Utilizing Composted Fish Waste Solids as a Medium Supplement for Flow-through Aquaponic Lettuce Production

Aquaculture systems produce nutrient rich effluent that must be managed in order to reduce environmental impact. Remediation through aquaponics, the integration of plant and fish culture, can reduce this impact and provide an opportunity to culture harvestable value-added crops. This experiment examined the food crop production capabilities of cold water (12°C) flow-through aquaponics utilizing the effluent produced in a raceway aquaculture system.  Currently, production consists of floating rafts to support the plants and these rafts contain vermiculite (plant growing medium) as the growing substrate for the plants.  This plant growing medium is an extra cost to the grower and a more sustainable substitute would be desirable.  One such replacement could be developed from the fish waste solids that are produced from the aquaculture system, which would eliminate the extra production cost of purchasing the necessary plant growing medium.  This research investigated the feasibility of using composted fish waste solids as an amendment to standard plant growing media for aquaponic lettuce production.  The solids were collected and concentrated from the flow-through aquaculture raceway quiescent zones utilizing Geotube® (geotextile) containers.  Fish waste solids were collected in the Geotube® for approximately 6 months and then removed from the aquaculture system.  The contained fish waste solids were allowed to compost for 6 months.  The composted fish solids were removed from the Geotube® and were incorporated into fine grade vermiculite at rates of 0, 25, 50 and 75% compost/medium volume.  Lactuca sativa ‘Red Sails’ (Red Sails lettuce) were direct seeded into speedling trays (128 cell trays, 3.8cm²/cell) into the different media formulations for a 12-week trial.   Lettuce was grown in flow-through channels 0.5m wide x 2.5m long inside a high tunnel utilizing the fish wastewater effluent as the aquaponics water source.  At the end of 12 weeks, total harvestable weights per media treatment were collected.  Results indicate that composted fish waste solids significantly increase the total harvestable weights.  The 50 and 75% fish waste solid medium incorporations significantly produced more total harvestable lettuce as compared to 0, 25 or 100%.  There was a drop in total harvestable weights from 75 to 100% fish waste solid media incorporation.  This composting system has great potential to provide a more environmentally sound and sustainable method of fish waste solids management with the added benefit of value-added use.