This report gives an estimate of biological production based on dissolved nutrients and particulate organic matter from Norwegian aquaculture. It provides an estimate of the quantitative importance this has in the marine food chain, where nutrients form the basis for marine algae production, while animals can benefit from organic material directly.
Norwegian aquaculture production of salmonids represents about 30% of total world aquaculture production of marine finfish (FishstatJ, www.FAO.org). Production is very efficient, also in terms of turnover of feed to edible food. Norwegian aquaculture used almost 1.5 million tonnes of fish feed in 2012. The protein retention (protein retention = protein in fish x 100 / protein in the diet) was in 2012 at 45%. Emissions from ranching of salmon can be divided into three categories; waste feed (nutrients), soluble nutrients and fecal matter (inorganic /organic nutrient). Waste feed can be eaten directly by fish living around the fish farms. 3
We do not have measurements of the proportion waste feed that is eaten by wild fish, but based on an estimate that about 50% consumed by wild fish, this represents an additional production of 50 000 tonnes of harvestable fish. Inorganic nutrients provide a basis for algae production and as such food up through the food chain (trophic level). Cod is located at trophic level 4.4. Since approximately 90% of the energy is lost in each trophic level, the additional production of fish at trophic level 4.4 would be between 1,000 and 2,000 tonnes at best. Nutrients will thus not provide an additional production of harvestable fish that is significant. We find the same proportion when we look at feces. Here again, one or two levels up through the food chain and the amount of harvestable fish will be negligible. However, if we estimate the maximal production capacity of blue mussel, a primary consumer of phytoplankton, the enhanced food availability will allow an additional production in the scale of 350 000 tonnes.