Combined models of growth, waste production, dispersal and deposition from cage-cultured Atlantic salmon to predict benthic enrichment

Abstract

Models of particulate waste production and deposition can be used in performance-based management approaches as cost-effective tools to assess environmental effects of open-pen finfish aquaculture. XLDEPMOD is an MS Excel® spreadsheet-based depositional model for predicting particulate organic carbon (POC) waste production and sedimentation from net-pen cultured finfish. Calculations are based on temperature-dependent fish growth and mass-balance calculations of feed input, growth, respiration and 3 size classes of feces. Depth-average and near-bottom directional currents are used to determine waste dispersion by fitted Gaussian distribution functions. Near-bottom velocity and substrate-based resuspension thresholds and loss of deposited waste due to decomposition and consumption by wild fish and invertebrates are used to calculate net POC sedimentation. The model was applied to 2 Atlantic salmon farms in southwestern Bay of Fundy, Canada. Sensitivity analysis showed that reduction in waste flux due to resuspension depends on the magnitude of current and wave-driven bottom shear and mass fractions of feces with different settling velocities. Depending on depth, current speed, substrate type and fecal mass fractions, resuspension can remove up to 80% of deposited waste from under net-pens. Steep gradients with high rates (>5 g POC m-2 d-1) of sedimentation predicted under and close to cages and lower rates (<1 g POC m-2 d-1) >50 m away are consistent with published DEPOMOD results and sediment trap observations at the farm sites. The model can be used by regulators to determine if acceptable environmental standards for benthic impacts due to waste deposition from salmon aquaculture are being maintained.