Monitoring BOD outputs from land-based fish farms

Waste production from fish farm sites presents a number of problems for regulatory authorities; sites are often distant from laboratories, whilst waste output fluctuates rapidly during the day in response to both the activity of the fish and production methods employed at any site (i.e. feeding regime, tank cleaning). Two land-based sites in NW Scotland, producing Atlantic salmon (Salmo salar) juveniles were visited regularly over a 4-month period, and outputs measured on a diurnal cycle to determine the most appropriate strategy for sampling such waste discharges. The BOD₅ of discrete samples taken at hourly intervals and a single composite sample for the whole 24-h period were compared for the pre- and post-treatment outflow sites for BOD₅ content. The BOD₅ value of the untreated composite sample was shown to exceed that of the mean of all 24 1-h discrete samples in a highly significant manner. It is suggested that the presence of solid particles has a disproportionate effect on BOD₅ value of composite samples.