Quantitative estimates of the eutrophication effects of fish farming on fjords

Abstract

This paper deals with the effects of fish farming on eutrophication of the upper layers (above the sill) and the increased oxygen consumption in the basin water (the water dammed by the sill) of fjords. The study is based upon measurements from a large number of Norwegian fjords as well as upon recent theoretical developments. We found that the rate of biochemical decomposition of the “farm sediment” (composed of excess food and faeces) seems to be proportional to the mass (the thickness) of the sediment. About 10% of the sediment mass is broken down annually. This implies that the mass of the farm sediment reaches an equilibrium only after several tens of years of fish production. A framework for computations of the environmental loading with respect to nutrients and oxygen demand caused by the farm sediment is presented. We conclude that fish farming causes only quite small effects of eutrophication in the upper layers, at least of Norwegian fjords. The reason for this is that the intensive water exchange between the coastal water and the fjords tends to equalize the conditions in the two regions. If a fish farm is located over a sill basin, the farm sediment will increase the oxygen consumption in the basin water. A method to compute the response of the minimum oxygen concentration in sill basins to fish farming (the “ R-method”) is presented. This method can also be used to compute how to increase the water exchange by technical means, thereby increasing the oxygen supply and the minimum oxygen concentrations in the basin water.