Development and application of dynamic models for environmental management of aquaculture in South East Asia

Abstract

With the world’s population set to reach 9 billion by the mid 21st century food security has never been more important. Increased competition regarding land for agricultural use and over fished seas means it falls to aquaculture to meet the global demands for protein requirements. The largest supply of aquaculture products are cultivated in South East Asia where the industry has seen rapid expansion, particularly of pond production in the past 50 years. This initial expansion has come at a cost with mangrove losses and eutrophication of natural water sources resulting. The impact of these not only affects other stakeholders, including domestic users, but effects will be felt by the aquaculture industry. Indiscriminate release of effluents to the surrounding water reduces the water quality for other users and may impact on the farm discharging the water originally. Poor water quality can then result in poor growth rates and increased mortalities reducing the profitability of the farm and endangering the livelihood of the farmer. If aquaculture is to meet the global food demand it is important that current and future enterprises are developed with sustainability at the fore front. This study investigates the nutrient dynamics in pond culture in South East Asia, focussing initially on four countries outlined by the SEAT (Sustainable Ethical Aquaculture Trade) project, including Thailand, Vietnam, China and Bangladesh. Within the four countries the main species cultured for export were identified resulting in tilapia, shrimp, pangasiid catfish and prawn. Following a farmer survey designed to collect a large volume of data over a range of topics including, water management, social, economic and ethical perceptions, dynamic models were developed, using Powersim Studio 8© (Powersim, Norway), for a generic fish and shrimp ponds separately. The models draw on data from the survey combined with other literature sources to provide outputs for Total Nitrogen and Total Phosphorus in water and sediment as well as dissolved oxygen in the pond water. One of the biggest challenges facing this study was the objective selection of relevant sites for case studies to apply the models to. With such a large preselected set of sites (200 per species per country) it was important that the method be capable of handling such large datasets. Thusly it was decided that a multivariate method be used due to the removal of any pre judgement of the data relevant to the study. In order to investigate the nutrient dynamics water management data was used in the multivariate analysis to identify any similarity between the practices occurring on farms. The case studies in this project focus on Thailand and Vietnam, covering tilapia, shrimp and pangasius. Prawn farms were disregarded as, through the survey, it was discovered most production was for domestic trade. The models were adapted to each farm case study expanding the boundary from pond level to farm level, providing an output for each pond in terms of nutrients in the water and production levels and the farm as a whole for dissolved oxygen and sediment accumulation. The results of the models suggest the culture species to be taking up much of the TN added followed by the accumulation in sediments in shrimp ponds, while TP is mostly taken up by sediments. The fish case studies suggest that most of the TN is discharged to the environment followed by uptake. While Total phosphorus shows similar results to shrimp, accumulating in the sediment. The models presented in this study can be used to estimate outputs from farms of similar water management strategies and can assist in the determination of where improvements can be made to reduce the potential for eutrophication of natural water sources.