## 4 Discussion
### Fish and Plant Growth Performance  
The present study investigated aquaponics in brackish water using salt-tolerant native Mediterranean fish and plant resources (gilthead sea bream and rock samphire) with high commercial and nutritional value. Sea bream is included among the most important aquaculture species cultivated in the Mediterranean region. According to [PERSON] et al. [55], gilthead sea bream, as a natural eurylahine species, shows increased adaptability to salinity fluctuations allowing it to grow well at low salinities thus occupying an important role in brackish water aquaponic systems [7]. On the other hand, rock samphire, a natural halophyte of the Mediterranean region, is suggested herein as an appropriate species for brackish water aquaponic systems, especially at low salinities (8 ppt) [7]. According to the results of the present study, salinity affected both the specific growth rate (SGR) of gilthead sea bream and the growth performance of rock samphire. Moreover, the results show that gilthead sea bream in the brackish aquaponics system had a high SGR at salinities 20 ppt and 8 ppt (SGR8\({}_{\text{ppt}}\) = SGR20\({}_{\text{ppt}}\) = 3.6%/day) than 14 ppt, which was lower (SGR14\({}_{\text{ppt}}\) = 3.1%/d), high survival rate (S\({}_{\text{sptpt}}\) = 96%, S\({}_{\text{20 ppt}}\) = 90%, and S\({}_{\text{14 ppt}}\) = 97%), and high FCR. Previous studies [7] reported that gilthead sea bream cultured in a brackish aquaponic system with 20 ppt and 8 ppt salinities show SGR 3.1%/d for both salinities (8 ppt, 20 ppt) which is in agreement to the SGR values of the 14 ppt salinity treatment and lower than 20 ppt and 8 ppt treatments observed in the present study. Other studies [37] reported that 20 g gilthead sea bream cultured in a brackish aquaculture system for 100 days show better growth performance in 12 ppt (1.99%/d) compared to 28 ppt (1.91%/d) and 6 ppt (1.79%/d); however, these values are lower than the SGR findings reported in the present study. At 12 ppt salinity, eurylahine fish, such as gilthead sea bream, seem to operate close to the isotonic environment [37], and therefore the energy cost required to supply its osmoregulatory requirements would be lower compared to higher salinities (28 ppt) or lower salinities (6 ppt), thus saving energy to cover other physiological functions, such as growth [56]. The effect of salinity on growth rate can be used as a mediator between osmotic energy cost and ionic regulation by rectifying the energy required for fish growth [56]. In previous studies [57], it was reported that hormones, such as prolactin, cortisol, and the growth hormone contribute to the adaptation of the species to a hypoosmotic or hyperosmotic living environment. The findings of the present study show that no significant differences (ANOVA, \(p\) > 0.05) were detected in weight gain (WG) and body weight increase (BWI %), and these were lower compared to those reported previously in brackish aquaponic systems at 20 ppt and 8 ppt salinities [7]. At the end of the experiment, there were no significant differences in the survival rates of gilthead sea bream (90% for salinity 20 ppt, 97% for salinity 14 ppt, and 96% for salinity 8 ppt). Rock samphire showed significantly better growth performance in 8 ppt in terms of plant height, fresh weight of the aerial part, and height gain rate compared to 20 ppt and 14 ppt salinities. The results from the present study are in agreement with those reportedby [PERSON] et al. [7] and [PERSON] [46] and actually confirm its suitability for brackish aquaporics with controlled salinity levels. In cultivating plants, it is known that high salinities may significantly reduce leaf growth due to the inhibition of cell division induced [58]. Some studies indicate that some crops, such as _Fragaria_ spp. and _Lactuca sativa_, can be grown in brackish aquaporics systems at 1.2 ppt salinity showing fairly good growth [44]. However, natural halophytes can survive and reproduce in salt concentrations around 200 mmol/L NaCl or even higher (>12 ppt) [36; 59].