In fact, the stage of the growing period seems to be an influencing variable. In a study involving the hydroponic growth of tomato , wilt due to Fusarium was reported to be most severe in the intermediate or late growth stage of the growing period . For this reason, the duration of the trial was longer than initially expected, so the size of the plants increased and with it their nutritional requirements. This situation led to an imbalance in the aquaponic systems which were unable to provide adequate levels of nutrients to the plants. As a consequence, these plants suffered a nutritional deficit that made them more sensitive to Fusarium wilt . Hence, the hydroponic plants being better nourished, could be more resistant to biotic or abiotic stresses. This effect was also observed by the fact that the disease severity was lower in the plants near the entrance of the water with the nutrients , despite being closer to the inoculated plants . As is the case of Pythium aphanidermatum. This produced 100% mortality in spinach that was grown in water at 30 ◦C, but 0% of mortality was observed in crops that were in water at 20 ◦C . As another example, Pythium dissotocum caused the wilting of 100% of the plants when the water was at 30 ◦C compared to the 69% that wilted when the water was at 20 ◦C . These differences in disease prevalence and severity are likely related to optimal growth temperatures for these pathogens, with higher infection and mortality rates as a result of water temperatures being more favourable for pathogen development . In our study, the maximum values of water temperature reached 25.1 ◦C in the aquaponic system and 26.8 ◦C in the hydroponic system, therefore below 30 ◦C. However, neither Fol nor P. cactorum were harmful to goldfishes in our tests since no mortality or disease symptoms were observed. In relation to the water chemical parameters, aquaponic systems remained within the limits recommended by the FAO ,ebb and flow table with a pH between 6 and 7 and a nitrate concentration between 5 and150 mg L− 1 , in order to maintain the well-being of the plants, fish and bacteria.
The electrical conductivity was kept close to 1500 μS cm− 1 in the hydroponic systems, not exceeding 1700 μS cm− 1 . Though the recommended pH in which the plants have a greater availability of nutrients is between 5.5 and 6.5 , it was kept slightly over those values in order to maintain conditions similar to aquaponic systems. Further studies are required to confirm higher suppressiveness in aquaponic systems compared to hydroponic systems, trying to maintain an adequate balance in the contribution of nutrients in each system. The study of several pathosystems could help to confirm the potential suppressiveness in aquaponic systems. Likewise, it would be interesting to take into account different types of fish, such as tilapia, which is fequently employed in aquaponic systems. Sustainable production of quality food under premises of circular economy reducing the flow of resources out of the system is crucial for ensuring food security while maintaining non-renewable resources such as phosphorus and reducing environmental impacts of agricultural practices . This is particularly relevant in areas of increasing population such as part of the Mediterranean basin.In this sense, initiatives that urge the development of policies, missions and incentives to spread circular economy practices, in particular in food production, throughout the world are increasingly abundant and insistent . Intensive early production of horticultural products is not only important for providing quality food out of the typical growing seasons but also crucial for ensuring economical sustainability of horticultural farmers due to the high prices of these early products. A paradigmatic example is early production of strawberry which, in the European Union, is mainly located in Southern Spain, concretely in the province of Huelva. This crop is a powerful economic engine for this area, comprising about 94% of total production in Spain . The total area dedicated in Spain to strawberry production in 2018 was 6603 ha, yielding 334,767 t of fruit, most of the production being destined to the international markets . In 2018, this exportation generated a value of 461.3 million €. The progression of the prices of the fruit shows a decline throughout the campaign, so that the initial values condition the subsequent prices. According to this, early production in these agrosystems is critical in order to guarantee their profitability.
In conventional horticulture, strawberry is usually produced in a monocropping system, i.e. it is consecutively grown in the same field plots year after year, which increases the risk of soil borne diseases , making soil disinfection a necessary practice . Hydroponic production has demonstrated to be an interesting option due to its independence from soil microbiological status, while required minerals are applied to plants with fertigation. In this context, the soilless production, as in other intensive horticultural areas, in Spain is progressively increasing and currently amounts to near 6% of the total production surface in the main strawberry production region. However, other authors report that the influence of substrates on fruit quality and yield is less relevant than the effect of strawberry cultivars . Although soilless production may enhance the resource efficiency, horticultural systems demand high rates of external inputs, in particular, water and nutrients. In this regard, the EIP-AGRI Focus Group on Circular Horticulture concluded that aquaponics is emerging as one of the most important areas of sustainable agriculture which meets the philosophy of circular economy . These systems not only decrease the need of external nutrient supply but also reduce the overall water discharge and increase water use efficiency in agricultural products. Aquaponics is a production system that synergistically combines the simultaneous growing of plants in soilless media and fish in recirculating aquaculture systems . Plants improve their growth by using metabolic waste from fish and unconsumed feed, which are transformed by a bacterial community into easily assimilated nutrients , reducing discharge to the environment and extending water use. Aquaponics has less environmental impact than conventional aquaculture and agriculture.This production system may have higher productivities and less resource consumption than conventional land-based systems . Therefore, aquaponics is a production system aimed at reducing inputs as well as minimising pollution whilst maximising production efficiency and stability, hence increasing revenues . Strawberry cropping in aquaponic systems may be an interesting alternative since it is a profitable crop when early produced which can be managed under soilless conditions.
