Management of marine fish farming in the sub-tropical environment: A modelling approach

Two deterministic models were applied to simulate hydrographic and water quality conditions within a sub-tropical marine fish culture site, where trash fish is used as feed. A two-dimensional, two-layer hydrodynamic model of tidal flows and salt transport calculated the water level, velocity and salinity in each grid cell of 50 m square in each layer within the culture area approximately every 30 s during a tidal cycle. Results from this flow model provided hydraulic data for input into a three-dimensional tidal water quality model, which was run to simulate water quality due to specified pollutant loadings from the marine fish culture operations. Simulated output from the models agreed reasonably well with observed field data, except for dissolved oxygen and nitrate levels in surface waters. This could be due to a conservative estimation of re-aeration rates in the model system. In applying the models to sub-tropical waters of Hong Kong, the simulation predicted the extent of pollution and area affected under varying fish stock and pollutant loadings. It was shown that impacts of fish culture activities on water quality at the test culture site were localized owing to strong advection by tidal residual flows. The higher simulated level of organic nitrogen in the bottom waters over the year at the test fish culture site was due to increased release of organic nitrogen through decay of wasted feed from the sea bed and from trash fish feed. Models of marine fish culture and environmental effects have applications in determining acceptable stocking levels, and the effects of changing feed and culture species, and for locating suitable culture sites in coastal areas.