Predicting the effect of cages on nutrient status of Scottish freshwater lochs using mass-balance models

Abstract

Cage farming of salmonids in floating cage systems has become widespread in the Scottish Highlands over the past 15 years. Determination of wastewater discharges and composition from such facilities, unlike land-based ones, has had to rely exclusively upon results of published studies of waste outputs. To limit impacts of such activities, regulatory authorities have used such data together with lake nutrient mass-balance models to assess likely increase in phosphorus (P) due to both organic and inorganic loading of cage farm losses. Results of biannual surveys between 1988 and 1991 of P levels in six lochs used for cage farming in Scotland, together with estimates of nutrient input on the basis of feeding and fish biomass data, are presented. Results demonstrate that in small basins, where cage farms are the only substantial source of P. model predictions of rises in P level reflect actual changes in water quality. However, in larger, more complex basins, where cage farms represent one of a number of sources of P, and the mass of P present in the nutrient cycle is far greater, use of such models is found to be inappropriate to assess the impact of such operations. Clear effects on loch nutrient levels are observed in the immediate vicinity of the cages. It is concluded that whilst such mass-balance models may estimate total contributions from cage farms accurately, use of such models to determine annual nutrient dynamics of complex sites requires more robust sampling programmes than have at present been implemented.