Empirical measurements of parameters critical to modelling benthic impacts of freshwater salmonid cage aquaculture

Although the waste dispersion models used in salmonid marine cage aquaculture could potentially be applied in freshwater cage aquaculture, much of relevant data on the waste particle parameters are unavailable. The present study provides new data on the wet densities of diets, the fall velocities of diets, the fall velocities of faecal matter and waste particle size. A significant increase of densities caused by the immersion time in water was observed, The wet density obtained from 60-s immersion time was found to be the effective settling density of diets. Fall velocities of salmonid diets were non-Stokesian and ranged from 0.02 to 0.12 m s(-1). These fall velocities did not significantly vary with water temperature. Fractionation of collected solid wastes from experimental tanks revealed that almost 60% by mass was retained on 2000-, 1000- and 500-mu m meshes. The fall velocities of these fractions were found to vary between 0.015 and 0.030 m s(-1). The waste fraction that passed through a 500-mu m mesh had an estimated median diameter of 114.4 mu m. The observed fall velocities of both diets and solid wastes were lower than those commonly used in modelling waste dispersion in the marine environment. However, because of the relatively slow current velocities in most freshwater environments, it is expected that the wastes will lend to accumulate locally at cages.