Fish farming is a significant local source of discharges of nutrient salts. The vast majority of Norwegian aquaculture production takes place in net cages in the sea. Feed spills and excrement are not collected but are released directly into the water (State of the Environment Norway).This produces a fertilising effect in the waters, and if the discharges are too large in proportion to what the waters carrying capasity, water quality will decline. This will have consequences for the surrounding environment and the environmental condition of the fish farm. Discharges of nutrient salts, primarily nitrates and phosphates, and organic materials come from feed spills, fish excrement and dead fish.

Reduced oxygen content
Organic material from fish farms sinks to the bottom and is decomposed by bacteria. This biodegradation process requires oxygen. The discharges thus reduce the oxygen content of the surrounding water, and in extreme cases we may see a total lack of oxygen in the water at the bottom. Discharges of nutrient salts of nitrogen and phosphorus lead to increased algae growth and biomass production in the surrounding water. This increased biomass production leads to a further addition of organic material to the bottom of the fjord, which in turn requires increased amounts of oxygen for bacterial decomposition.

Nitrogen and phosphorus
Two types of nutrient salt, those of nitrogen and phosphorus, represent most of the nutrient salt discharges connected with fish farming (Braaten, 1992). Generally it is salts of phosphorus that are mostly responsible for eutrophication in fresh water, whereas nitrogen salts normally will be more important in salt water (Braaten, 1992). Many of the fish farms are located in fjords and in areas with a large inflow of fresh water, and detailed knowledge is required of the area in question in order to say anything about which nutrient salts are limiting primary production. A fish farm that produces 500 tonnes of salmon, that is, a small farm, releases nutrient salts equivalents to a town of 5,000 to 7,500 inhabitants (State of the Environment Norway). According to the Norwegian Institute for Water Research (NIVA), aquaculture is responsible for 60% and 25%, respectively, of discharges of phosphorus and nitrogen salts along the coast from Lindesnes northward.

Organic material
From fish feed, it is proteins, fats and carbohydrates that constitute nearly all of the organic material (Einen & Mørkøre, 1996).When the organic material hits the bottom, it can be "biodeposited" on the surface of the sediment. Biodeposition means that the sedimentated material is taken up by benthic fauna that are nourished by filtering particles from the water (such as mussels), and the excrement from such fauna is deposited on the seabed (Wasermann, 1994). To prevent a reduction in water quality due to high consumption of oxygen for decomposition we are dependent on adequate currents that can spread the organic material over a larger area. The energy content of the organic material is crucial for the quantity of oxygen that must be added to the water to decompose the organic material (Åsgård & Storebakken, 1993). That is why organic material is converted to energy content in the table below.

Increasing discharges
Since the beginning of the 1990s we have seen a steady increase in discharges of nitrogen and phosphorus salts and organic material from Norwegian aquaculture. Despite some reductions in discharges per tonne of fish produced, the growth in volume leads to a continuous increase in total discharges.

If the assumed increase to 1,050,000 tonnes of salmonids produced in 2010 comes about (Almås, K. et al., 1999), the discharges of nitrogen and phosphorus will climb to 47,355 tonnes and 10,290 tonnes, respectively – an increase of 63% and 62%, respectively. This assumes, however, that fish farming is conducted in the same manner as in 2005

From Lindesnes and northward, aquaculture contributes heavily to the total discharges of nitrogen and phosphorus salts. The discharges of these nutrient salts and organic materials have increased in step with the growth in volume of Norway’s farming of salmonids. Even though the industry’s measures to improve feed quality and reduce feed spills have reduced the discharges per tonne of fish produced, this reduction has been counteracted by an increased production volume. Continued growth in the farming of salmonids in addition to scaling up the production of other farmed species will lead to increased discharges of nutrient salts and organic material to Norway’s coastal and fjord areas. Even though fish farming is responsible for a large part of these discharges, these discharges do not necessarily constitute a large addition of nutrients compared with natural levels, and compared with what is added by ocean currents from foreign discharge sources.

These discharges are harmful only if they exceed the carrying capacity of the area in question. As long as the discharges do not exceed this, they may have a positive impact on the productivity in the area and not inflict any harm on the environment.

The challenge linked to discharges of this type is therefore to calculate the carrying capacity of the location and adjust fish farming activities accordingly. The MOM system (Modelling – Ongrowing Fish Farms – Monitoring) will be an important tool in these efforts.