The Big-Freeze Affect - By Simon Scott

Happy New Year and wow what a start to 2010 - Icy winds from the Northeast, thick blankets of snow across the whole country, frozen lakes and temperatures staying below zero day after day! With the exception of Martin Locke, who has just smashed the World Record with his magnificent fish from Rainbow in France, January 2010 is not going to feature heavily in many carp angler’s photo albums!!
Just about every lake in the country has been frozen solid for at least two weeks and apart from a few die-hard river anglers most of us have had no hope of going fishing. Of course life still goes on under the thick layer of ice. So while we have been enjoying the opportunity to throw snowballs, what have the carp been up to and are there any dangers for fish stuck under the ice for such an extended period?

Carp like most fish are cold blooded. This means that their body temperature is the same as the water that they are swimming in, which currently means they are VERY cold! During the last two or three weeks water temperatures below the ice will have hovered just above freezing. For a carp, that has evolved to live in warmer climates, temperatures below ten degrees centigrade are cold. This is different to other species that we commonly fish for in the winter months such as pike or grayling, which are far better suited to life in cool water.
As the temperature of the carp falls, its metabolism slows right down. Once the temperature has dropped below ten degrees the metabolism of the carp will slow down significantly and this affects their behaviour. Unlike us, carp cannot increase their activity to warm themselves up so they actually reduce their movement as the water temperature falls. In the prolonged extremely cold weather conditions that we are currently experiencing carp will virtually shut down. They minimise any feeding activity and may even stop feeding completely for long periods. They often rest on the lake bed and will not move for several days becoming virtually torpid. If the fish do move a little, the periods of activity are often really short, lasting a few minutes and in my experience are commonly at dusk.

Carp that are close to torpidity will tend to group up together, often in the deeper water where the water temperature is likely to be more stable. In my experience carp will often bunch up tightly near to features such as sunken branches or amongst the roots of lilies. While undertaking netting operations during the winter months I have frequently come across carp tightly grouped close to snags or under overhanging trees. It is always amazing and interesting to find these groups of over-wintering fish. They are frequently stacked several feet deep and because they are so cold they are staggeringly sleepy. On some occasions if the water is really cold I have been able to pick fish up and lift them from the water and they have hardly flinched!

Recently an interesting scientific study undertaken in European carp rearing ponds, using fish tracking technology, showed that cold water carp are actually more active than had been previously thought. During the winter undisturbed carp in the study kept to localised areas of their lake. Although they moved about in the winter, the movement of the fish was restricted to these small areas. It is almost as if the carp have a small winter territory and will not venture beyond it. The research also showed that this activity continues to go on under ice, but for only short periods, and that the carp in the study were even feeding at water temperatures as low as 3oC! It is important to note that the volume of food consumed at these near freezing temperatures is tiny.

So are there any risks to carp that are trapped under the ice for long periods such as those that we are having now? As I have already mentioned carp are not really adapted to live in temperatures this low. But having said that a healthy fish, which reached the winter in peak condition should be able to withstand very long periods of extreme cold. The really harsh conditions that we are currently having are however likely to cause old or out of condition fish problems and could potentially lead to mortalities in some situations.

Probably the biggest risk to coarse fish trapped under the ice is from shortage of oxygen. Although the metabolism of the carp at present will be very slow indeed, it is still ticking over and therefore they still require a little oxygen to survive. Ice effectively acts like a plastic bag over the surface of a lake. It prevents essential oxygen diffusing into the lake and also traps unwanted gases for escaping into the atmosphere.

Although very cold water will hold plenty of oxygen, as soon as it has iced over the amount of dissolved oxygen it is holding will slowly start to decrease as it is used up by all of the aquatic life within the pond. This not only includes the fish, but also all the amphibians, aquatic bugs, bacteria and microbes beneath the surface. For example decaying weed beds and rotting leaves will use up a considerable volume of dissolved oxygen over time.

In the majority of lakes the oxygen content beneath the ice will fall very slowly and therefore they should not have become critical YET. However, in lakes that are heavily stocked with fish or that contain high levels of rotting weed or silt that will be using up the oxygen, things may have started to become critical. In these cases carp will often be observed coming up under the ice in an attempt to reach the surface and of course some oxygen. Once they have reached this stage the clock is really ticking and unless the weather conditions change quickly or the fishery manager can break some of the ice up to allow oxygen to enter the water mortalities are likely to occur.

Hopefully within the next week the weather will start to warm a little and the frosty fingers of this winter will lessen their grip on our country. Let’s hope that as the ice melts we have not had too many casualties and that the spring brings us some warm winds and excellent fishing conditions ….I cannot wait!!

Simon Scott