In northern and southern regions near polar areas, ice in the water may be a problem for the development of cage aquaculture, especially in fresh and brackish water, but also in the sea. The problems are of three types:
· Surface ice
· Drift ice
· Icing up.
Surface ice is mainly a problem where there is a supply of fresh water which reduces the salinity of the top layer of the water. This causes the freezing point to change to close to 0°C, and the water surface might freeze. This will, of course, also be the condition on a freshwater site. From time to time there may be a thin layer of ice on the surface that is so sharp that it is able to cut the nets on the cages.
Drift ice does not normally present any problem for traditional aquaculture sites, but may occur near the polar areas from where it is released.
Icing up of parts of the farm above the surface might be a problem. Icing up occurs when sea spray or supercooled rain hits a construction cooled to below freezing point. When the water hits the construction, it will immediately freeze and coat the construction with ice. This can also happen with air-craft under certain weather conditions and is the reason they are de-iced before take off. The same phenomenon can be observed on fishing vessels working in polar areas; under unfavourable conditions the vessel may become totally covered with ice, the amount and weight which can be so large that the vessel will sink. When sea cages ice up the same thing can happen; the loads may be so large that construction breakage occurs. The weight of the ice can exceed the buoyancy of the collar and the cage will sink below the surface. Here, however, the ice will melt after a time and the cage will surface again. Windy conditions and relatively low temperatures may cause icing by sea spray. If the temperature is very low the water may freeze in the air before it hits the construction. Ice may cause supercooling of the water and possibly also fish death.
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