Tropical Shrimp Biofloc Aquaculture within Greenhouses in the Mediterranean: Preconditions, Perspectives, and a Prototype Description

Abstract

Biofloc technology (BFT) offers an innovative eco-friendly approach that is particularly applicable in shrimp farming. Penaeus vannamei is the most important seafood species in terms of global economic value. Nevertheless, its increasing global demand highlights the necessity for sustainable production of P. vannamei shrimps outside their native range, assuring the avoidance of genetic pollution risk. Towards this direction, the present study focuses on the feasibility of tropical shrimp species aquaculture in indoor systems evaluating BFT application in temperate zones. The achievability of P. vannamei cultivation inside greenhouses in temperate latitudes is thoroughly examined and a representative experimental biofloc setup for P. vannamei within a greenhouse in Northern Greece is demonstrated. Nevertheless, there are two major limitations, related to economy and ecology, namely the energy demand for high seawater temperature and the fact that most reared shrimps are non-indigenous species setting risk for genetic pollution, respectively. To overcome the former, energy-saving measures such as tank and greenhouse insulation in combination with a microclimate chamber construction were implemented to optimize water temperature at minimal cost. Concerning the latter, there is clear evidence that P. vannamei populations cannot be established in the Mediterranean, setting aside any environmental risk. Overall, based on the developed and tested pilot prototype, employment of optimal management practices, innovative manufacturing and clean energy alternatives, and the utilization of ecosystem services could reduce the environmental impact and maximize the profitability of biofloc operations. These actions could probably permit sustainable and economically viable farming of P. vannamei employing BFT within greenhouses in the Mediterranean.

1. Introduction

Global food production is challenging to expand sustainably in order to meet the increasing demands of the growing human population [1]. Following the increasing significance of accessible protein sources, shrimp has become an economical and vital seafood product that corresponds to approximately 15% of the total value of the world’s traded seafood products [2]. Aquaculture stands out as the fastest-growing food production sector globally [3]. Apart from preventing the overexploitation of natural fishery stocks, aquaculture also possesses the most significant capability to meet the escalating global food demand [4].

Meeting these demands entails a heavy reliance on natural resources such as land, water, and nutrients, which are presently being utilized unsustainably by modern agricultural practices [5]. As a result, benefits arising from the intensification and growth of aquaculture are compensated by environmental concerns regarding energy consumption, carbon emission, habitat destruction, loss of biodiversity, and water pollution [6,7,8,9,10] together with social concerns concerning food safety [11] and animal welfare [12], leading to the need to address these issues within the concept of sustainable aquaculture development [13].

Biofloc technology (BFT) has been developed as a promising aquaculture method for sustainable food production that could align with the Food and Agriculture Organisation’s (FAO) Sustainable Development Goals (SDGs) associated with food security [14]. The sustainable approach of BFT relies on the minimum water exchange and on the microbial communities, which assimilate nitrogen compounds, generating in situ microbial protein [15]. The accumulation of nitrogenous compounds is usually controlled by conducting partial water exchanges. However, this procedure can promote eutrophication of adjacent areas and facilitate pathogen introduction into the aquaculture facilities [16]. Biofloc is applied in closed systems and thus ensures a controlled release of effluents to the neighboring aquatic ecosystems [17]. Nitrogenous waste in BFT systems remain at safe levels for long periods even under zero water replacements, possibly due to their conversion into microbial biomass through heterotrophic nitrification [18]. Since water renewals are unnecessary, the water footprint in BFT systems can be ten times lower than in water exchange systems [19]. The great reduction in water demand supported by BFT favors both the production and the environment, also decreasing the risk of pathogen introduction into the culture alongside improvements in the quality of effluents [20].

