The aquaponic principle is the coupling of animal aquaculture (e.g. fish) with plant production (e.g. vegetables) for saving resources. At present, various definitions of aquaponics exist, some bearing the risk of misinterpretation by dismissing the original meaning or being contradictory. In addition, there is no standard terminology for the aspects of coupling between the aquaponic subsystems. In this study, we addressed both issues. (1) We developed new or revised definitions that are summarised by: Aquaponic farming comprises aquaponics (which couples tank‐based animal aquaculture with hydroponics) and trans‐aquaponics, which extends aquaponics to tankless aquaculture as well as non‐hydroponics plant cultivation methods. Within our conceptual system, the term aquaponics corresponds to the definitions of FAO and EU. (2) A system analysis approach was utilised to explore different aquaponic setups aiming to better describe the way aquaponic subsystems are connected. We introduced the new terms ‘coupling type’ and ‘coupling degree’, where the former qualitatively characterises the water‐mediated connections of aquaponic subsystems. A system with on‐demand nutrient water supply for the independent operating plant cultivation is an ‘on‐demand coupled system’ and we propose to deprecate the counterintuitive term ‘decoupled system’ for this coupling type. The coupling degree comprises a set of parameters to quantitatively determine the coupling's efficiency of internal streams, for example, water and nutrients. This new framework forms a basis for improved communication, provides a uniform metric for comparing aquaponic facilities, and offers criteria for facility optimisation. In future system descriptions, it will simplify evaluation of the coupling's contribution to sustainability of aquaponics.

With the general objective of optimizing internal nutrient recycling, circular multitrophic food production systems, e.g., combining fish, plant, and insect larvae production, rely on the quality and composition of sustainable nutritional inputs. Therefore, differences in dissolved and solid nutrient excretion patterns produced by Nile tilapia (Oreochromis niloticus) reared in recirculating aquaculture systems (RAS) with 5% daily water exchange and fed black soldier fly meal (BSFM), poultry by-product meal (PM), poultry blood meal (PBM) and fish meal (FM) as single protein sources were investigated to evaluate the potential for creating specific fish meal-free diets. Fish fed the FM and PM diet showed the significantly best (p < 0.05) and among each other similar (p > 0.05) growth performance (specific growth rate (SGR): 2.12 ± 0.04/2.05 ± 0.11; feed conversion ratio (FCR): 0.86 ± 0.03/0.92 ± 0.01), whereas the PBM diet caused significantly reduced performance (SGR: 1.30 ± 0.02; FCR: 1.79 ± 0.05) in comparison to the FM/PM diet as well as the BSF diet (SGR: 1.76 ± 0.07; FCR: 1.11 ± 0.05). The FM and PM diet resulted in a faster increase and significantly higher dissolved nitrogen and phosphorus levels, while the BSF diet caused faster accumulation and significantly elevated levels of dissolved potassium, magnesium, and copper. The PBM diet resulted in the feces with the significantly highest nutrient density (gross energy, crude protein, and amino acids) but overall much lower dissolved nutrient levels in the water. Results are discussed with regard to implications for developing circular multitrophic food production systems.

The utilisation of insect meal-based fish feed as a substitute for conventional fish meal-based fish feed is considered as a promising innovative alternative to boost circularity in aquaculture and aquaponics. Basic research on its use in 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, were available on the growth performance of lettuce. Therefore, this study aimed to compare the effect of reusing fish waste water from tilapia culture (as a base for the nutrient solution) fed with a fish meal-based diet (FM) and a BSF meal-based diet on resource use and lettuce growth in decoupled aquaponic systems. A conventional hydroponics nutrient solution (HP) served as control, and inorganic fertilisers were added to all nutrient solutions to reach comparable target concentrations. The experiment was conducted in a controlled climate chamber in nine separate hydroponics units, three per treatment. Lettuce fresh and dry weight, number of leaves, relative leaf chlorophyll concentration, water consumption, and the usage of inorganic fertilisers were measured. Micro- and macronutrients in the nutrient solutions were monitored in time series. Similar lettuce yield was seen in all treatments, with no significant effects on fresh and dry weight, the number of leaves, and relative chlorophyll values. Water use per plant was also similar between treatments, while the amount of total inorganic fertiliser required was 32% lower in FM and BSF compared to HP. Higher sodium concentrations were found in the FM nutrient solutions compared to BSF and HP. The results confirm that BSF-based diet is a promising alternative to FM-based diet in aquaponics with no negative effects on lettuce growth. Additionally, BSF-based diet might be beneficial in intensive, professional aquaponics applications due to the lower sodium concentration in the nutrient solution.

