Introduction
Nowadays, the principle of sustainability is widely used by practically all the economic sectors and most of activities presume to be sustainable. Sustainability is not a final condition nor an achievable goal; sustainability is a way ahead and a way of economic and social development. Under this scenario, sustainable strategies for improving blue food economies are essential to design a new approach for transitioning towards more responsible, comprehensive, exploitable, and favourably positive impact-generating food production systems. Among them, aquaponics presents an innovation in traditional aquaculture systems by combining fish farming and hydroponics (the soilless cultivation of plants). Potential advantages of aquaponics include enhanced sustainability, less resource consumption, and fewer environmental impacts compared to conventional aquaculture and agriculture practices (Baganz et al., 2022). The objective of this study was to evaluate the farming of M. cephalus in an aquaponic marine coupled system and compared to a conventional recirculating aquaculture system (RAS) in terms productivity, farming costs, fish performance and operational welfare indicators (OWIs). This approach will double the sustainability of aquaculture farming practices by testing a low-trophic fish species like M. cephalus in an aquaponic system in order to improve the sustainability of this practice by using the nutrient waste from the fish culture system as a nutrient source for growing the halophyte Salicornia spp.
Materials and Methods
Two different trials were conducted in this study. In a first one, the assay was set-up for testing the viability of growing salicornia in combination with a low trophic species under seawater conditions (35 ppt) in a coupled aquaponic system. We used three independent aquaponic systems in which we grew-up 100 flathead grey mullet fish (50-52 g in BW) per tank at an initial stocking rate of 3.5 kg/m3 that was connected to the plant unit that contained wild seedlings of Salicornia spp. in 6.4 m2 (141-145 of plants per unit). The second trial consisted of rearing flathead mullet in conventional RAS unit. For this purpose, 381 fish (30-50 g in BW) were evenly distributed among the three experimental tanks connected to the RAS unit (127 fish per tank; initial stocked biomass = 2.5 kg/m3).
In all trials, fish were fed the same commercial diet (Nutra MP, Skretting; 55% crude proteins, 17.5% crude lipids). Fish were fed a feed ration of 2.5% of the stocked biomass using automatic belt feeders. In both trials, water quality parameters (temperature, oxygen, salinity) were measured daily, whereas nitrogenous compounds in water (ammonia, nitrites, and nitrates) were measured twice a week by our technicians. At the end of the trial, all animals were measured in length and BW, the apparent feed conversion ratio was determined, and the aquaponics plant yield (APY, kg/m2/year) was calculated taking into consideration the yield of salicornia per m2 in each aquaponic unit and the duration of the experimental period. Operational welfare indicators (OWIs) related to skin and fin conditions were recorded and compared between both farming systems.
Results and Discussion
In aquaponic systems, the increase of stocked fish biomass grew from 3.5 kg to 10.7 kg per tank (9.8 - 12.2 kg/tank), growth that was coupled with an increase in the stock density, values that increased from 3.5 kg/m3 to final stocking values that ranged from 6.5 to 8.1 kg/m3. This increase in biomass was a result of an increase in the average individual weight of flathead grey mullets that reached 183.7 to 195.3 g (SGR = 0.94-0.99% of increase in body weight per day). Mortality rates in all three aquaponic systems was lower than 5% (98.4± 1.5%). Regarding the yield of salicornia, plants in all systems grew very well with survival rates higher than 95%. The final yield (plant biomass) per aquaponic unit ranged from 43.1 to 50.1 kg (6.7-7.8 kg/m2). The increase in plant biomass from the beginning to the end of the trial ranged from 290 to 346 times. Flathead grey mullet reared on RAS units grew linearly from 39.2 g to 143.6 and 145.1 g, depending on the tank considered (Figure 16). Values of SGR were similar among the three replicate tanks (1.1% BW/day) and apparent FCR values ranged from 2.1 to 2.3. Fish survival was 96.1 ± 3.1%. The analysis of the OWIs of flathead grey mullet reared in the coupled marine aquaponic system and RAS revealed that this species adapted very well to both farming systems since no moderate or severe lesions were observed in animals. Most of the alterations were considered as very mild (disarrangement of fins) or loss of few scales over the skin. The loss of scales may not be attributed to rearing conditions since this species is very sensitive to handling and generally, some scales are lost when fishing, sedating, and measuring fish.
The energy cost (kW) for running each aquaponic unit based on the consumption of the water pump (0.1 kW/h/unit) and air blower (0.3 kW/h/unit) for the duration of the trial (142 days) was estimated at 340.8 kW/unit for the water pump 1,022.4 kW/unit. In total, each aquaponic unit consumed 1,363.2 kW, which represented an energy cost per kilogram of salicornia of 0.029 ± 0.002 kW/kg. In contrast, the electric costs of the RAS unit composed of a water pump, air blower, UV lamps and heat exchanger was 1.0 kW/h, 2.5 times higher than that for the coupled aquaponic system.
Conclusion
Flathead grey mullet showed good adaptability to farming conditions in RAS and aquaponic systems, even though the species is more difficult to handle and acclimate to intensive rearing conditions than initially expected. Fish performed similarly in terms of growth performance, FCR and OWIs between both tested farming systems. Coupled aquaponic systems are a sustainable strategy for the combined production of fish and salicornia with a minimal use of water (<5% of water renewal), an efficient use of land and water, and a reduced cost in terms of electricity (aquaponics are 2.5 times cheaper in running costs than RAS units). Furthermore, growing plants coupled to fish rearing provides a sustainable economic profit to the fish farmer by producing a high quality fresh product to the consumer. Present results indicated that this technology might be applied to commercial, or community based urban food production, industrial scale production in rural areas, small scale farming in developing countries (backyard aquaculture systems) or as systems for education.
References
Baganz, G. F., Junge, R., Portella, M. C., Goddek, S., Keesman, K.J., Baganz, D., Kloas, W. (2022). The aquaponic principle—It is all about coupling. Rev. Aquac. 14, 252-264.