Biological flocs (bioflocs) are aggregates consisting of microalgae, fungi, zooplankton, bacteria, and particulate organic matter exchanging chemical pollutants for nitrogen compounds. These aggregates have a great nutritional profile [14]. Since shrimps consume bioflocs as natural food, their protein requirements when reared in BFT systems can be remarkably reduced, limiting the reliance on high-protein commercial feed and, thus, reducing the production expenses [21]. Flocs are formed inside the culture units and contribute to shrimps’ growth and health. The development of a diverse and stable microbial load inside a biofloc system is crucial in order to enable the systematic decomposition of chemical pollutants and the formation of high-quality flocs [22]. Avnimelech [23] found that farming the Pacific white leg shrimp Penaeus vannamei in biofloc can yield a 30% reduction in the operating costs due to the decreased amount of feeds. Dorothy et al. [24] reported faster growth and higher tissue protein content of P. vannamei due to the additional protein generated from recycled waste within a zero-water-exchange BFT culture system. Moreover, ex situ biofloc supplements decreased the levels of nitrogen compounds and phosphate in the water column and increased the growth performance and the nutritional value of P. vannamei [25]. Similarly to P. vannamei, biofloc was found to be an important food source for species such as the shrimp Litopenaeus stylirostris [26] and the freshwater prawn Macrobrachium rosenbergii [19,27].

During the last 15 years, special notice has been given to BFT as a promising option for sustainable shrimp farming. Until 2020, over 550 scientific documents were published regarding the application of BFT. A great proportion of these documents are focused on shrimp, and most of the attention was paid to P. vannamei [14]. The shrimp P. vannamei’s production has expanded rapidly in recent years and exhibited a 53% increase between 2015 and 2020, being accountable for 51.7% of global shrimp production [4]. This species has several appealing characteristics such as the ability to tolerate a high range of salinities, great survival even in high densities, and rapid growth. The best growth of P. vannamei is achieved at seawater temperatures of 26–32 °C [28]. Consequently, farming of this species mostly takes place in tropical regions around the globe. Low temperatures occurring in subtropical or temperate climates can strongly affect the culture viability, minimizing the shrimps’ growth and causing mortalities during the colder months [29].

2. Penaeus vannamei in the European Market—Shrimp Aquaculture in the EU

The tendency regarding seafood consumption and fishery product preferences relies on regional factors and traditions [30,31]. A great percentage of white leg shrimp that is imported to Europe is consumed in Southern European countries. In Turkey, marine food consumption in regions located in the Mediterranean is greater than the country’s average per capita consumption [32], while recent research pointed out that seafood consumption is still prevailing in coastal regions of Turkey [33]. Portuguese people are among the greatest seafood consumers globally, and shrimp is among the most preferred items in Portugal [34]. In Belgium, the Netherlands, and Denmark, only 25% of the consumers choose seafood at least twice a week, while in Spain, this number was found to be 75% by Pieniak et al. [35].

The European market has always been more peculiar in comparison with USA and Japanese markets. Europe is more concerned with a wide range of issues including sustainable farming, traceability, antibiotics, and heavy metals. Such concerns have led to restrictions on trading farmed shrimp from many Asian countries as well as from Ecuador [36]. Ecolabels, such as Aquaculture Stewardship Council (ASC) certification, which indicate that a seafood product originates from sustainable practices, are increasingly common. From 2014, when the ASC implemented the standards for shrimp, the number of ASC-certified shrimp products increased from 500 products in 2014 to 7500 in 2018. It is more than clear that sustainability certificates will become a market entry necessity in Europe.

In Europe, only small volumes of Pacific white shrimp are produced, so most P. vannamei is imported. The greatest European importers of Pacific white shrimp are Spain, France, the United Kingdom, Italy, the Netherlands, Germany, and Belgium, representing almost 80% of the total European P. vannamei imports. Throughout Europe, Lofstedt et al. [37] found that less than half of the 31 examined countries produced national seafood supplies that meet the dietary recommendations for marine product consumption. Moreover, countries with coastlines and traditional fish-eating cultures (France and countries of Northern Europe) were found to have sufficient supplies, in contrast to landlocked countries of Central and Eastern Europe.

