## 4 Discussion
On the other hand, [PERSON] et al. [19] analyzed the effect of different stocking densities (20, 40, 60, and 80 org m\({}^{-3}\)) and reported 100% survivals at densities of 80 org/m\({}^{3}\), different from that found in the present investigation. This difference can be attributed to the duration of their study, which had a duration of 60 days, and the present one which had a duration of 157 days, with the negative effects originating from the chronic stress of overcrowding at high stocking densities [32]. Despite the above, and considering the existing information, the survival obtained is acceptable for all fish culture systems, suggesting the possibility of maintaining good fish health in the aquaponic system tested. The blood parameters evaluated in this study (Table 3) provide valuable information on the health status of the fish and the physiological response of _D. latifrons_ to different stocking densities. Blood glucose levels were outside the reference range for _D. latifrons_ reported by [PERSON] et al. [33] (34.63-68.30 mg dL\({}^{-1}\)) with the highest glucose levels at the highest stocking density (T3, 70.6 \(\pm\) 23.3 mg dL\({}^{-1}\)) and the lowest (T1, 69.9 \(\pm\) 12.9 mg dL\({}^{-1}\)). This indicates that an intermediate stocking density may be less stressful for this species. Blood glucose levels are one of the most commonly used parameters to measure stress response [34]. Increased blood glucose levels with increasing stocking density were reported in channel catfish _Ictalurus punctatus_[21], rainbow trout _Oncorhynchus mykiss_[35], tilapia _Oreochromis niloticus_[23], and koi carp _Cyprinus carpio_ var. _Koi_[25]. The significant increase in globulins in the T1 treatment (2.2 \(\pm\) 0.7 g dL\({}^{-1}\)) and the A/G ratio generated at the highest stocking density could indicate a better immune response at low stocking densities [36; 37]. The measurement of blood globulin level is important because it is an indicator of fish health [38] and plays a key role in the immune response [37], mainly in stressful conditions such as high stocking density, the presence of pathogens, feeding interruptions, and environmental factors [39]. A decrease in globulin level in the higher stocking density treatment may indicate compromised immunity [37]. Previously, a decrease in globulin level with increasing stocking density was reported in major carp _Labco rolita_[37], Gangetic mystus _Mystus cavasius_[40], and _O. niloticus_[41]. On the other hand, the significant decrease in triglycerides in the T3 treatment (250.8 \(\pm\) 36.6 g dL\({}^{-1}\)), could be related to a higher mobilization of energy reserves in response to the chronic stress of high culture densities. Referring to this, some researchers comment that high culture densities may activate triglyceride mobilization to cope with increased energy demand, because triglycerides are another energy substrate used to carry out the compensatory response to stress [22; 42]. A decrease in blood triglyceride levels at high stocking densities was reported in beluga _Huso huso_[43], _O. niloticus_[22; 23], and _O. mykiss_[35]. However, it was reported that culture density does not affect triglyceride levels in senegalese sole _Solea senegalensis_[44] or, contrary to the present work, [PERSON] et al. [40] reported higher triglyceride concentration in _M. cavasius_ at high stocking densities. These differences in results could be due to the fish species and their differences in physiological responses, as well as differences in experimental conditions and duration. In addition, regarding the parameters that relate to the immune system, it is important to take into account that they may occur at different times and intensities [45]. In our study, cortisol levels show an increasing trend as culture density increases, with values of 51.9 \(\pm\) 20.2 ng mL\({}^{-1}\) at T1 (10 org m\({}^{-3}\)), 64.5 \(\pm\) 14.7 ng mL\({}^{-1}\) at T2 (30 org m\({}^{-3}\)), and 71.9 \(\pm\) 35.4 ng mL\({}^{-1}\) at T3 (50 org m\({}^{-3}\)).