Paleolimnological evidence for variable impacts of fish farms on the water quality of Scottish freshwater lochs

Since the 1980s, fish farming (aquaculture) has been an important contributor to Scotland's economy, but there are concerns that nutrient-rich food waste and excreta from these farms are causing eutrophication. Water quality monitoring preceding the arrival and subsequent expansion of the industry is limited. Therefore, to better understand the impacts of in-lake fish farms on the quality of freshwater ecosystems, we examined the diatom records in sediment cores from seven freshwater lochs in Scotland over a timescale of c.100-200 years, spanning the period before and after installation of the fish cages at these sites. At three lochs (A, C, E) we observed marked diatom assemblage shifts indicative of eutrophication, coincident with arrival of the fish farms, at two lochs (B, G) there was evidence of enrichment over a longer timescale although with some further enrichment occurring with the advent of the fish farms, and at the other two lochs (D, F), diatom shifts were subtle and showed no sign of eutrophication. Thus, while marked ecological shifts are shown to occur with the arrival of fish farms in some sites, this is not always the case. The natural background conditions, the scale of operations, the siting of the fish cages in relation to location of inflows and outflows, the role of flushing rate and additional sources of nutrients are discussed as potential factors for the variable impacts observed across the seven lochs. Such factors should be considered when planning future installation and expansion of fish farms to ensure sustainable development of these resources. Our study provides an understanding of baseline conditions and long-term water quality trajectories in freshwater lakes with fish farms and demonstrates the value of paleolimnology in supporting management decisions with respect to fisheries.