Aquaponics (AP) and biofloc technology (BFT) systems rely heavily on bacterial communities to break down organic matter and cycle nutrients that are essential for fish and plant growth. The functional roles of bacterial communities in aquaculture systems are critical to their sustainable operation. Currently, the research on the combination of BFT and AP systems called FLOCponics (FP) is lacking, thereby hindering our ability to optimize their performance. Here, several characteristics (productivity of fish and crops, physicochemical properties of water, and bacterial community) in FP systems cultivating Japanese eel (Anguilla japonica) and leaf lettuce Caipira (Lactuca sativa) were compared to those in the BFT system. Additionally, the effect of fish density on the FP system was investigated. The results indicated that the FP system was more productive than the BFT system. Fish growth rate was highest in the FP system (52.6%), and the average body weight of eels was 168.2 ± 26.8 g in the FP system compared to 140.3 ± 27.0 g in the control (BFT, 5 kg/m²). However, increasing fish density resulted in lower growth rates, with a growth rate of 20.6% observed in the high-density (20 kg/m²) experimental group. The bacterial composition was also significantly different between the systems and fish densities, suggesting that bacterial communities may be closely related to the performance of the aquaponics system.

The Japanese eel (Anguilla japonica) is a valuable aquaculture species in East Asia. However, sex differentiation under culture conditions is often male-biased, complicating the production of female broodstock. This study evaluated the effects of dietary soy isoflavone (SI) supplementation on growth performance, sex differentiation, and sex-specific gene expression in juvenile eels. Juvenile eels (initial mean weight: 1.25 ± 0.02 g) were randomly assigned to 15 tanks, each containing 100 g eels, in a 30-week feeding trial. The control diet (SI0) contained 75% jack mackerel meal. The graded levels (2.5, 5, 7.5, and 10%) of α-Starch were included at the expense of SI, referred to as SI0, SI2.5, SI5, SI7.5, and SI10 diets, respectively. Fish were hand-fed to satiation in triplicate groups of each diet, twice daily for 30 weeks. The SI2.5 and SI5 groups exhibited significantly enhanced final weights and specific growth rates compared to those in the control group (SI0). Histological analysis revealed the presence of differentiating gonads with oogonial proliferation and previtellogenesis in the SI-treated groups, particularly at the 2.5% and 5% inclusion levels. Additionally, the expression of key female-related genes (vasa, cyp19a1a, foxl2a, zp3, and zar1) was significantly upregulated in these groups. In contrast, relatively high SI levels (7.5% and 10%) did not provide any further benefits. These results suggest that low-dose supplementation of dietary SI can enhance feminization and promote growth in juvenile eels, thus offering a potential natural alternative to synthetic hormone treatment for broodstock development.

In this study, we sought to improve the productivity of Far Eastern catfish (Silurus asotus) and tropical eel (Anguilla bicolor), which are high-value fish species in the Republic of Korea, as well as that of associated crops by applying biofloc technology (BFT)-based aquaponics systems. The following three systems were used: the flow-through system (FTS), BFT, and BFT aquaponics system (BAPs). Caipira lettuce (Lactuca sativa) was utilized and hydroponics (HP) was implemented to compare crop productivity. After 42 days of treatment, the BAPs and BFT systems improved fish productivity, with weight gain rates of 134.47 ± 1.80% in BAPs-cat, 130.38 ± 0.95% in BFT, and 114.21 ± 6.62% in FTS for S. asotus, and 70.61 ± 3.26% in BAPs-eel, 62.37 ± 7.04% in BFT, and 47.83 ± 1.09% in FTS for A. bicolor. During the experiment, the total ammonia nitrogen and NO₂⁻-N concentrations were stable in all plots. In the case of NO₃⁻-N, BFT showed an increasing tendency while both BAPs showed a decrease compared with that of the BFT. BAPs-cat (total weight: 224.1 ± 6.37 g) and HP (220.3 ± 7.17 g) resulted in similar growth. However, in BAPs-eel was 187.7 ± 3.46 g due to root degradation. Water content analysis showed that BAPs-cat and BAPs-eel contained sufficient K, Ca, P, and S, which are important for crop growth. Overall, the effect of BAPs on fish growth was higher than that of FTS. This study reveals that integrating BFT with aquaponics improves productivity for high-value fish and associated crops while maintaining stable water quality. This method offers sustainable, efficient production, reduces environmental impact, and provides insights for future research in sustainable aquaculture practices.

