Predictive Models and Eutrophication Effects of Fish Farms

Abstract: Aquaculture has become one of the fastest growing food industries in the world. Like many other industries, aquaculture, and especially fish farms may cause negative effects on the environment, such as eutrophication, which is recognised as a major threat to aquatic ecosystems. In this thesis such effects are studied in a number of lakes and Baltic coastal areas. The ecosystem scale in focus is an entire lake or defined coastal area in the size range 0.5-25 km2. One important aim is to develop practically useful models that estimate the eutrophication effects as a function of fish farm load. Such models should be useful for the licensing authorities when giving permissions for fish farms.It was concluded that fish farms might increase the surface water concentrations of total and dissolved phosphorus, total nitrogen and chlorophyll in lakes. Furthermore, traditional models overestimate the fish farm impact on lakes. Therefore, new models especially valid for fish farm emissions in lakes were developed.A method to assess the sensitivity of coastal areas to an increased organic load from, e.g., fish farms was developed. Moreover, models to predict the enhanced phosphorus and chlorophyll-a concentrations and the decreased Secchi depth caused by fish farms were developed for coastal areas in the Åland archipelago. From extensive field studies in the Åland archipelago it was concluded that fish farms could cause increased total phosphorus concentrations, periphytic growth and phytoplankton biomass. However, those effects were generally only observed in small and semi-enclosed bays. Finally, the sediment quality beneath coastal fish farms was found to be a key parameter for the water quality, as well as for the recovery time after closure of fish farms.