Phosphorus Emissions from Fish Farms: Observed and Predicted Effects

During the last decades, fish farming has been a rapidly increasing industry in many European and North American countries. Sweden has a large potential for aquaculture, but there is also a strong concern about the effects on the environment that an increased aquaculture production may cause. This thesis focuses on the eutrophication effects of fish farming in lakes and in coastal areas of the Baltic Sea. Possible eutrophication effects related to marine fish farm emissions were found in small and semi-enclosed bays in the Åland archipelago of the Baltic Sea. Fish farming did not have any measurable large scale effect on TN in the Åland archipelago, but there were indications of a large scale effect on TP. Nutrient concentrations and other eutrophication indicators in lakes with fish farms correlated well with the variations in farm load among the lakes and with distance from the fish farms within lakes. In a majority of the observed cases the effects on phosphorus concentrations were smaller than expected from the most commonly used models. Besides overestimating the effects, these old models described the variation between lakes adequately. It was therefore possible to construct farm specific steady state models based on the same model structures, that should be simple and accurate enough to be useful. The observed effects on phosphorus concentrations tended to be higher in lakes where fish farms had been active for a long time, compared to lakes with new fish farms. Earlier investigations have shown that there is a considerable time lag in the response of changed phosphorus loads in many lakes. Empirical data and model simulations suggest that this is the case also for fish farm lakes.