Fishery products are an integral part of the Mediterranean diet. Concerning Greece, most of the shrimp production comes from fisheries and imports. P. vannamei is imported in Greece, mostly from India and Ecuador [38]. Several attempts have been made to farm shrimp in Greece, yet there is still no commercial cultivation unit according to the annual aquaculture report for 2023 by the HAPO (Hellenic Aquaculture Producers Organization). Considering the dominance of tropical species in global shrimp aquaculture, an important causative factor is the local climate conditions. The Mediterranean climate possibly allows the production of P. vannamei for a five-to-seventh-month period; however, pond temperatures in January/February in most regions of the North Mediterranean might be as low as 7–8 °C [39]. Such temperatures hinder the development of commercial tropical shrimp farming by restricting the production to one harvest per year and therefore making the production costs unaffordable. The development of indoor aquaculture systems could enable shrimp productivity throughout the year and reduce environmental impacts, simultaneously providing a level of biosecurity that cannot be achieved in open ponds [40].

3. Indoor Aquaculture Systems: Advantages, Disadvantages, Innovations, and Economic Viability in the Mediterranean

Indoor aquaculture systems are highly capital-intensive; de Almeida et al. [41], investigating the economic viability of greenhouse biofloc systems in Brazil, proposed feed (57.54%), salaries and charges (17.06%), and electric energy (10.7%) as the highest cost factors. The energy demands for artificial aeration are many times higher in BFT systems compared to ponds and RAS systems [42], as sufficient mixing and oxygenation of water are important for biofloc build-up, ammonia nitrification, and the promotion of microbial growth [23]. Nevertheless, renewable energy sources for aeration systems could be used [43]. Temperature has a vital influence on the growth performance of penaeid shrimps [44]. P.vannamei requires temperatures between 28 and 32 °C for optimal growth. Such temperatures throughout the year require very high energy expenditure in greenhouses when insulation is low [45]. Energy for water heating can be significantly reduced, either by treatment of the effluents in retention ponds and their subsequent recycling back to the tanks or by controlling the levels of total ammonia nitrogen by promoting the heterotrophic bacteria of the culture system. In the case of zero water exchange, a biofloc system could significantly increase the technical efficiency (the ability to gain a maximum output from a given set of inputs) of a farm [46]. Short ceiling constructions can also reduce the energy costs of an indoor aquaculture system [47], as can an investment in effective insulation.

Indoor systems should efficiently use all the available space of the facility to take advantage of economies of scale wherever possible. P. vannamei aquaculture appears to generate more profit when large-scale production is coupled with intensive operation [48,49]. This can be achieved either by increasing the carrying capacities of the system, which maximizes annual production, with investments in filtration and aeration systems, or by employing a multi-phase production strategy, which decreases the amount of time that shrimps are raised at low densities. Both aforementioned strategies appear to produce cost-effective margins in shrimp production [47]. In addition, resource use (land, water, energy, wild fish) per metric ton of shrimp seems to decline when production intensity is higher [50].

By favoring the growth of heterotrophic bacteria in BFT systems (e.g., through adding a carbon source), the water quality is maintained, allowing high densities inside the culture unit [51]. Increasing shrimp density in super-intensive biofloc systems boosts profit and reduces land exploitation. Furthermore, limited water exchange decreases eutrophication of the adjacent shores and prevents disease transmittance between farmed and wild animals [52]. On the other hand, high densities require excessive amounts of feeds which result in large loads of nutrients, which have to be recycled by microbes. Microbial composition is a crucial part of a biofloc system. Microbial communities are extremely complex and more difficult to control compared to RAS systems [53]. Alterations in the bacterial community structure can trigger disease outbreaks, affect the performance of the farmed organism [54], and deteriorate water quality [55].

Intensive shrimp aquaculture sustainability also possesses a high risk of failure due to diseases [56]. For instance, white spot disease (WSD) was found to be responsible for vast economic losses in the Asian shrimp aquaculture industry. Optimal management practices focusing on improving the water quality and controlling water exchanges can significantly reduce WSD prevalence [57]. Technical efficiency could be greatly improved by focusing on farmers’ training [58]. Dimension-wise analysis by Kumaran et al. [56] showed that for P. vannamei farming, even if it was economically profitable, improvements are needed mainly in environmental and institutional dimensions in India. Other technological innovations that can reduce technical efficiencies in intensive biofloc systems include high-density polyethylene lining [46], although the investment in this infrastructure is high. Rego et al. [59], analyzing the financial viability concerning the insertion of biofloc technology in the aquaculture of P. vannamei, concluded that BFT systems were a favorable alternative to traditional systems which operate in northeastern Brazil. Noguera-Muñoz et al. [60] reported that technology-driven, super-intensive systems for whiteleg shrimp production in Nayarit (Mexico) were fully viable concerning environmental, social, and economic aspects. Financial viability was reported for several whiteleg shrimp farming operations within BFT systems [41,61].

