Mixing downstream of a submerged fish cage: a numerical study

As part of the overall Hawaii Open Ocean Research Project (HOARP) a fish cage of biconical shape (OceanSpar SeaStation 3000) has been placed in the waters of the coast of Ewa, HI, to enable research on the effect of cage culture of fish in tropical waters. One of the goals of HOARP is to examine the nature of the waste product stream of the fully submerged fish cage. HOARP activities began as a research effort but now are conducted in conjunction with a commercial operator that has three cages installed. For our analysis of the impact of the farm, an estimate of the mixing downstream of an individual cage is needed to assess the potential water quality impact of the farm operation to nearby waters. Federal regulations (40 CFR 122.24) require aquaculture systems to obtain a NPDES permit if they annually produce more than 45 454 kg harvest weight (about 100 000 lbs.) of warm water fish (CWA sections 318 and 402). The current farm off Ewa meets this criterion, thus a "mixing zone" needs to be established. A CFD simulation was made to evaluate the mixing downstream of the fish cage. This mixing consists of two parts, the turbulence generated by water passing through the cage, and the vortical flow generated by the general obstruction to water flow that is caused by the cage. The small scale turbulence increases the effective diffusion, and thus increases dilution of the pollutant, within the mixing zone. 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.