Carrying capacity for finfish aquaculture, Part II – Rapid assessment using hydrodynamic and semi-analytic solutions

Abstract

The carrying capacity for aquaculture cage farming in Spencer Gulf (South Australia) is based in part on guidelines that the maximum feed rates and nutrient flux into the lease regions are determined such that the maximum nutrient concentration c does not exceed a prescribed value (say cP) to ensure ecosystem health – ecological carrying capacity. The goal of this study is to allow the rapid estimation of maximum nutrient fluxes and feed rates at new lease sites. Spencer Gulf is chosen as a case study although the methodology should find application in other regions around the world. In part I of this study, semi-analytic solutions were obtained to show that to a good approximation the maximum nutrient flux (feed rate) F can be simply estimated from: F=cP/T* where T* is a flushing time scale of the cage or lease region. In this study a 3-dimensional hydrodynamic model for Spencer Gulf is used to determine the parameters needed to estimate T* and thus F and feed rates at every model cell in the gulf. The parameters needed include the vector mean speed (U), r.m.s. tidal amplitude (UK) and the mean shear dispersion diffusivity (KS). As a case study, these parameters and T*, are estimated by three-monthly, winter averages. Results show the vector mean speed to be very small (U∼0.01m/s), tidal velocities large (UK∼0.3–1m/s) and the associated shear dispersion coefficients very large (KS∼10–100m2/s). Flushing at the scale of the lease (600m) and in the upper gulf is generally dominated by diffusive affects for which the maximum nutrient flux (and feed rates) is largest. The results should find application in other finite source flux problems in the coastal oceans including desalination plants and ocean outfalls.