Developing efficient Integrated Multitrophic Aquaculture (IMTA) models.

Fani Lamprianidou:

The constantly increasing demand for seafood, during a period of saturation for the fisheries sector, can only be met by sustainable growth of the aquaculture sector. This growth is limited by the environmental impacts and economic requirements of intensive monoculture of fed species. Integrating bioremediatory organisms, which extract particulate organic matter or dissolved inorganic nutrients, within monocultures of fed species has the potential of reducing the particulate and soluble waste loads of effluents whilst producing a low input protein source that may also increase the farm income.

The objective of this project is to develop a matrix for designing IMTA systems on any site and to use this matrix for designing an efficient IMTA system for a specific locality. The model developed estimates the nutrient bioremediation potential and productivity of an IMTA consisting of salmon, sea cucumbers and seaweed by qualifying and quantifying the matter and energy flux within the IMTA and its surrounding environment. The desired IMTA design will achieve high bioremediation efficiency in limited space, higher growth rates of extractive organisms than in monocultures and will increase farm revenues. This way the developed IMTA system will provide both environmental and socioeconomic benefits and will thus contribute towards a more sustainable and productive form of aquaculture.

At a salmon monoculture site 38% of feed nitrogen (N) and 30% of feed phosphorous (P) is incorporated into fish biomass while the remaining nutrients are released to the environment. The model predicts that a salmon farm producing 1000 t of salmon in an 18 month period, releases approximately 37 t of dissolved nitrogen during the at-sea stage of production. This is in agreement with the DIN release rate by salmon farms in Scotland, which ranges between 35 and 45 kg N t-1 of salmon produced (Davies, 2002). During an 18 month production period 470 t of seaweed of the genus Porphyra can be harvested, by harvesting in a way that sustains the total biomass to maximum 50t wet weight at all times. This way, 35.4 t of dissolved nitrogen can be assimilated by the seaweed biomass and removed from the ecosystem via the process of seaweed harvesting. Consequently, the addition of seaweed, of the genus Porphyra, to a salmon culture sytem increases the dissolved nutrient retention of feed N by 80.2%.

The aim was to build a dynamic interactive model that could act as a matrix for creating IMTA systems on sites with different environmental characteristics or IMTA systems composed of different species combinations. The modelling platform used was Powersim™, which is a graphical interface object-oriented modelling software that allows us to conceptualize, build and test system dynamics models.

For the development stage of the model the assumption was made that there is homogenous distribution of released DIN within a defined “IMTA area” and no further nutrient transfer. In order to establish the spatial limits and temporal patterns of the nutrient and particulate wastes released from the salmon farm and thus convert the closed IMTA to an open-water system, the waste release-assimilation model will be merged with Geographic Information System (GIS). The GIS system will enabel estimation of the quantity of waste on site at any given time and spatial optimization of the IMTA structural design by placing the extractive species in the areas that allow maximum waste uptake. Future work will also include field trials performed for verification of the modelling output.

Acknowledgements: Fani is funded by the institute of Aquaculture, MASTS and private support.