Chris J Cromey

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

Abstract 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.

Validation of a fish farm waste resuspension model by use of a particulate tracer discharged from a point source in a coastal environment

To validate a resuspension model of particulate material (salmonid farm wastes), a UV fluorescent particle tracer was selected with similar settling characteristics. Tracer was introduced to the seabed (water depth ≈30 m) and sediment samples taken on days 0, 3, 10, 17 and 30 to measure the horizontal and vertical distribution of tracer in sediments. A concentric sampling grid was established at radii of 25, 50, 100, 150, 200, 400, 700 and 1, 000 m from the source on transects 30° apart. The bulk of the deployed tracer was initially concentrated in an area 25 m radius from the release point; tracer was observed to steadily decrease to zero over a period of 30 days. In a 200 m region measured from the release point in the direction of the residual current, the redeposition of tracer was low. A Lagrangian particle tracking model was validated using these observed data by varying resuspension model parameters within limits to obtain the best agreement between spatial and temporal distributions. The validated model generally gave good predictions of total mass budgets (±7% of total tracer released), particulary where tracer concentrations were high near the release point. Best fit model parameters (critical erosion shear stress=0.018 N m−2, erodibility constan=60 g m−2 d−1) are at the low end of reported parameters for coastal resuspension models. Such a low critical erosion shear stress indicates that the frequency of resuspension and deposition events for freshly deposited material is high.

New developments in the remote measurement of currents and waves at the Scottish Association for Marine Science

In our last presentation to this conference in 1999, we outlined the incorporation of GPS and adaptive sampling techniques into a variety of Lagrangian drifters (the GPS-Argos Drifter, the Smart Buoy and the Mini-Drifter), each tailored to address a particular scientific question and optimised for given space and time scales. In this paper we will describe further developments which have led to the successful deployment of innovative ice buoys in both polar regions. These buoys have exploited the high resolution of post-processed GPS techniques, wave spectral data from on board accelerometers, and the enhanced bandwidth of new satellite communications systems to yield valuable new insights into the formation and deformation of young pack ice. At the other end of the scale, mini drifters are now routinely used to estimate diffusion parameters close to pollution sources as part of an ongoing modelling effort to quantify the fate of pollutants.