Abstract
Closed recirculation aquaculture systems (RAS) in combination with integrated multi-trophic aquaculture (IMTA) are considered best management practices, but high material costs and difficult maintenance still hinder their implementation, especially in developing countries and the tropics. Few case studies of such systems with tropical species exist. For the first time, an extremely low-budget system was tested combining the halophyte sea purslane Sesuvium portulacastrum and a detritivore, sandfish Holothuria scabra, with finfish milkfish Chanos chanos over 8 wk on Zanzibar, Tanzania. In a 2 m3 RAS, milkfish and sea purslane showed good growth, producing an average (±SD) of 1147 ± 79 g fish and 1261 ± 95 g plant biomass, while sea cucumber growth was variable at 92 ± 68 g. The system operated without filter units and did not discharge any solid or dissolved waste. Water quality remained tolerable and ammonia levels were reliably decreased to <1 mg l-1. A NO2- peak occurred within the first 30 d, indicating good biofilter performance of the different system compartments. Changes in dissolved inorganic nitrogen (DIN) species support the notion that the sea cucumber tank was the main site of nitrification, while the hydroponic halophyte tank acted as a net sink of NO3-. A nitrogen budget accounted for 63.7 ± 5.3% of the nitrogen added to the system as fish feed. Increasing the plant to fish biomass ratio to 5:1 would fully treat the DIN load. The experiment provides proof-of-concept of a simple pilot-scale RAS, integrating tropical species at 3 trophic levels.
Highlights
Aquaculture is expanding rapidly but with significant environmental impacts (Holmer et al 2003, Primavera 2006, Herbeck et al 2014)
As our agricultural food production is heavily reliant on glycophytes, salttolerant halophytes are promising as vegetable crops or biomass for biofuel production (Buhmann & Papenbrock 2013)
This study provides a proof-of-concept of an integrated multi-trophic aquaculture (IMTA) recirculation aquaculture systems (RAS) setup, integrating milkfish C. chanos, sea cucumber H. scabra and, for the first time, sea purslane S. portulacastrum in a tropical country
Summary
Aquaculture is expanding rapidly but with significant environmental impacts (Holmer et al 2003, Primavera 2006, Herbeck et al 2014). Halophytes are a possible extractive species for integrated aquaculture, but research focusing on them in this context has been limited (Custódio et al 2017). Different plant species and types of natural and constructed wetlands are already being successfully used for phytoremediation of municipal and industrial wastewater or contaminated soil (Verhoeven & Meuleman 1999, Vymazal 2010). Their use can have great economic benefits compared to conventional water treatment, as some species are nutritious and desired on the gourmet market (Cardoch et al 2000, Ventura et al 2011). Very little is known about its nutrient tolerance and biofiltration capacity, but previous studies suggest that it can remediate aquaculture wastewater (Slama et al 2006, Boxman et al 2017)
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