Turbulence and flow field alterations inside a fish sea cage and its wake

Abstract

Measurements were performed inside and in the wake of a commercial salmon sea cage. The key instrumentation included the following: sea bottom-mounted acoustic Doppler current profilers providing continuous concurrent flow velocity and turbulence information about the water columns; vessel-mounted acoustic current profilers mapping the flow pattern around the wake of the cage in a selected incoming flow; a microstructure profiler measuring the fluctuations in vertical shear in the dissipation range; an acoustic Doppler velocimeter measuring the velocity inside the sea cage; dissolved oxygen sensors and echosounders measuring the distribution of fish inside the cage. The measurements have performed with stocked and emptied sea-cage. The results showed simultaneous strong flow reductions in the wake near the cage and high turbulence in the upper part of the water column, both of which were generated by the sea cage. Measurements inside the cage showed that although the schooling fish reduced the flow, there was no evidence that they generate secondary radial and vertical flows.