A fish cage of bi-conical shape (OceanSpar SeaStation 3000) has been placed in the waters of the coast of Ewa, Hawaii. As part of the HOARP Phase II research program, the cage was stocked with 133,000 moi (Pacific threadfin). One of the goals of the work is to examine the nature of the waste product stream of the fully-submerged fish cage. Federal regulations (40 CFR 122.24) require aquaculture systems to obtain a NPDES permit if they annually produce more than 45,454 harvest weight kilograms (about 100,000 pounds) of warm water fish (CWA sections 318 & 402). Should the fish cage in Ewa meet this criterion, it would need to meet the NPDES regulation criteria. This requires that a ‘mixing zone’ be established downstream of the cage. A CFD simulation was made to find the intensity of turbulence downstream of the fish cage. This turbulence increases the diffusion of the pollutant and the size of the mixing zone as a result. Parametric studies have been made to examine the effect of current speed on the initial mixing thereby enabling an estimation of the total mixing downstream of the cage. This estimate of the effective diffusion is essential for the establishment of the boundaries of the mixing zone.

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