# Mediterranean Aquaponics: Fasting and Refeeding in a Polyculture Aquaponic System
###### Abstract  
The use of Mediterranean euraluralyhaline fish and halophytes in aquaponics presents a sustainable and alternative approach to food production. The present study investigates the effect of compensatory growth on sea bass (_Dicentrurchus labtrax_) and Baltic prawn (_Palamon adserus_) co-cultivated with the halophytic glasswort (_Salicorina europaea_). Three autonomous systems were established, each containing forty-five sea bass, nine Baltic prawns, and eight glasswort plants, with different feeding regimes for each treatment: (i) daily feeding (treatment A), (ii) three days of feeding per week followed by four days of fasting (treatment B), and (iii) feeding for seven days followed by seven days of fasting (treatment C). The growth performance of the fish was significantly higher in treatment B. Conversely, the feed conversion ratio (FCR) was notably higher in treatment A. As for the prawns, their final body weight and length were similar across all treatments. The glasswort plants also demonstrated significantly improved growth in treatment B. These results indicate that the incorporation of feeding and fasting cycles can be an effective feed management strategy for polyculture aquaponic systems. Additionally, food deprivation had a positive impact on the growth performance of both glasswort and prawns. RAS system; multirophic; compensatory growth; glasswort; sea bass; Baltic prawn  
So far, aquaponics systems are usually used for freshwater cultivation, but they can also be designed for marine fish or prawn species [8; 9]. In this case, the use of euryahline species is proposed, as they can adapt to a wide range of salinities [10], allowing the cultivation of a greater number of plants that would otherwise be limited [11]. In this respect, salt-tolerant halophytes are promising candidates for saltwater aquaponics [10]. Glasswort (_Salicornia europaea_, [PERSON] 1753) is a succulent, annual, halophytic plant that exhibits a cylindrical growth pattern with regular branching [12]. The plant is devoid of leaves; the internodes comprise a photosynthetic outer cortex encasing a central pit. At the conclusion of the growing season, each green internode produces six flowers in two cyme clusters of three each, with each flower setting one seed [13]. The Baltic prawn (_Palaemon adspersus_, [PERSON], 1837) is a species that is widely distributed in protected brackish waters. It can be found in bays, lagoons, and estuaries in the eastern Atlantic, Baltic, Mediterranean, Black Sea, Caspian, and Aral Sea, as well as in Atlantic Canada [14]. Baltic prawns are utilized as live food for aquaculture [15], though they are predominantly employed as live bait for commercial, artisanal, and sport fisheries [16]. The commercial cultivation of these shrimp could classify them as one of the most promising species for Greek aquaculture. World aquaculture production reached a new record of 130.9 million tons in 2022, with Europe contributing 3.53 million tons [17]. Sea bass (_Dicentruchus lubrax_, [PERSON], 1758) is the most important fish species farmed in the Mediterranean. In Greece, the production of sea bass in 2021 was 54,000 tons [18]. Sea bass are optimal for brackish water aquaponics as they are euryahline species and can tolerate a wide range of salinities [5; 7; 19]. Compensatory growth is an innovative method of reducing the cost of feed and maintaining the growth of farmed organisms [20]. In rearing experiments using the compensation growth technique, greater growth has been observed in fish that were deprived of food compared to fish that were fed daily [21]. With total or partial food deprivation, excessive lipid accumulation by fish is reduced [22]. This leads to increased growth rates and the optimization of food consumption by organisms [23; 24]. This work is significant in that it demonstrates the beneficial impact of growth compensation on the cultivation of two or more organisms in closed aquaponics systems.