The effects of semi-intensive (100 kg m⁻³), intensive (200 kg m⁻³), and super-intensive (400 kg m⁻³) stocking densities on the growth and welfare of African catfish (Clarias gariepinus) were investigated under commercial production conditions. Plasma cortisol, glucose, and selected transcripts following a stress challenge, lactate, as well as skin lesions, were analyzed at regular intervals (from 12 g juveniles to 1.5–2.0 kg). The fish grew well, but after 23 weeks, the semi-intensively stocked fish had a mean final weight of 1830.5 g, significantly higher than the super-intensively stocked fish with 1615.4 g, and considerably higher than the intensively stocked fish with 1664.8 g (p > 0.05). Cortisol and glucose responses significantly differed between stressed and unstressed fish, but not between treatment groups. An unforeseen external stressor (nearby demolition noise) caused stress responses among all treatment groups, but was similarly coped with. Mortality ranged between 3.8–9.2%. In the juveniles, skin lesions were reduced under intensive or super-intensive densities, with the least under semi-intensive densities in outgrown fish. Expression profiles of 22 genes were compared in the spleen at semi-intensive and super-intensive densities. The transcript concentrations of most genes remained unchanged, except for slc39a8 and mtf1, which were significantly downregulated in stressed catfish under semi-intensive conditions. We demonstrated that African catfish growth performance and welfare depend on age and stocking density, also reacting to demolition noise. This supports farm management to optimize stocking densities during the grow-out of African catfish in RAS and suggests avoiding external stress.

Fingerlings (0.23 g) and juveniles (267.04 g) of African catfish (Clarias gariepinus) were reared for 32 days under experimental aquarium conditions and were exposed to either 0.75 mg/L or 3.0 mg/L diethylenetriaminepentaacetic acid-iron(II) (Fe-DTPA) and 3.0 mg/L or 12.0 mg/L Fe-DTPA in the water, respectively. These treatment groups were compared to a control group without additional Fe-DTPA. The growth, mortality, ethological indicators (activity, agonistic interactions, air-breathing), leukocyte distribution, histopathological changes in liver and gills, and genetic biomarkers were evaluated for each group. While the growth, mortality, and behavior were not significantly different between the groups, the lymphocyte count in the fish’s blood increased significantly in all groups during the course of the experiment, but independently from the treatments. A similar trend (p > 0.05) was observed in monocytes. The number of granulocytes decreased significantly, but independently from the treatments. These changes indicated the possibility of an ongoing immune response in the fish from all treatments that might be caused by the increasing aggressive behavior of the fish. However, the Fe-DTPA treatments did not cause a notable suppression or enhancement of the immune reactions. Fe³⁺ accumulations in liver tissues were detected at the tested concentrations, and further changes occurred in the cells of the gills. Gene-expression biochips were used to simultaneously quantify the transcript levels of 34 genes associated with iron metabolism and stress physiology in head kidney samples. The obtained gene-expression profiles did not reveal any significant differences across either the different treatments or the time points. The results indicate that Fe-DTPA supplementation in the tested concentrations can be considered relatively harmless for the health and welfare of African catfish.

In this study, the growth and welfare of the African catfish (Clarias gariepinus, Burchell 1822) were investigated under industrial farming conditions. For this purpose, the growing success (cm, g) and typical stress related parameters (glucose-, lactate-, cortisol-concentrations, growth hormone, HSI-liver index) were investigated on the African catfish (102–841 g) in relation to an external stressor (working light and noise) and different feeding regimes (day, night, and day and night feeding) over 83 days. As no significant effects were found among the experimental feeding treatments in relation to the growth performance and investigated stress parameters, the time of feeding seems to have less impact to the production success and stress reactions as suggested before. Regarding our results, the effect of feeding conditioning could have played a strong factor likewise the ageing process of the reared fish species which is known to be rather photophobic. Therefore, the factor of conditioning and its influence to the time shift in feeding regimes and the impact of noise and light stressors during feeding should be investigated separately in future experiments to obtain further results in this context and clarify the validity of the best feeding conditions for African catfish.

As nutrient-rich water in aquaponic systems cannot supply growing plants with all the required trace elements, supplementation with specific fertilizers is performed to make up for this deficit. While chelated fertilizers such as ethylenediaminetetraacetic acid–zinc disodium complex (Zn-EDTA) are becoming more popular in this context for improving plant growth in aquaponic systems, little is known about their effects on fish. During two experiments, a total of 576 individuals of catfish fry (0.19 g) and fingerlings (220.01 g) of the African catfish (Clarias gariepinus; Burchell, 1822) were kept separately for 32 days under experimental aquarium conditions. The fry was exposed to 0.125 and 0.5 mg/L, while the fingerlings were exposed to 0.5 and 2.0 mg/L Zn-EDTA in a plantless aquaponic system. The third treatment group consisted of a control group without Zn-EDTA. The growth, mortality, and ethological indicators were assessed for all growth stages, while the leukocyte distribution and histopathological changes were additionally determined for the fingerlings. As the feed intake in the experiment was limited, the investigations were focused on the effects of Zn-EDTA and not on the growth process of a respective fish growth stage. While the growth, mortality, and behavior were not significantly different in both growing stages, the number of mature neutrophils changed significantly in all treatments in fingerlings. Zn was not detected in the histologically investigated organs at the tested concentrations using the staining method. However, morphological alterations of the gill epithelium were found on the secondary lamellae. Quantitative multiplex PCR was used to simultaneously evaluate the expression of 17 genes related to Zn metabolism and stress physiology in head kidney samples. The transcripts of several selected genes changed by up to 70-fold. Due to high individual variances, only the copy numbers of the KMT2A (lysine-specific methyltransferase 2a) gene were significantly different across treatment groups and sampling points. However, the present results indicate that the addition of Zn-EDTA at the tested concentrations can be considered relatively benign for the health and welfare of C. gariepinus, as no toxic effects of Zn-EDTA were observed in moderately hard to hard water.