Natural populations of P. vannamei inhabit environments where water temperatures generally exceed 20 °C all year round [12]. Pond cultures’ productivity in Southern Brazil is restricted to 6–8 months [62], while in most regions of the NE Mediterranean, tropical shrimp species could be cultured from early spring to late autumn, meaning that productivity is restricted to 5–6 months [63,64]. The establishment of biofloc farms for P. vannamei inside greenhouses may offer a range of potential benefits. Greenhouses offer a controlled environment which could reduce energy costs as they utilize natural sunlight and protect from adverse weather conditions. Biofloc systems inside a greenhouse have the potential to increase productive periods in higher latitudes, especially when eliminating water renewals which lead to decreased energy for heating [65,66]. The Mediterranean climate offers a prospect for the production to be expanded year-round, increasing the overall yield. Greenhouses can contribute to minimal water exchange through constantly recording water quality parameters, leading to efficient water and nutrient management.

Many studies revealed that the proximate composition and heavy metal concentration in fish species collected from traditional sources are above the guideline limits of various authorities. More specifically, concentration of As, Pb, and Cd in Anabas testudineus and Heteropneustes fossilis were found to exceed the FAO/WHO-permissible limits [67], while in another study, Ullah et al. [68] observed that the concentrations of Pb and Cd in Tebualosa ilisha and Dorosoma cepedianum were higher than the acceptable limit for human consumption. Similar results were obtained from Saint Martin Island, where the concentrations of Pb in the species Rastelliger kanagurta and Sargocentron rubrum were above the food safety guidelines of marine fish and crustaceans [69]. Concerning the Mediterranean Sea, high levels of Hd, Cd, and Pb were found in Thunnus alalunga, where some samples had higher values than the EU limits (2006) [70]. From a more comprehensive study, including 16 fish species from the north-east Mediterranean, higher Zn levels than acceptable by TFC and FAO/WHO limits occurred [71]. Thus, benefits of BFT include sustainability, productivity, higher mineral concentrations (Na, K, Ca, and Mg), and significantly lower toxic element concentrations (As, Pb, Cd, Cr, Fe, Zn, Mn, Cu, Ni, and Al) in cultured aquatic organisms (i.e., tilapia, koi, shing, and magur) compared with seafood coming from the local market [72]. Finally, P. vannamei’s nutritional value does seem to be affected in BFT systems [61]. Thus, it was observed that the consumption of fish species from biofloc fish farms was more suitable than that of local markets, mainly due to lower concentrations of heavy metals, a fact that added value and quality to the final product [61].

Despite all of the benefits, greenhouse biofloc’s profitability depends on various factors. The initial investments in infrastructure, system setup, and equipment are significant [47], while maintaining the greenhouse environment, despite the energy savings, involves operational costs for daily management of the system. Market demand for P. vannamei should be analyzed, since profit relies on the ability to produce shrimp at competitive prices, while fluctuations in market prices can affect the overall financial viability. Further, greenhouse farming and shrimp aquaculture should be compliant with local regulations, which is crucial for long-term sustainability. Success of a novel operation such as biofloc farming requires expertise; thus, collaboration with experts (e.g., on operating procedures, economists) or farmer training is crucial.

4. Presence of Non-Indigenous Marine Shrimp Species in the Mediterranean—The Case of Penaeus vannamei

According to the CABI (Centre for Agriculture and Biosciences International), there are multiple escapes of non-native cultured organisms into the environment. Biological introductions promoted by humans are continuously observed in the Eastern Mediterranean, which is a crucial biodiversity hotspot [73]. Considering only the land ecosystems of the Mediterranean basin, very high costs have been reported from invasive species by Kourantidou et al. [74]. Most of the aforementioned costs were assigned to damages or losses and only a small percentage was attributed to management practices [74].

