DEPOMOD - modelling the deposition and biological effects of waste solids from marine cage farms.

To enable better predictive capability of the impact from large marine cage fish farms on the benthos and improved objectivity in the regulatory decision-making process, a computer particle tracking model DEPOMOD was developed. DEPOMOD predicts the solids accumulation on the seabed arising from a fish farm and associated changes in the benthic faunal community. The grid generation module allows the user to set up a grid containing information on depth, cage and sampling station positions for the area of interest. Given the information on wastage rates of fish food and faeces and hydrodynamics of the area, the initial deposition of particles on the seabed can then be predicted with the particle tracking model. The resuspension model then redistributes particles according to near-bed current flow fields to predict the net solids accumulated on the seabed within the grid area. From quantitative relationships between benthic community descriptors and solids accumulation, predictions of the level of benthic community impact can then be made. The particle tracking model was validated using sediment trap studies. Model predictions of flux (g/m(2)/day) generally agreed well with field data with an accuracy of +/- 20% and +/- 13% for a dispersive and depositional site, respectively. Using parameters from the validated resuspension model (rare among models in this field), semi-empirical quantitative relationships between predicted solids accumulation (g/m(2)/year) and observed Infaunal Trophic Index (ITI) and total individual abundance were established using data from numerous Scottish marine fish farms. A submodel was also validated for predicting feed input throughout a growing cycle for planning purposes. DEPOMOD may be used for assessing the potential impact of a farm throughout a growing cycle, or if the biomass consent is increased. It may also be used in the site selection process of a new farm to investigate the proposed farm position and biomass levels. Prediction of the dispersion of particulates during use of in-feed medicines may also be undertaken. (C) 2002 Elsevier Science B.V. All rights reserved.