EXPERIMENTAL INVESTIGATION OF CANADIAN FISH FARM HYDRODYNAMIC WAKE PROPERTIES AND ITS IMPLICATIONS FOR INTEGRATED MULTI-TROPHIC AQUACULTURE (IMTA)  

Hydrodynamic experiments on 1:15 scale model arrays of both circular cages typically used on the East Coast of Canada, and square cages typically used on the West Coast of Canada have been completed in the large recirculating flume tank located at the Fisheries and Marine Institute of Memorial University in St. John's, Newfoundland. Scale model cages were designed with a high amount of detail from cages used in industry, and a global force ratio scaling technique was developed to ensure geometric similarity between cages of model scale and full scale. A wake study for both cage arrays was completed to determine flow velocities, flow topology and turbulence in the wake. With knowledge of wake properties downstream of fish cage arrays, temporal and spatial patterns of released particulate and dissolved nutrients can potentially be predicted to guide in the placement of organic and inorganic extractive species for optimized recapture efficiencies.

Swoffer current meters were used to capture planes of wake velocity measurements at several downstream distances from the cage arrays. Individual isolated cages and 4 cage arrays of both circular and square cages were tested for comparison. The circular cage array was deployed at two different cage spacings, representative of typical salmon farms, while the square cage array was deployed at the only industry standard. Results show high velocity deficits behind the cages, causing accelerations of flow underneath and around the sides of the cages. Large scale turbulence is generated near the bottom of the cages, due to the presence of a shear layer in the wake. Turbulence intensity is found to spread into the wake and intensify with increasing downstream distance from the cages. This turbulence in the wake of the fish cage arrays will cause heavy mixing of released nutrients from cages. Dye release was also used to observe the large scale flow topology within and in the wake of the fish cage arrays, and to verify the results obtained from wake velocity measurements.

Hydrodynamic results from the wake study present some implications for IMTA component placement. Large velocity deficits observed behind fish cages suggest that organic extractive species, such as mussels, should be placed as close to individual cages as possible, so that particulate matter can be intercepted before it sinks to the seafloor. It has also been shown that mussels have higher particulate extraction efficiencies at lower flow velocities. Placing co-cultured species infrastructure close to cages will cause further hydrodynamic implications, such as increased drag and velocity deficits, and should be carefully considered. Inorganic extractive species, such as kelps, should be placed further downstream in the wake of a fish farm to benefit from increased flow velocities due to the recovery of the wake. Dissolved inorganic nutrients will be transported within the wake and spread with increasing distance from the farm. In case of an isolated farm, the balance between nutrient concentration and flow velocity must be carefully considered when placing inorganic extractive species and their infrastructures.