Sustainable aquaculture in Saldanha Bay, South Africa

Abstract

Saldanha Bay, located near the coastal town of Saldanha, in Western Cape Province of South Africa, possesses excellent conditions for mussel and oyster aquaculture. Its linkage with the adjacent upwelling current system provides a very productive environment for phytoplankton growth, and this has led to the development of a vibrant shellfish aquaculture industry. The main objectives of this work are to develop a model which simulates the main ecological processes within the Bay, to determine the Bay’s carrying capacity for mussel and oyster production, and to produce a management tool for decision making. Bivalve aquaculture has great growth potential and may be important for human food security as mankind faces a projected need of an additional 30 X 106 tonnes per year of aquatic products by 2050. Bivalve aquaculture is organically extractive, and can additionally provide significant ecosystem services in top-down control of eutrophication, and creation of structure for stimulating biodiversity. When managed properly, this form of aquaculture has a very low environmental footprint, mainly associated with organic enrichment of the sediment. This impact is even less relevant in upwelling systems such as Saldanha Bay where particles tend to be flushed out in the surface layer, and in all cases it must be borne in mind that by definition shellfish aquaculture results in a net removal of seston from the water column. This model was developed using EcoWin an object oriented approach to ecological modelling. The model for Saldanha Bay was set up using oceanographic and water quality data collected from Saldanha Bay, and culture practice information provided by local shellfish farmers. The first step was the construction and calibration of the ecological model, in order to provide a general description of the biogeochemical behaviour of the Bay, followed by the addition of the shellfish aquaculture component. EcoWin successfully reproduced the key ecological processes, correctly simulating a mean phytoplankton biomass of 7.5 chl a L-1. The aquaculture module simulated an annual harvested biomass of about 3000 t y-1, in good agreement with reported yield. Six production scenarios were explored, for illustrative purposes: - Increase in stocking density of shellfish - Two alternatives for aquaculture development in particular areas of Saldanha Bay - Prediction of the maximum production capacity of the Bay. These results were analysed in terms of their impacts and potential.This study demonstrates the relevance of aquaculture-oriented ecological models in evaluating different stakeholder-defined development scenarios, and their utility in avoiding the social and environmental impacts of testing different scenarios in situ. The present application of EcoWin shows great potential for supporting both water managers and industry in Saldanha Bay.