By the 2000s, in the Mediterranean Sea, 159 non-native crustaceans were reported [75]. More specifically, more than 70 invasive decapod species have been documented since 2014 in the Eastern Mediterranean and many of them were introduced from the Suez Canal [76]. Furthermore, the Mediterranean Sea is highly vulnerable to non-native crustacean introductions as it is characterized by a 70% successful establishment rate in the eastern part of the basin [77]. Examples of well-adopted alien shrimp/prawn species in the Mediterranean Sea include Penaeus semisulcatus and Palaemon macrodactylus (Rathbun, 1902). The former is native to the Indo-West-Pacific and invaded the Mediterranean through the Suez Canal. P. macrodactylus was initially detected on the Balearic Islands in 2009 [78] and was later found up to the Northern Adriatic [79]. A remarkably successful shrimp invader in the Mediterranean Sea is Penaeus aztecus. This species was initially detected in Southern Turkey in 2010 [80]. Currently, it is spread almost throughout the Mediterranean basin [81], including Greece [82,83], and it has already been implemented for farming in Egypt [84]. P. aztecus is native to the NW Atlantic and Gulf of Mexico, which means that their indigenous populations are adapted to similar latitudes as P. vannamei. This fact raises concerns regarding the possible escapes of farmed P. vannamei into the Mediterranean.

P. vannamei represents the most important penaeid shrimp species farmed worldwide [85], with its native distribution range being the Eastern Pacific coast from the Gulf of California, Mexico, to Tumbes, North of Peru [86]. Although there are studies indicating the potential competition of P. vannamei with native shrimp species for food [87,88], and while P. vannamei has been introduced for farming into more than 30 countries [36], no reproductively active population of this species beyond its natural habitat has been recorded [89]. Perez-Enriquez et al. [90] found that farmed stocks of P. vannamei in Sinaloa (Mexico) had a different genetic composition compared to the native wild specimens using a set of 14 nuclear genetic markers, and they concluded that this species possesses a low introgression risk. Increased attention has been paid to escape events of P. vannamei in many locations where the recipient and native environments exhibit many similarities, i.e., the southern Gulf of Mexico [89], the USA [91], Brazil [92,93], Vietnam [94,95], and India [96]. However, even under these favorable circumstances, the establishment of a self-sustaining population has not been sufficiently documented in any of the aforementioned cases. Recently, a report from Panutrakul and Senanan [97] mentioned substantial numbers of P. vannamei specimens along the east coast of Thailand, where it was introduced for commercial aquaculture 1998.

Around the Eastern Mediterranean, P. vannamei has been cultured for more than two decades in Israel and Egypt [98,99]. Nevertheless, there is no report of a specimen in the wild until today. Moreover, 64% of P. vannamei production takes place in coastal areas where this species is not native [100]. Generally, shrimp escapes from traditional aquaculture facilities may occur during storms and floods [90]. Taking into consideration the high vulnerability of the Mediterranean Sea to non-native crustacean introduction, land-based cultivation systems would be even safer for minimizing the introgression risk. More specifically, many studies have shown minimal adverse impacts associated with the escape and spread of cultivated species [101]. Additionally, biofloc as a closed system has the advantage of a decreased release of wastewater into adjacent aquatic ecosystems, preventing the escape of nutrients, organic matter, pathogens, and farmed animals [17]. Thus, it is evident that even though P. vannamei seems unable to establish viable populations in the Mediterranean Sea, BFT offers an extra obstacle to the possibility of escape of cultured P. vannamei individuals in natural waters.

