Tropomod is one of a suite of particle tracking models (including Depomod and Meramod) which simulates the dispersion of waste feed and faecal particles from aquaculture to the seabed. The model is used to predict sediment quality footprint below fish cages in order to be able to ensure that severe impact to the sediment is prevented and to optimise farm layout and production within the different aquaculture zones.
The simulation begins with particles released from a defined cage volume, the mass of particles released being dependent on husbandry data (feed ration, feed type, etc.) and numerous waste conversion coefficients (e.g. % uneaten feed, feed digestibility, etc.). Particles have settling characteristics according to their size or type (i.e. feed pellet size, trash-fish, etc.), or in the case of faecal particles, according to the main species being farmed in that aquaculture zone. Particles in deeper areas are dispersed further away from the cages. Coarse, fast settling particles (e.g. feed pellets) deposit on the seabed close to the cages, whereas finer feeds which settle more slowly disperse more widely.
Particles settle down through the water column and are advected horizontally according to local current velocity and direction. In addition, particles are dispersed more randomly using a random walk model with horizontal and vertical dispersion coefficients from drogue surveys. The particles settle on the bed and create a deposition ‘footprint’, the severity and extent of this footprint being useful for farm management and strategy.
Tropomod has been used in Singapore and the Philippines for planning new fish cage culture development as well as optimising existing cage culture development to ensure sustainability of development.
Where farms have closely spaced cages and poor feeding practices waste flux to the bed is high, resulting in a severe deposition footprint and high environmental impact close to the cages. Conversely, farms with good feeding practices (e.g. careful feeding, high-quality feed, etc.) and located in dispersive areas with appropriate spacing between cages, often have larger extent deposition footprints but these are less severe.
This paper will give details of the model and it’s use for predicting sediment impact for locating and sizing new farms, use for locating extractive species in conjunction with cage culture (IMTA), use for optimising small scale cage culture by reducing cumulative sediment impact, identifying operational management options for reducing sediment impact, etc. based on some of the previous research project outputs in South East Asia.