DEPOMOD model

The farming of Atlantic salmon in the UK is a major industry (~ £250 m p.a., SSGA). In spite of the adverse economic climate, and many pessimistic predictions, the industry continues to expand rapidly. Salmon farming is now dominated by a few large companies and as margins are driven down, economies of scale are sought to retain profitability.

Increasingly regulators find themselves seeking better predictive capability for large fish farms and improved objectivity in the decision making process. Similarly, fish farmers need methods for assessing the suitability of new sites, or the potential for expansion at existing sites. DEPOMOD aims to be a modelling tool that is of value to both the industry and the regulator, providing assistance in the decision making process.

DEPOMOD (v1.5) used the model BenOss as its starting point, following a similar modular structure but includes a fish growth model and in–feed treatment module. Consideration of the differences between sewage outfalls discharging in coastal areas and fish farms in sea lochs was required throughout the project. In sea lochs, biological communities present are quite different; the gradients of organic input are extremely steep with very large changes occurring in relatively short distances making sampling station selection critical; fish food and faeces have quite different behaviours in sea-water relative to sewage solids and are of different composition. In addition, fish farm sites are generally more stratified than sites where sewage outfalls are sited requiring a greater appreciation of vertical current shear and its effects on particle dispersion.

Model validation has been undertaken in three stages. Validation of initial deposition of material from farms was undertaken with sediment trap studies. The resuspension model was then validated using a particulate tracer discharged from a point source in a sea loch. DEPOMOD was then used to establish semi–empirical relationships between predicted solids accumulation and several measured benthic indices at fish farms. Further development of the model has resulted in DEPOMOD (v2) which has modelling capability for two in–feed treatment chemicals.

The DEPOMOD project was funded by NERC MAFF Link Aquaculture, Marine Harvest McConnell, Scottish Environment Protection Agency and Scottish Salmon Growers Association. Scottish Environmental Advisory Services were also participants in the project. The model is a pc–based Windows 95TM application available from DML

Names of DEPOMOD project staff