5. Description of an Experimental Penaeus vannamei Aquaculture Unit within a Greenhouse in Temperate Latitude (North Greece)

As an experimental setup, we developed and examined the feasibility of a P. vannamei biofloc prototype in North Greece. The external side of the greenhouse was made of panes of glass, and 15% of the vertical sides were 6 mm thick polycarbonate. To achieve better insulation, two plastic greenhouse films of 80% and 100% permeability were placed above the aforementioned materials (Figure 1) when the outside temperature was lower than 18 °C. Two tanks were constructed from modular galvanized sheets (40 × 125 cm and 2 mm thickness) (Figure 2A,B) by attaching them together to form the desired shape (Figure 2C,D). Polypropylene film was placed inside the frame of the galvanized sheets, forming a total area of 16 m² capable of holding 15,000 L of water for each tank (Figure 2C,D). the tanks were insulated with special felt of 2–3 mm width together with extruded polystyrene (30 mm width) and polypropylene (PP) membrane (1 mm width).

Inside the greenhouse, a second microclimate chamber was built around each tank (Figure 3A–C). Filters were positioned inside the microclimate chamber for extra energy saving. The Chamber’s margins were 1 m away from the three sides of the tank, and from the side where the filters were placed, the distance was 4 m. The microclimate chamber was short (3 m high), creating a small volume of air above the tanks, thus maintaining a relatively saturated atmosphere that prevented further evaporation and, subsequently, rapid cooling of the tank water. The metal frame of the chamber was covered by a plastic greenhouse membrane, while the roof had a double layer of plastic greenhouse membrane (Figure 3C). Inside the greenhouse, there were also dark tanks filled with water which were heated by sunlight during the day (Figure 3D). This energy was re-emitted inside the greenhouse during nighttime, assisting the warming needs of the greenhouse space. Every tank contained heat exchangers suitable for seawater. Electric heating pumps and propane burners were employed for extra heating whenever needed. Heating of the system was mainly based on propane.

During the summer, due to the absence of a shading system, water misting was used to chill the microclimate chambers. The primary control method of cooling involved the tilting windows of the roof (Figure 1), which were remotely opened through the remote-control system when the greenhouse temperature was approaching 42 °C. If the internal temperature exceeded 45 °C with opened doors, opened windows, and the use of wet panels, then the misting system was activated. The wet panel system was utilized in two ways, either through simple dampening or by dampening combined with the use of the greenhouse fans. Water oxygenation was achieved by an air circuit which was supplied by side channel blowers with air filters ending at the air lifts attached to the tanks.

After filling the tanks, selected Nitrosomonas, Nitrobacter, biofloc starter, and probiotic bacteria were introduced into the system and the system started its transition from clearwater to biofloc after 4 weeks (Figure 4). The temperature during this period was 30.2 ± 1.17 °C. Three filters were used in the system: a sedimentation tank, a protein skimmer of 500 L operating using the venturi effect, and a denitrification reactor of 1000 L. Moreover, a denitrification reactor containing 250 L of polypropylene (PP) filter media and siporax was unsuccessfully tried. The anerobic bacteria inside the denitrification tank had very slow assimilation rates, while significant time was needed for the maturation of the filter. The 1000 L denitrification reactor could perform degradation of around 40 ppm of nitrates after 2 h of retention in water containing 350 ppm of NO₃⁻.

P. vannamei postlarvae (PL12) were immersed in the two tanks during the fifth week. The animals were fed 4–6 times daily on 6% of their body weight initially and later on 4% with commercial feed (SKRETTING PL#4). The culture period was 112 days and started on 5 May 2023. Throughout this period, water temperatures were inside the range of 29.4–32.3 °C. Physicochemical parameters including temperature, oxygen concentration, pH, salinity, total ammonium nitrogen (TAN), and nitrite and nitrate ion levels were constantly monitored. Remote monitoring and a remote-control system for the abiotic factors were established by a tracking device along with the main automation system based on a PLC device which recorded oxygen levels, pH, and temperature. Molasses and sugar were added to the system occasionally for maintaining the C:N ratio greater than 15:1 to sustain the heterotrophic condition.