In fact, soilless production is gaining interest as mentioned above to avoid the risks ascribed to monocrop production on soil. Nevertheless, very few studies about strawberry aquaponic production are available, flood table and mainly focus on operational factors such as substrates used or fish densities employed . Both the early strawberry production and the quality of the fruit, which are crucial factors affecting crop profitability, have not been assessed in previous research. In this regard, it is crucial to know how the root anchoring system , whether bare roots or inert growing media such as the widely used rockwool may affect crop traits. Thus, this work aimed at the evaluation of the yield and quality of the combined early production of strawberry and tench , by means of coupled aquaponic systems. In them, commercial hydroponic bands were employed, and the differences of using or not a rockwool substrate were assessed as a relevant aspect which may affect crop performance, quality, and precocity, as well as resource consumption in the system.For this study, three identical coupled aquaponic systems were installed inside a greenhouse located at the School of Agricultural Engineering , encompassing a total surface of 36 m2 . Each system was composed of a 1 m3 cylindrical fish tank, where water was aerated using a 400 L⋅h− 1 air pump for ensuring a correct level of oxygen dissolved in the water for the fish. Water temperature was controlled by means of a 300 W heater . From the fish tank, water was conducted by gravity to a handmade PVC biofilter of 50.26 L capacity , filled with ceramic rings as filter media. Then, water was divided into two NGS multilayer channels with 1.5 m length and 0.8 m separation, with 12 holes per channel to hold plants . The NGS is a modification of the nutrient film technique hydroponic system, consisting of a series of interconnected layers, which favour small cascades that support the increase of oxygen availability and elude length limitations in NFT channels . NGS’ multi-layer DUO is specially designed for strawberry crops, and it is composed of 3 polythene bands forming a superior level, with 2 lines of holes following a zigzag pattern to hold the plants, and two inner levels to room the root system and to collect the nutritive solution while favouring aeration . At the end of both hydroponic lines, water flowed down into a 100 L sump tank, where a single submerged water pump was located in order to return the water to the fish tank, closing the loop. With the aim of facilitating the growth and colonization of nitrifying bacteria inside aquaponic components, the systems remained in operation, without fish or plants, for a period of six weeks. During this time, ammonia was artificially introduced in the tanks to speed up the process. Tench was chosen as fish species for this aquaponic prototype, since it is fully adapted to the Iberian Peninsula’s climate and is recognized for its great resistance to changes in water quality in extensive and intensive regimes , being an adequate candidate for aquaponics production systems in this area .
The study was started with an initial population of 366 tench fingerlings, counting up a total biomass of about 2100 g. The fish were provided by the Regional Aquaculture Center “Las Vegas del Guadiana”, a public company belonging to the Junta de Extremadura, located in Villafranco del Guadiana . The acclimatization phase began at the end of November and consisted in the introduction of a fish biomass of 700 g in each tank. The number of specimens in each system was homogenized, leaving 122 fish in each tank. Low density of fish was selected according to the recommendations of Villarroel et al. and to simplify systems management. The cultivar of strawberries used was ‘Primoris FNM’, a short day cultivar, which begins fruit production about eight or ten days before most of medium cycle varieties. Runners were acquired from Fresas Nuevos Materiales, S.A. , after having gone through a high-altitude nursery to meet the low temperature requirements for the start of flowering, in the same conditions as theseedlings that are usually planted in crops in southern Spain. Half of the runners were placed in a seedbed with perlite and the other half in rockwool blocks. Once the first three leaves appeared, the seedlings were transplanted to the hydroponic lines, after elimination of perlite with running water. The study started from a total of 72 strawberry seedlings, of which 36 plants did not use any substrate and 36 were established on rockwool’s blocks. The strawberries were transplanted into the aquaponic two days after the introduction of fish, in order to ensure adequate nutrients contents for plants. Distribution of plants according to the substrate type followed a random blocks pattern, as it can be observed in Fig. 2. Each of the fish tanks was connected to two hydroponic lines and each of the lines contained two blocks of 6 plants with different substrate types . The development of the plants was monitored throughout the crop cycle, weekly recording different parameters regarding leaves, flowers and fruits. The width and length of the leaves were measured, as well as the chlorophyll content in old and new leaves using a SPAD-502 chlorophyll meter . In addition, a count was made of the flower buds, number of flowers and fruits and weight of fruits per plant. Regarding strawberry quality parameters, immediately after harvest, the firmness of the fruit was analysed using a PCE-PTR 200 Forge Gauce penetrometer ; and soluble solids were measured using a hand-refractometer RHC-200ATC applied to fruit juice.