In order to further close nutrient cycles of aquaponic systems, it could be possible to integrate a third trophic level in the form of insect larvae production (i.e., black soldier fly larvae) to recycle internal waste streams into valuable nutrients. This would present opportunities to formulate sustainable circular aquafeeds that combine these internally available nutrients with complementary external raw materials. The ingredient composition of feeds for such circular multitrophic food production systems (CMFS) may affect fish performance as well as excretion of important dissolved plant nutrients such as N, P and K. Hence, fish meal from catfish processing (CM) as base ingredient was combined with variable levels of poultry by-product meal (PM) and black soldier fly larvae meal (BSFM) into three marine-ingredient-free experimental diets corresponding to hypothetical production scenarios of a CMFS that aims to integrate aquaponics with insect larvae production. These experimental diets and a commercial diet (COM) were compared using isonitrogenous and isolipidic formulations. They were fed to African catfish (Clarias gariepinus) in recirculating aquaculture systems (RAS) and evaluated concerning growth performance and nutrient excretion. All diets resulted in similar total inorganic nitrogen (TIN) excretion, whereas the increase of dietary PM inclusion from 0% (BSF diet) to 20% (MIX diet) and to 41% (PM diet) and concomitant reduction of BSFM inclusion led to increasingly higher soluble reactive phosphorus (SRP) excretion per unit of feed compared to the COM diet. While the PM diet enabled the best growth and feed conversion performance, the MIX and especially the BSF diet produced more similar performance to the COM diet, which generated the highest dissolved K excretion. The MIX and the PM diet resulted in the highest Ca and P, yet lower N content in the fish feces. Results indicate that combining CM with elevated levels of PM in the diet of African catfish could improve growth performance and reduce the need for P fertilization in aquaponics when compared to industrial diets optimized for low environmental impact. Findings are discussed regarding their implications for CMFS and aquaponic feed formulation.

In aquaponics and circular multitrophic food production systems, dietary protein source, as well as fish species choice, particularly in cases of different nutritional physiology, could be factors affecting excreted nutrient profiles. Accordingly, growth performance, dissolved nutrient accumulation and feces nutrient profiles were evaluated for African catfish (Clarias gariepinus) reared in recirculating aquaculture systems (RAS) and fed single protein source diets based on black soldier fly larvae meal (BSF), poultry by-product meal (PM), poultry blood meal (PBM) and fish meal (FM) and the results were compared to previous findings for Nile tilapia (Oreochromis niloticus). All diets resulted in significantly different growth performances of African catfish, with FM producing the best growth performance, followed by PM, BSF and PBM. PM resulted in the highest soluble reactive phosphorus concentrations (SRP) in the RAS water; whereas, BSF resulted in the highest K, Mg and Cu concentrations. The highest feces nutrient density was recorded for PBM; whereas, FM and PM yielded the lowest feces nutrient density. Comparing African catfish to Nile tilapia revealed that the former showed significantly better growth performance with FM and PM, however, significantly weaker performance with BSF. Although dissolved K accumulation was similar between species across diets, significant differences were recorded for total inorganic nitrogen and SRP production per unit of feed for individual diets. Despite similar feces nutrient profiles, African catfish produce significantly less feces dry matter per unit of feed for each diet compared to Nile tilapia. Findings are discussed regarding their implications for aquafeed development in the context of circular multitrophic food production systems.

Societal Impact Statement: Refining circular multitrophic food production methods, which integrate plant, fish, and insect outputs, is imperative for environmental sustainability. Our findings suggest that the right protein choices in fish feed, like black soldier fly meal and poultry meal, can notably enhance the nutrient profile of fish waste water. This, in turn, is conducive for hydroponic cultivation, enhancing the nutritional attributes of plants like basil and lettuce. As we reduce environmental impact and optimize resource use, it is evident that our food ecosystems are deeply intertwined. Harnessing these synergies could redefine our approach to food production, paving the way for a more sustainable global future.Summary: Optimization of nutrient use efficiencies in circular multitrophic food production systems (i.e., plant, fish, and insect production) is crucial for sustainability. This study tested how protein ingredient choice in fish feed influences the plant nutritional value of the fish waste water when used for hydroponic crop production. Waste water samples were obtained from recirculating aquaculture systems (RAS) in which Nile tilapia (Oreochromis niloticus) were fed different single protein source diets—black soldier fly meal (BSFM), poultry by‐product meal (PM), poultry blood meal (PBM), and fish meal. Water was analyzed for plant nutrients and used for lettuce and basil cultivation to evaluate their suitability for hydroponic crop production—viz. yield, mineral nutrient, and selected secondary metabolite levels. BSFM RAS water had the highest concentrations of K, Mg, and micronutrients (Cu, Mn, Mo, Zn) of the RAS waters, whereas PM RAS water contained the highest P concentration and had a mean pH of 6.5 closer to the optimum pH for hydroponic plants. These RAS waters consequently lead to the highest yields in basil and lettuce indicating the importance of the aforementioned factors. From a plant production perspective, BSFM appears promising as a protein source in fish feeds for aquaponics. Usage of RAS waters for plant production helps reduce (i) environmental impact of RAS water and (ii) resource input in plant production. Longer term RAS trials should be conducted to determine the maximum nutrient concentrations achievable during fish production with diets including BSFM as the main protein source.