PHOSPHORUS LEACHING FROM ATLANTIC SALMON DIETS

This paper examines the leaching of phosphorus from two freshwater Atlantic salmon diets and from the faeces offish fed on these two diets. A standard diet and a low-phosphorus diet are examined. The forms of phosphorus in leachate are described and the leaching rates are given. Dissolved reactive phosphorus, a form which is readily available for phytoplankton growth, is shown to dominate pellet leachate whereas particulate phosphorus forms a greater proportion of faecal leachate, possibly associated with the break-up of faecal particles. The relative importance of phosphorus leachate to the overall budget of an Atlantic salmon cage farm is estimated as less than 10% of the overall load of phosphorus to the water column. There is scope for improvement in the stability or leaching characteristics of feed and faeces to reduce environmental phosphorus loadings from freshwater Atlantic salmon farming.