to be found in deeper pools or still waters where they can remain out of the main current.
In the spring, summer and fall fish are constantly looking out for predators from the air or land. Kingfishers, osprey, herons, cormorants and eagles all fish. In the winter fish can forget about winged predators but the land based ones are still around. I remember seeing a pile of fish scales on the ice of Giants Lake in Guysborough County where an otter had set up a feeding station. Whether it is on a lake or river the Red Cross recommends that you require a minimum of 15 cm( 6 in) of good ice for one person while a group of people will require a minimum of 20 cm(8in). Snowmobiles need 25 cm(10 in) of ice before they venture out.