Sustainable aquaculture requires innovative systems that enhance production efficiency while minimizing environmental impact. Among emerging technologies, biofloc technology (BFT) and recirculating aquaculture Systems (RAS) are widely adopted for intensive fish culture; however, their integration with aquaponics remains underexplored. This study aimed to compare the growth performance and water quality in aquaponic (AP) systems combining Japanese eel (A. japonica) and caipira lettuce (L. sativa) using BFT and RAS technologies for 28 days. This study compared four aquaculture configurations: BFT, RAS, BFT-AP (BFT with aquaponics), RAS-AP (RAS with aquaponics). Results showed that aquaponics integration seems to be improve the fish growth and apparent feed efficiency. Eels reared in BFT-AP achieved the highest final weight, weight gain, specific growth rate and apparent feed efficiency, which were relatively greater than in BFT, RAS, or RAS-AP systems. The growth of caipira lettuce was markedly enhanced in BFT-AP, with total biomass nearly four times that of RAS-AP. Electrical conductivity and total dissolved solids were relatively higher in BFT-AP than in RAS-AP, indicating nutrient enrichment beneficial for plants. In conclusion, integrating aquaponics with BFT substantially enhanced both eel and production of caipira lettuce. The BFT-AP configuration represents a more efficient and ecologically balanced model for sustainable aquaculture intensification compared with traditional RASs.

17α-methyltestosterone (MT) is a synthetic steroid used to induce masculinization when administered during the larval stage of fish. However, the side effects of MT on eel are still poorly understood and, in this study, we examined the various effects of MT on juvenile eel A. japonica (100.63 ± 8.56 mm total length (TL)). To further investigate growth and sex differentiation, juvenile eels (n = 1000) were exposed to 25 µg/g MT for 6 months. We analyzed growth-related factors, sex steroid hormones, skin pigmentation, and color-related gene expression. Through this study, we found a 90% sex conversion of juvenile eels to males using MT treatment. In the MT-treated eel group (285.97 ± 26.21 mm TL) where sexual maturity was induced, spermatogonia stages were observed in the gonads. In contrast, the control group (395.97 ± 27.72 mm TL) exhibited an 80% immaturity rate, with only 20% of the subjects that were rapidly developing displaying early oogonia. ELISA analysis results showed that the level of growth hormone, which is known to be secreted from spermatogonia, did not change as a result of MT treatment. We confirmed that MT delayed growth and caused morphological changes, particularly a shortened snout length and pigmentation of the fin. The total length, body weight, and snout length were considerably lower in the experimental group than in the control group. In addition, in histological analysis we also observed that some of the MT-treated group (5 out of 10 fish) showed liver atrophy and inflammation, and physiological analysis showed that the cortisol concentration increased in the MT-treated eels. Interestingly, we found that some pigment color-related genes, such as melanocortin 1 receptor (MC1R), tyrosinase (Tyr), and dopachrome tautomerase (DCT), were significantly overexpressed in the fins of MT-treated eels. These results suggest that the treatment of A. japonica larvae with MT induced masculinization but also causes growth side effects from the use of synthetic hormones.

FLOCponics systems combine biofloc-based aquaculture with nutrient recycling and are considered sustainable for intensive aquaculture. However, fungal community dynamics in these systems and their links to water quality remain poorly understood. In this study, we examined temporal changes in fungal communities in a FLOCponics system-based Japanese eel aquaculture by integrating metabarcoding with weekly water quality monitoring. Electrical conductivity (EC), nitrite, and pH displayed significant temporal variation, whereas other water quality parameters remained stable. Metabarcoding yielded 217 fungal operational taxonomic units across all samples. Alpha diversity indices exhibited no significant differences among weeks, indicating stable richness and evenness. In contrast, beta diversity analyses revealed consistent temporal shifts in fungal community composition with time exerting a significant effect after accounting for tank identity. Community changes were significantly EC-associated, with associations to nitrite and nitrate. Variation partitioning indicated that time and water quality primarily explained variation through their overlapping contribution rather than independent effects. Beta diversity decomposition revealed that temporal differences were driven primarily by species replacement rather than nestedness. Environmentally responsive taxa were mainly composed of Rozellomycota, with fewer Ascomycota, including Metapochonia. These results demonstrate structured short-term temporal fungal community reassembly in FLOCponics systems and highlight fungal turnover as a potential indicator of water quality-linked microbial dynamics in sustainable aquaculture.