The culture of P. vannamei was performed without any water exchange. Water was pumped from Nea Michaniona (Thermaikos Gulf) and freshwater was added to reach 29 ppt salinity. Fresh water was added only to replace evaporation and sludge removal losses. The aquaculture water salinity was between 29 and 31 ppt, and the pH was stable at 8 with minor fluctuations. The mean suspended solids’ concentration was 17 mL/L as measured by the Imhoff cone method. Dissolved oxygen levels were continuously above 5 ppm. Regarding the nitrogen compounds, total ammonia nitrogen (TAN) exhibited spikes, sometimes reaching 2–3 mg/L, but in every case, the levels were rapidly reduced with the direct addition of a carbon source to the tank according to Avnimelech [102]. Nitrite levels initially reached 5 mg/L, and later, they were always lower than 2 mg/L. Mortalities reached 32% and were mostly evidenced after ammonia or nitrite spikes and due to a problem in the aeration system that lasted 7 h, also during the first days. The nitrate concentration reached 350 mg/L, which is high but cannot trigger severe mortalities of this species, although it can surely affect the weight gain [103] and thus the productivity of the system.

Approximately 1800 specimens of twelve-day-old P. vannamei postlarvae (PL12) were placed into two tanks on 5 May 2023. The initial densities in the tanks were 120 PL/m³ or 112.5 PL/m². Despite the problems in aeration and the temperature fluctuations which frequently exceeded 30 °C, as well as the partially maturated biofloc in the beginning of the culture period, the shrimps reached 23.5 g of weight (Figure 5) from the initial 8.5 mg within 112 days. The mean specific growth rate of weight was 7.07% ± 0.58 per day and, accordingly, the weight gain was 1.47 ± 0.21 g/week. Taking into consideration the 32% mortalities, productivity after 112 days (25 August 2023) was 1.8 Kg/m² or 1.92 Kg/m³. These results are comparable to other growth rates reported for this species. For example, Wyban et al. [104] reported 5.14%/day and 1.45 g/week weight SGR and weight gain, respectively, in simulated pond conditions at 30 °C for P. vannamei when providing ad libitum feed to the shrimps. Ponce-Palafox et al. [105] found that PL18 individuals stocked at 500 shrimps/m³ with ad libitum feeding at 30 °C had about 7.4% weight SGR per day at 30–35‰ salinity.

Comparing our results to biofloc systems, Lara et al. [106], working on 1.14 g shrimps at a density of 400 shrimps per m³ and 30.85‰ salinity for 21 days, reported a 6.53%/day SGR of weight and 1.04 g/week weight gain. Kumar et al. [107] documented 1.78 kg/m² productivity in a biofloc system established in lined earthen ponds when stocking 60 PL15 P. vannamei shrimps per m² and on a 3–5% feeding regime after 120 days. A productivity of 1.55 kg/m² in a lined tank biofloc system which was located inside a greenhouse was found by Lara et al. [108]. These researchers observed the growth of medium-sized shrimps of 4.3 g (initial weight) for 33 days at 29.5 °C at a density of 140 shrimps/m² which had 1.73 g/week weight gain. This system had higher NO₂⁻ levels (9 mg/L), but the TAN levels (1 mg/L) and NO₃⁻ levels (15 mg/L) were both lower compared to our system. Baloi et al. [109], working on 3.3 g individuals in a biofloc system with no water exchange and a stocking density of 300 shrimps/m³ for 40 days, found a weight gain of 1.2 g per week and a productivity of 2.7 Kg/m². It should be underlined that super-intensive P. vannamei farming in biofloc can yield to a production of 4.09 to 10 Kg/m² [62,66,110]. Reaching these yields mainly heavily depends on high-technology resources.

