Reservoirs for hydropower have a unique condition for the establishment of fish farming operations since the regulation has meant that these reservoirs, as well as being naturally nutrient-poor, have also caused a reduction in the level of nutrients in the water. The reservoirs thereby have a greater space for nutrient loads compared with unaffected lakes. On the other hand, the difficulty in calculating the fish farming potential in these reservoirs consists in the fact that good documenting data is usually lacking, that the mass balance models for calculating the fish farming potential are not adapted to this type of lake, and that nor are the Environmental Protection Agency's assessment bases completely adapted to the large, nutrient-poor reservoirs that often also have big regulation amplitudes and are thereby deemed to be heavily modified water.
The fish farming potential in ten Jämtland reservoirs has been calculated on the basis of samples taken within the framework of Aquabest, work package 2, which aims to "develop methods for the identification of suitable localities for fish and mussel farms within the Baltic Sea region". The total fish farming potential in the ten reservoirs that have formed part of this survey is significant and amounted to a total of almost 25 000 tonnes. However, these calculations have been performed on individual reservoirs and it is not possible to utilise the total volume since several of the reservoirs are right next to one another. The establishment of a fish farm leads to an addition of nutrients to the water, some of which will be transported downstream and thereby reduce the fish farming potential in the areas downstream.
The report also mentions and discusses several of the questions that may arise when calculating fish farming potential, primarily in large, nutrient-poor water reservoirs that are close to mountains. Examples of this are the handling of the values that fall below the reporting limit on analysis, choice of documenting data, relevant background levels and calculation models.
The conclusion is that it is not currently possible to establish a uniform guideline that is suitable for all of Sweden's lakes for the way in which documenting data shall be implemented. Each lake and/or reservoir is unique as regards its conditions and because some conditions can also change with time, the calculation of the fish farming potential must be adapted to the relevant conditions and the documenting data available.
This report constitutes a good basis for those who are planning to establish fish farms in Jämtland and for the authorities who deal with these applications since it discusses and highlights many of the questions that arise when calculating fish farming potential. This report may also facilitate the handling of the licence application process for planned fish farming activities, primarily in reservoirs, for other interested parties and authorities in other parts of the