Impacts of organic discharge and nutrients

The impacts of the discharges from fish farming are largely local and correspond to the impacts of other forms of organic environmental load (Frogh & Schanning, 1991; Braaten. 1992; Tvedten, 1996).

Organic material that is deposited on the seabed is decomposed by bacteria that use oxygen. As the oxygen level in the surrounding waters falls, local biodiversity is reduced (Tvedten et al., 1996). If the seabed is overburdened, when the oxygen has been used up and because toxic hydrogen sulphide is formed in the bottom sediments, a so-called "rotten" seabed without animal life develops. Rising hydrogen sulphide gas may harm the fish in the fish farms. In Norway, damage has been shown to fish gills assumed to be due to gas formation in sediments.

Any adverse impacts due to eutrophication from a location are reversible. Studies done by Frogh & Schanning (1991) show that locations to which large quantities of organic material were previously added and had highly anaerobic sediments can recover to an almost natural state after a rehabilitation period of between three and five years. The length of the rehabilitation period that is necessary will depend on local topographical conditions.

It has been shown that the currents with dissolved nutrients from aquaculture discharges go out into the Norwegian Sea and thence end up in the Barents Sea. However, the contribution is so small that changes in concentration are not measurable (Hillestad et al., 1996). The amount of dissolved nutrients can however have a negative effect locally in a limited fjord area. It is suggested, for example, through NIVAs seaweed project that increased discharges of nitrogen and phosphorus prevents important seaweed species to re-establish. In years with high water temperature the large seaweed plants dies. Increased concentration of nutrients favours other smaller species (thread algae) that displaces the natural flora.

In recent years there have been reports of wild fish with reduced quality as a result of that they have eaten excess feed from fish farms. In the County of Rogaland, this has caused problems for fishermen who fish saithe in the fjords. The feed used for farmed salmon is rich in fat and the high fat content probably reduces the quality of wild fish.

Good locations important
In the case of nutrient salts and organic material, the carrying capacity of a location, i.e. how much the location can tolerate of a type of environmental load, is dependent on depths, current, seafloor conditions and what is defined as acceptable environmental conditions.

The depth conditions at a locality are important. Aure and Ervik (2002) have discovered that the quantity of fish in a standard fish farm can be increased from 60 to 250 tonnes when the depth is increased from 30 to 80 metres, i.e. that carrying capacity quadruples. The favourable depth conditions along the Norwegian coast are also one of the main reasons for the success that aquaculture has had in Norway.

The design of the fish farms is also important with regard to the carrying capacity of a location. The difference between a compact fish farm (a farm where the cages are in rows on both sides of a central walkway) and a freestanding fish farm is relatively big. Based on a current speed under the cages of 4 cm/sec., calculations show that the local carrying capacity increases from approximately 100 to 300 tonnes when using free-standing cages as opposed to compact fish farms. (Aure, J. et al., 2002)