Energy consumption after 112 days of cultivation in the two tanks was calculated at 7099.2 Kwh for the air and water pumps. The water pump consumed 66% of this energy. More efficient water aeration and pumping could be used in our system considering that energy consumption for aeration (two paddle-wheel aerators and a bubble aeration system) in a much bigger (30 m diameter) circular pond stocked with 150 P. vannamei PL20/m² was calculated as 3834 kwh for 90 days [111]. In total, 896 Kwh was needed for 112 days of heating the system with propane from 5th of May to 25th of August. Our experiments ran until 17th of November, and from 26 August until 17 November, the propane burner operated for only 20 min per day to achieve 29–30 °C inside the water tank without using the outside membrane for greenhouse insulation. This means that for another 84 days, the 25.6 kWh propane burner consumed 716.8 kWh for heating, and overall, 1612.8 kWh of energy for heating with propane was required for 6.5 months for the two tanks. Regarding the overall energy expenditures of the completed 112-day experiment, including electricity in both tanks, the total energy consumption was estimated at 7995.2 kwh. Therefore, CO₂ eq emissions were about 2442.1 Kg (average emissions in Greece per kWh were about 0.344 Kg in 2022 according to statista.com) for electricity and 187 Kg for heating, with 2629.1 Kg of CO₂ eq in total. Moreover, for 6.5 months, heating with propane was estimated to have produced about 336.5 Kg of CO₂ eq for each tank.

The observed energy expenditure and environmental impact of our system was very high, considering that in Ecuador, India, Indonesia, Thailand, and Vietnam shrimp farms, the mean energy requirements to produce 1 ton of P. vannamei was found to be between 15,555 and 27,444 kWh [112], and the production of a ton of live-weight shrimp generated 4657.2 kg of CO₂ eq in a BFT system [9]. This is mainly attributed to inappropriate design of the water pumping and aeration system or the relatively small tanks, and not the heating procedure. The microclimate chamber saved a considerable amount of energy. Also, taking into account the estimated 1612.8 kWh required by the system for heating (based on propane) for 6.5 months, heating will probably not be a problem in commercial BFT farms inside a greenhouse in temperate regions when innovations regarding energy saving are applied. Intensive and super-intensive aquaculture systems could be more profitable and sustainable using clean energy alternatives instead of conventional fossil fuel sources and utilizing ecosystem services such as geothermal water or underground cool water [113]. Using a solar photovoltaic system could also reduce the negative environmental impact by eliminating carbon dioxide, sulfur dioxide, and nitrogen oxide emissions and therefore allowing for more sustainable shrimp aquaculture [111].

6. Conclusions

BFT potentially offers a variety of benefits including reduced environmental impact, lower escape risk, lower production costs, and greater production yields. Furthermore, there are many studies indicating lower concentrations of heavy metal in the fish products coming from biofloc in comparison with traditional sources. The use of BFT is mainly limited to tropical and subtropical climates, as more temperate environments would demand greater energy for heating to maintain both the microbial community and the tropical shrimp species, which potentially worsens the environmental impacts [114]. These impacts may be mitigated by introducing passive heating systems, on-site generation of renewable energy, and good insulation. Maintaining optimal shrimp growth in cold seasons in temperate climates may pose challenges, yet this can be attained through energy-saving manufacturing innovations. Short ceiling designs such as the microclimate chamber constructed in this study and good insulation can significantly reduce the energy needed for heating. Cheap and clean energy sources such as photovoltaic solar panel systems or underground warm water should also be employed for cost-effective production [115]. P. vannamei appears to grow better at temperatures greater than 30 °C when its body weight is lower than 5 g, and when the weight exceeds 5 g, the optimum temperature for growth is about 27 °C [104]. P. vannamei can also be kept at low or intermediate temperatures during the cold season, under restricted feeding, to induce partial compensatory growth when the temperature is elevated again [116]. Consequently, in temperate climates, better management practices should include small- or medium-sized rearing during the warmer months and larger shrimps during colder periods and a restricted feeding regime during the colder months.

Shrimp aquaculture development in Mediterranean countries offers a great opportunity for increasing the domestic production to cover the requirements of the natives and the increasing demands arising from tourism during summer, presenting promising prospects. The Mediterranean climate provides suitable temperatures for a five-to-seven-month period, which can be expanded inside greenhouses with relatively low energy demands for heating, while the large coastline ensures seawater accessibility. With sustainable management practices, shrimp farming could complement Greece’s tourism industry, as local, responsibly sourced seafood will be attractive to tourists. The development of a new industry, especially when aligning with sustainable practices, could attract investments as well as advocacy from government initiatives. While these advantages exist, strong research and innovation are needed to implement the latest technologies and the best management practices to take advantage of economies of scale and ensure the long-term success of shrimp farming in Greece.