What happens to fish when a lake freezes?

Ice is annoying for anglers but it can be a matter of life and death for fish. Here’s what’s going on when water stiffens

“When a lake begins to thaw the coldest water will now begin to sink”

THE maritime climate of the British Isles mean that we experience milder winters than our latitude would suggest. Even so, every few years we suffer a really cold winter, normally when the Atlantic jet stream moves further west out to sea. The jet stream blows across the Atlantic and brings with it warm air from the tropics. When it moves offshore it enables colder air from more northerly climes to cover the British Isles, bringing with it colder winters and sustained periods of sub-zero weather. Fortunately, very cold winters are a rare event. But when they do occur they can have a profound effect on fisheries, particularly stillwaters. Smaller bodies of water will see their surface water temperature fall below zero and ice will begin to form in the margins. Over time, the rest of the surface will freeze. An unusual property of water is that it reaches its maximum density at 4oC. Below this temperature water begins to expand again. By the time ice forms the density is considerably less than that of liquid water, so the ice floats. Without this feature of water, life below zero would be almost impossible for aquatic organisms, because it means that even though the surface can be covered with a thick layer of ice the water underneath will remain liquid. The ice actually acts as an insulator, slowing the cooling of the water underneath and so ensuring that only very shallow water bodies will freeze solid in climates such as ours. With liquid water, albeit very cold, always being available, higher organisms such as fish can survive. Cold water and a covering of ice does little harm to our fish in its own right. It can lead to other consequences, however, that can result in fishkills. A covering of ice stops the exchange of gasses at the water surface that normally occurs. Often significant amounts of oxygen are absorbed at the surface and poisonous carbon dioxide is released. Ammonia, produced from the breakdown of waste, is also released at the surface. When ice prevents this gas exchange from happening, the levels of poisonous gases can rise quite rapidly, even though the metabolism of the fish and other organisms is relatively low in the cold water. Eventually, this can lead to fish-kills caused by a build-up of poisonous gases or a crash in oxygen levels. Ice-induced fish-kills are most likely to occur in stillwaters that have a high loading of waste, either because they contain a lot of decaying organic matter such as tree leaves, or because they contain a high stock of fish and other organisms. Both of these scenarios can see the water composition change over time and lead to toxic conditions. Fortunately, ice rarely forms for long enough for fisheries to be at serious risk. Also, it is worth remembering that ice is rarely of the same thickness right across the surface of a lake and even where the ice at the margins may be several inches thick, there may be areas where it is much thinner. Areas that catch the bulk of the weak sunlight are likely to have much thinner ice, for example. Underwater springs, which may have a slightly higher water temperature, can also produce areas of thin ice. It is this unpredictability in the thickness of the ice that often leads to animals and humans getting into trouble when walking on it. Beneath ice conditions for fish can be surprisingly good. With the water temperature stable and sunlight passing through clear ice, fish can often be seen just below the surface soaking up the weak heat from the sun. The water clarity also tends to become exceptionally clear when the surface is frozen because the water is completely sheltered from any wind. Fish will not necessarily be in the deepest water in the depths of winter, and particularly when the lake is covered with ice. Expect them to be close to cover, such as weedbeds and overhanging trees in areas that receive the most sunlight. If the lakebed is a dark colour, such as silt, then so much the better as this will absorb more of the sun’s heat. When a lake begins to thaw, the process of freezing reverses and the coldest water will now begin to sink, turning over the water underneath and often chilling the fish. This can be why the fishing is poor straight after a venue has become ice-free. If you are able to fish just as the ice is beginning to break up, you might be in for a bumper day because the water will still be relatively stable, encouraging fish to feed.