Integrated multi-trophic aquaculture : Nutrient retention efficiency and valorisation of waste nutrients

Abstract

With the increasing demand for fish and seafood, further growth of the aquaculture sector is foreseen. This growth associates with ecological concerns, including pressure on natural resources, a growing demand for high-quality aquafeeds and an increased waste production (i.e. uneaten feed, metabolic waste and faeces) with potential detrimental effects if discharged into the environment. Therefore, sustainable aquaculture approaches are needed, to keep up with the increasing demand for food and resources while minimizing adverse impacts on the environment. Integrated Multi-Trophic Aquaculture (IMTA) is an approach with the ambition to fulfil this need. In IMTA systems, fed species aquaculture (i.e. dependent on external feed supply) is combined with extractive species aquaculture (i.e. extract nutrients from their environment), so that waste resulting from fed species is recycled in extractive species biomass. This approach strives to reduce aquaculture waste losses by improving resource use efficiencies and transforming linear monocultures into circular systems.

The general concepts and principles of IMTA have been extensively addressed in the literature. Still, the environmental benefits of IMTA are mainly conceptually described and insufficiently quantified. The aim of this thesis is to investigate nutrient retention efficiencies in different marine IMTA systems (i.e. open-water sea cages, land-based flow-through and recirculating aquaculture systems [RAS]) by quantifying the fluxes involved in nitrogen (N), phosphorus (P) and carbon (C) retention and exploring the impact of biological and environmental factors on retention efficiency and waste valorisation.

Based on an extensive literature review, this thesis shows that it is feasible to recycle 22-32% N, 41% P and 7-37% C of the feed input via extractive species in a four-species marine land-based closed IMTA systems, while in an open-water IMTA system this is 7% for N, 16% for P and 2-12% for C. In addition to the literature review, experimental work with seaweed and benthic deposit feeders was done to address specific knowledge gaps. Under moderate to high nitrogen (0.5-5 mM) and phosphorus (0.01-0.1 mM) concentrations, performance of the seaweed species Ulva spp. is not influenced by (unfavourable) stoichiometry and high nitrate concentrations do not limit phosphorus uptake. This is promising for closed IMTA systems with Ulva spp. as extractive species. Orthophosphate concentrations of 0.9 mM are toxic for Ulva spp., and these conditions should be avoided in the design and management of closed IMTA systems. Two benthic polychaete species were studied, Capitella sp. and Ophryotrocha craigsmithi and both species are interesting for IMTA systems, as they show good bioremediation and growth potential when fed fresh salmon faeces. Moreover, albeit the salmon faeces fed to the polychaetes contained relatively low polyunsaturated fatty acid levels, the worms studied were rich in polyunsaturated fatty acids. This indicates that the polychaetes are able to convert low-quality fish faeces into a high-quality resource, containing for fish essential fatty acids. Potentially, this makes them a valuable resource for fish feed formulation. Overall this thesis creates more insight in the (re)cycling of fed nutrients by extractive species in IMTA, and the waste valorisation potential of seaweed and polychaetes.