Like all other human activities aquaculture affect the surrounding environment. Production of Atlantic salmon and rainbow trout has become a major industry in Norway. The continuous growth requires that the industry is persistently trying to solve the industry’s environmental challenges. This section discusses some of the direct discharges from fish farms along the Norwegian coast.
Organic material, nitrogen and phosphorus
A large fish farm has a relatively large discharge to the fjord in the form of excrements and excess feed. Decomposition of organic material released to the water fish farming can result in oxygen depletion in the fjord environment. Discharge of nutrients like nitrogen and phosphorus reinforces this process by fertilizing the biomass production in the water.
Changes in water quality may have great impacts on biological diversity. Such local harmful effects appear when the areas carrying capacity is exceeded. A fjord area can withstand a certain amount of this type of discharges and it is important to adjust farming activities so that this limit is not breached. Most of the farms today are placed on deep localities with good water replacement, and the problem of overloading sea areas has been reduced in relation to the early years of Norwegian aquaculture. The growth in Norwegian production of farmed fish however is resulting in ever increasing discharges from farms. Before more intensified use of fjords for farming activities can be accepted there should be established a permanent mandatory system that ensures that production is adjusted to the fjords carrying capacity.
Today’s fish farms contain a lot of fish. In these dense populations of fish there will always be a danger for outbreaks of various diseases or parasites. The most important measures against disease will always be prevention methods that ensure good water quality and vaccination of fish. When an outbreak occur the farmers uses drugs given either through food or directly into the water by so-called bath treatment. Medication use in farming can be divided into three main substances; antibiotics, sea lice chemicals and anaesthesia agents.
When using drugs in an open cage some of these substances will be released into the local environment where they can cause harmful effects.
To ensure good water quality in a fish farm, it is important to have clean nets that provide good water exchange in the farm. Today mainly copper impregnation is used to keep the fouling of nets at bay. Copper is acutely toxic to many aquatic organisms and thereby prevent growth of algae on the nets. Approximately 200 tonnes of copper is used for this purpose annually in Norway. Estimates show that about 80 percent of the copper used leaches into the surrounding environment.
Some farmers have replaced the use of copper impregnation with different mechanical solutions without the use of chemicals. Studies at SINTEF Fisheries and Aquaculture show that these solutions, in addition to being less harmful to the environment, in many cases are economically advantageous for the farmer.