Small-scale aquaponics producers are often challenged by high fish feed costs. To offset this cost, it has been suggested that farmers reduce feed rates at the expense of plant production. However, a significant portion of the nutrients from feed are excreted by the fish and bound in solid waste. Mineralization is a process where aqueous microbes break down organic waste into dissolved nutrients that may be accessible to plants. Using mineralized waste in aquaponics may offset reductions in feed rates and allow farmers to maintain plant production while reducing costs.
This study was designed to explore the effects of added mineralized tilapia effluent on Bibb lettuce (Lactuca sativa var. capitate) growth at low feed rates in aquaponic systems. To test the effectiveness of mineralized aquaculture effluent on plant production, a 21-day research trial was conducted. Six 1500L aquaponic systems were stocked with 10 tilapia each (235g avg.) and fed at a rate of 15g of feed per square meter of growing area (2.7m2) per day. Three tanks were supplied with mineralized aquaculture effluent collected from the solid filters along with fish feed while the other 3 tanks were only provided with feed.
There were no significant differences found in water quality metrics or plant production. Nitrogenous compounds decreased over the study, indicating that feed rates of the fish were inadequate to keep up with the needs of the plants. While the mineralization process increased available nutrients in the effluent, the amount of mineralized effluent added back into the systems was not enough to increase average nutrient levels or plant production relative to the control systems. The low feed rate used in this study likely resulted in a low amount of solid waste generation, which even when mineralized did not generate a meaningful amount of nutrients in the short time frame of this study. Future research will examine the use of mineralization at higher feed rates and over long term.