Summary

1. Over the last twenty years, there have been emissions of great quantities of cooling water from the Nordic nuclear power stations located around the Baltic Sea. Even before the first reactor was taken into operation, research was underway on the ecological impact of temperature rises on the aquatic environment, for example at major coal- or oil-fired thermal power plants.

2. Swedish research on the effects of cooling water has been presented in a number of scientific reports (a selected list of these reports can be found at the back of this book). The results from previous research periods were presented in "Kylvatten - effekter på miljön [Cooling Water‑ Effects on the Environment]", 1974, "Värmekraft och miljö - några forskningsprojekt [Thermal Power and the Environment ‑ Some Research Projects]", 1980 and "Kylvattnets biologiska effekter [The Biological Effects of Cooling Water]", 1990 (all reports in Swedish).

3. Earlier research has shown that there are unwanted changes in cooling water recipients. However, these alterations are limited in extent and can be regarded as acceptable impacts.

4. The negative effects are instead outweighed by the changes which may be regarded as positive from the society's point of view, that is the increases in the biological productivity in those areas which are affected by the increased temperature. It is mainly these positive effects which are highlighted in this report. The results of earlier research are a prerequisite for the work presented here, but the details are available in previously issued reports (see references).

5. One general observation is that, in an international perspective, the Baltic Sea is less sensitive than other studied ecosystems which have been subjected to cooling water effluents. This probably relates to the fact that the Baltic Sea is a northern, brackish water sea with a limited number of species ‑ an environment which is not found in any other part of the world.

6. It has been assumed that warm water increases the rate of microbial decomposition of organic material on the bottoms. Consequently, it has been argued that by designing and locating emissions of cooling water appropriately it would be possible to clean waters which have previously been subjected to eutrophication. Studies of microbial processes in sediments in coastal areas with an influx of warm water have been carried out. These studies reveal, however, that not only temperature and water movements are important, but that other factors also control such processes. The method can not, therefore, be regarded as generally applicable.

7. The size of fish stocks in the coastal areas of the Baltic Sea is primarily determined by the survival rate of the fry and the size of the area available for recruitment. Both of these factors are directly related to temperature.

8. A model has been developed which makes it possible to predict stocks of so-called warm-water fish (e.g. perch and pike-perch) primarily on the basis of data on temperature conditions. This model provides satisfactory results in coastal areas, but is less reliable for lakes where the interaction between different species is of comparatively greater importance.

9. Directing cooling water to shallow areas means that large areas are heated. The temperature increase, which is moderate with good mixing and dispersal, is greatest during the spring and autumn. This fact can be used to improve the survival and growth of warm-water fish. If the main flow of cooling water is divided up into several smaller flows, simply by utilising the topography of an archipelago for example, the water movements will carry away fewer of the pelagic fish larvae such as those of pike-perch and herring.

10.Fish growth increases when the temperature rises provided that there is enough food, i.e. that food is not a limiting factor. The comprehensive studies that have been carried out have found no indications that there is insufficient food. This is an important conclusion for future investments in various forms of aquaculture in Nordic coastal waters.

11.High densities of fish fry, which are required for more intensive fish farming in cages or other facilities, demand food supplements. The fry of perch and pike-perch consume zooplankton. It is possible to increase the density of such food items by attracting plankton with the help of lights at night, or by filtering the cooling water flows and directing the filtrate to the fish cages. The success of such systems for collecting plankton depend on the ability to adapt the technical systems to the prevailing quantities of zooplankton.

12.Keeping high densities of pike-perch and perch fry is complicated by the fact that certain individuals become cannibals, and these rapidly and efficiently decimate stocks in enclosed spaces. Different methods which may reduce the effects of this cannibalism remain to be tested.

13.Releasing elvers is an effective way of increasing local populations of eel, at least within heated areas. Growth is also enhanced in such areas and almost all individuals become females, the sex with the highest growth rate. The release of elvers could be increased in extent in the cooling water recipients of the Baltic Sea, particularly if the rearing of imported elvers is initiated in plants supplied with waste heat.