Modelling aquaculture impacts

MOM (Modelling - Ongrowing fish farms - Monitoring) This model developed in Norway is a three component model for modelling organic impacts. The fish sub-model simulates the release of particulate material from the farm based on information on the feeding rate and composition of food. Uptake, retention and excretion are all calculated in relation to the temperature and size of the fish. The output from this sub-model provides the initial conditions for the dispersion sub-model which simulates dispersion and sedimentation rates of excess feed and faecal pellets. The sediment sub-model calculates the maximum decomposition at the seabed for a particular scenario and oxygen concentration in the benthic boundary layer is also calculated. The combination of these sub-models allows the calculation of maximum fish production that a site can sustain without adverse benthic effects.

AWATS - Aquaculture Waste Transport Simulator The AWATS model is a mathematical model to simulate tidal and wind-driven currents, waves, and the resulting dispersion of fish food and faecal matter in coastal areas. This model was one of the first aquaculture organic impact models to include complex models of the descriptions of spatially varying current around the study area. In addition, wind driven flow and waves are also included as processes having an effect on the subsequent dispersal of discharged particulate material. This model originally lacked resuspension which causes deposited particles to be re-entrained into near bed flows and advected away from the initial footprint area.

DEPOMOD This model is a lagrangian particle tracking model which predicts the dispersion of particulate wastes arising from aquaculture activities and associated benthic impacts. It was developed from a sewage dispersion model also developed by SAMS, but required extensive modification to data input requirements and validation for the fish farming environment. There are three main modules: particle tracking, resuspension benthic response. Predictions of solids accumulation (g m-2 yr-1) determine the benthic response using a relationship between solids accumulation and benthic indices validated for Scottish fish farms.

Current profiles in stratified waters are complex Current profiles in stratified waters are complex. Particles settling at different rates are subject to current shear and turbulence

MERAMOD Is a conversion of Depomod from cols water areas to temperate areas There is an addition of wild fish component to take into account the utilisation of waste feed by wild fish under the cages It also takes into account the different species and behaviour of faecal pellets in the water column. It is validated for cage farms in the Mediterranean

MERAMOD modules (I)

MERAMOD modules (II)

Crucial input data for modelling There are a number of key input data issues which need to be addressed when developing an existing model for application in a different environment. Although the principal physical processes can be applied to different areas, the input data used to drive these components need to be critically assessed.

Sediment trap experiments (model validation) Water column (WC) Upper (U) Lower (L) x6 x6 x6 or x12 1. Deploy 2. Retrieve, filter, dry 3. Calculate observed flux (total waste particulate material = g solids m-2 yr-1) 4. Check calculation 75 cm H:D = 5:1 ratio

Typical impact footprints Dispersive sites Strong currents Impact over a larger area (up to 100 m) but less intense Typical of fish farms in Scotland Depositional sites Weaker currents Impact over a limited area (up to 30 m) But more intense Typical of farms in Greece

Model validation Species Abundance Use benthic data to establish relationships between benthic indices and flux predictions Shannon Weiner Modelled flux (g m-2 yr-1)

Use of models in knowledge transfer Closely spaced cages Largely spaced

Sedimentation – 4 cages weak current Sensitive habitats

Sedimentation – 4 cages strong current Sensitive habitats