Introduction
The utilization of insect meal-based fish feed as substitute to conventional fish meal-based feed is considered as a promising innovative alternative to boost circularity in aquaculture and aquaponics (Shaw et al. 2022a). However, basic research on its use in aquaponics and particularly in decoupled aquaponics is limited. So far, no reports on the effects of fish waste water, derived from a recirculating aquaculture system using Black-Soldier-Fly (BSF) meal-based diets, on the growth performance of lettuce were available. Therefore, this study aimed to compare the effect of reusing fish waste water (as a base for the hydroponic nutrient solution) from tilapia culture fed with a fish meal-based diet (FM) and a black soldier fly meal-based diet (BSF) on lettuce growth and water nutrient profile in decoupled aquaponic systems.
Material & methods
The experiment was conducted in a controlled climate chamber in nine separate hydroponics units. A conventional hydroponics nutrient solution (HP, control) was compared to two different aquaponic treatments (FM & BSF), and inorganic fertilizers were added to all groups to reach comparable target concentrations. All treatment groups were tested in triplicates. Lettuce fresh and dry weight, number of leaves, SPAD values, water consumption, and the usage of inorganic fertilizers were measured. Abiotic parameters of the nutrient solutions, environmental conditions, and micro- and macronutrients in the nutrient solutions were monitored in time series.
Results
Similar lettuce yield was seen in all treatments, with no significant effects on fresh and dry weight, the number of leaves, and SPAD values. Water use per plant was also similar between treatments, while the amount of total inorganic fertilizer required for preparing the nutrient solutions was 32% lower in FM and BSF compared to HP (Fig. 1). For the water nutrient profile analysis, higher sodium concentrations were found in the FM-based nutrient solutions compared to BSF and HP.
Discussion
The potential of using BSF-based feeds aquaponics for a combined production of fish and plants in a decoupled aquaponic approach was clearly shown in this study, as it was not negatively affecting lettuce growth. Nevertheless, also fish growth performance needs to be considered to evaluate the overall sustainability, since the use of BSF-meal-based diets might have a negative effect on the growth of Nile tilapia compared to a fish meal-based diet (Shaw et al. 2022b). In addition, BSF-based diets might be beneficial over FM-based diets in intensive, professional aquaponics applications due to the lower sodium concentration in the nutrient solution. This is especially important if the nutrient solution is recirculated in the hydroponics unit, as higher sodium concentrations might negatively influence plant growth (Hernández-Salinas et al.2022).
The results confirm that BSF as ingredient for fish diets is a promising alternative to FM in aquaponics without any adverse effects on lettuce growth, but more research is needed to evaluate the overall applicability.
References
Shaw, C., Knopf, K., Kloas, W. (2022a). Fish Feeds in Aquaponics and Beyond: A Novel Concept to Evaluate Protein Sources in Diets for Circular Multitrophic Food Production Systems. Sustainability, 14, 4064. https://doi.org/10.3390/su14074064.
Shaw, C., Knopf, K., Kloas, W. (2022b). Toward Feeds for Circular Multitrophic Food Production Systems: Holistically Evaluating Growth Performance and Nutrient Excretion of African Catfish Fed Fish Meal-Free Diets in Comparison to Nile Tilapia. Sustainability, 14, 14252. https://doi.org/10.3390/su142114252.
Hernández-Salinas, M., Valdez-Aguilar, L. A., Alia-Tejacal, I., Alvarado-Camarillo, D., & Cartmill, A. D. (2022). Silicon enhances the tolerance to moderate NaCl-salinity in tomato grown in a hydroponic recirculating system. Journal of Plant Nutrition, 45(3), 413-425.