Development of a model for evaluating and optimizing the performance of integrated multitrophic aquaculture (IMTA) systems

Abstract

Earth’s population is expected to reach 9 billion by 2050. Ensuring food security for the growing world population is one of today’s society’s major challenges and responsibilities. Aquatic products have the potential to contribute significantly in the growing population’s dietary requirements. Since increasing the pressure on most natural fish stocks is now widely agreed not to be an option, the aquaculture sector needs to grow. The challenge is to increase aquaculture production without depleting natural resources or damaging the environment but also in a financially sustainable way. Integrated Multitrophic Aquaculture (IMTA) is one method of sustainable aquatic production. Integrating bioremediatory organisms that extract particulate organic matter or dissolved inorganic nutrients with monocultures of fed species has the potential of reducing the particulate and soluble waste loads from effluents, whilst producing a low-input protein source that may also increase the farm income. IMTA is a viable solution for mitigating the environmental impact of waste released from fish farms. The fish waste is exploited as a food source for lower trophic, extractive organisms giving an added value to the investment in feed. Studies up to now have shown that under experimental conditions as well as in small-scale commercial studies, various filter-feeding, deposit-feeding and grazing species can ingest fish waste particles. The aim now is to achieve IMTA optimization, where extractive organisms can ingest most of the finfish waste food and excretions. Any such design is likely to be complex incorporating a multidisciplinary approach, and therefore to date a reason why most studies have failed to prove the environmental and economic benefits of IMTA. Consequently, the aim of this study is to develop ways of selecting an ideal combination of species for a specific locality, manage the cultures in a way that ensures the maximum nutrient recycling feasible per unit of area; and ensure high growth rate of the extractive organisms while being financially beneficial. The approach taken was a combination of investigative literature reviews, computer modelling work and small-scale growth trials to determine the relative growth of extractive organisms fed fishfeed and waste, followed by the development of a systems-based model of interaction and growth efficiency for combinations of organisms within an IMTA system. This study starts by investigating, with small-scale laboratory experiments, the potential of two organic extractive species, the lugworm, Arenicola marina and the sea urchin, Psammechinus miliaris, as organic extractive components of IMTA systems. Their ability to consume and assimilate salmon faeces was evaluated as well as their remediation efficiency. This was done by comparing the carbon, nitrogen and phosphorus content of the pellet-faeces mixture to that of the sea urchin faeces and sea urchin gonad content. Their growth, gonadosomatic index (GSI) (for the sea urchins), tissue carbon, nitrogen and phosphorous content were compared between seaweed diets and a diet consisting of a mixture of salmon faeces and feed pellets. The results showed statistically significant gonad carbon content for the sea urchins fed with faeces. Similarly, statistically significant higher phosphorous content was found in the tissues of the lugworms fed with the mixture of salmon faeces and pellets than in the lugworms of the other two groups. The subsequent and main phase of this study was the development of a model for optimising IMTA performance. The modelling process included model development, run, optimization and risk assessment. The IMTA model developed consisted of Atlantic salmon Salmo salar, the sea urchin Paracentrotus lividus and the macroalgae Ulva sp.. It simulates the growth as well as the uptake and release of nitrogen by these organisms under environmental conditions of a hypothetical site on the west coast of Scotland. The aim of the model was to maximize the potential of IMTA in terms of productivity and to reduce the amount of nutrients that are released in the environment, and thus to contribute towards a more sustainable and productive form of aquaculture. The IMTA model developed can be re-parameterised to simulate the growth and nutrient uptake of different species and the growth and nutrient uptake under different environmental conditions. This capacity of the model was used in order to do a comparative study of the nitrogen bioremediation potential of three different invertebrate species, cultivated as part of an IMTA. These species were the lugworm (Arenicola marina), the blue mussel (Mytilus edulis) and the purple sea urchin (Paracentrotus lividus). The results of this comparative study showed that weight for weight, M. edulis is more efficient in removing POM than P. lividus that is in turn better than A. marina with regard to the amount of nitrogen they can assimilate. But in terms of cultivation area required for the production of the same total biomass, P. lividus was better at removing POM followed by M. edulis and then by A. marina.