Observation of three-dimensional flow structures and effluent transport around fish cages using a towed ADCP and free-fall multi-parameter profiler

A field campaign was conducted around salmon cages, using a combination of a towed ADCP and a free-fall multi-parameter profiler, in order to investigate flow structures and the possible distribution of effluent materials. Two transect observations showed that hydrographic conditions changed dramatically within 5 days, from highly stratified open water conditions to weak stratification. Three-dimensional observation revealed that flow was blocked behind the cages and that the blockage was reduced as the distance from the cages increased. The flow speed was positively correlated with the intensity of the backscattering signal. The R-2 value was high immediately behind the cages and decreased with distance from the cages. The flow time series behind the cages exhibited a k(-2) power law spectrum that was consistent with a typical internal wave spectrum. This suggests that eddies shed from the cages were highly influenced by stratification. We estimated the rate of kinetic energy dissipation, epsilon, from YODA Profiler data based on the Thorpe scale approach. We also estimated the eddy diffusivity coefficient, K rho. Both epsilon and K rho followed a lognormal probability density function. The mean K rho was consistent with a one-dimensional diffusion model assessed from the R-2 values of flow speed and backscatter intensity.