Aquacultural Engineering 97 (2022) 102242 2production in 2018 was 6603 ha, yielding 334,767 t of fruit, most of the production being destined to the international markets (around 65% of total production). 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 (extra-early pro- duction, before February 28th and early production, before March 31st) 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 soilborne diseases (De Cal et al., 2005), making soil disinfection a necessary practice (Borrero et al., 2017). The prohibition of the use of usual soil disinfectant (methyl bromide) led to the emergence of alternative strawberry production systems (L ́opez-Aranda et al., 2009; Martínez et al., 2017). Hydroponic production has demonstrated to be an interesting option due to its in- dependence from soil microbiological status, while required minerals are applied to plants with fertigation. In this context, the soilless pro- duction, as in other intensive horticultural areas, in Spain is progres- sivelyincreasingandcurrentlyamountstonear6%ofthetotal production surface in the main strawberry production region. The types of substrates used in soilless production may affect fruit yield and quality (Adak et al., 2018; Alsmairat et al., 2018; Marinou et al., 2013; Martínez et al., 2017; Wysocki et al., 2017). However, other authors report that the influence of substrates on fruit quality and yield is less relevant than the effect of strawberry cultivars (Palencia et al., 2016). 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 Cir- cular Horticulture concluded that aquaponics is emerging as one of the most important areas of sustainable agriculture which meets the phi- losophy of circular economy (EIP-AGRI, 2017). 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 prod- ucts. Aquaponics is a production system that synergistically combines the simultaneous growing of plants in soilless media (hydroponics) and fish in recirculating aquaculture systems (RAS). Plants improve their growth by using metabolic waste from fish and unconsumed feed, which are transformed by a bacterial community into easily assimilated nu- trients (i.e., nitrates, phosphates), reducing discharge to the environ- ment and extending water use (i.e., by removing dissolved nutrients through plant uptake, the water exchange rate can be reduced) (Rakocy et al., 2006). Aquaponics has less environmental impact than conven- tional aquaculture (potential contamination of aquifers with effluents, water requirements/water footprint, wastewater treatment cost, etc.) and agriculture (use of fertilisers such as phosphorus, water footprint, an excessive use of pesticides and fossil fuels, soil contaminants/diseases, etc.) (Eck et al., 2019; Joyce et al., 2019; Yep and Zheng, 2019). This production system may have higher productivities and less resource consumption than conventional land-based systems (Van Ginkel et al., 2017). Therefore, aquaponics is a production system aimed at reducing inputs as well as minimising pollution (e.g., wastewater) (Blidariu and Grozea, 2011) whilst maximising production efficiency and stability, hence increasing revenues (Tyson et al., 2011). 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, and mainly focus on operational factors such as substrates used or fish densities employed (Roosta and Afsharipoor, 2012; Villarroel et al., 2011). 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 (substrate type), 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 (Tinca tinca), by means of coupled aquaponicsystems.Inthem,commercialhydroponicbandswere 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. 2.Materials and methods 2.1.Aquaponic systems For this study, three identical coupled aquaponic systems (Fig. 1) were installed inside a greenhouse located at the School of Agricultural Engineering(UniversityofSeville,Seville,Spain;37◦21’6.45"N, 5◦56’12.35" W), encompassing a total surface of 36 m2. Each system was composed of a 1 m3cylindrical fish tank, where water was aerated using a 400 L⋅h1air pump (Air pump 400, Eheim GmbH&Co, Germany) 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 (aquar- ium heater 3639, EHEIM GmbH&Co KG, Germany). From the fish tank, water was conducted by gravity to a handmade PVC biofilter of 50.26 L capacity (200 mm in diameter and 1.60 m in height), filled with ceramic rings as filter media. Then, water was divided into two NGS (New Growing System, Almería, Spain) multilayer channels with 1.5 m length and 0.8 m separation, with 12 holes per channel to hold plants (Fig. 2). The NGS is a modification of the nutrient film technique (NFT) hydro- ponic 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 (Urrestarazu et al., 2005). 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 (NGS, 2019). At the end of both hydroponic lines, water flowed down into a 100 L sump tank, where a single submerged water pump was located (Compact 1000, EHEIM GmbH&Co. KG, Germany) 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 opera- tion, 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. 2.2.Fish and plants Tench (Tinca tinca L.) 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 (Pula et al., 2018), being an adequate candidate for aquaponics production systems in this area (Lobillo et al., 2014). 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 (Badajoz, Spain). 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 (366 in total) in each tank. Low density of fish was selected according to the recommendations of Vil- larroel et al. (2011) 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 NuevosMateriales,S.A.(FNM,Huelva,Spain),afterhavinggone throughahigh-altitudenurserytomeetthelowtemperaturere- quirements for the start of flowering, in the same conditions as the V.M. Fern ́andez-Caban ́as et al.
Aquacultural Engineering 97 (2022) 102242 3seedlings 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 (rockwool or bare root). 2.3.System operations and measurements performed 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 chlo- rophyll meter (Konica-Minolta, Japan). In addition, a count was made of the flower buds, number of flowers and fruits and weight of fruits per plant. Fig. 1.Coupled aquaponic systems used for the production of tench and strawberries in NGS bands. Main aquaponics components are 1) fish tank, 2) biofilter, 3) hydroponic channels, 4) sump (water pump inside) and 5) detail of multilayer band. Fig. 2.Schematics of the random distribution of blocks with different substrate type (rockwool or bare root) at each of the 6 lines in the essay. Numbered circles indicate plant positions. Arrows represent the direction of the water flows. V.M. Fern ́andez-Caban ́as et al.
Aquacultural Engineering 97 (2022) 102242 4Regarding strawberry quality parameters, immediately after harvest, the firmness of the fruit was analysed using a PCE-PTR 200 Forge Gauce penetrometer(PCE-Inst.,Spain);andsolublesolids(ºBrix)were measured using a hand-refractometer RHC-200ATC (Huake, China) applied to fruit juice. Fish weight was recorded monthly by extraction of all animals from each tank. In this process, changes in total biomass of fish were deter- mined to calculate different growth parameters as average daily growth rate (ADGR), specific growth rate (SGR) and feed conversion ratio (FCR), according to Lobillo et al. (2014). Average fish weight (AFW) was calculated by dividing the total fish weight (TFW) by the number of animals. ADGR was calculated as the ratio between average fish weight (AFW) increment and elapsed time. SGR was calculated as 100 x (lnAFW2- lnAFW1) / (T2- T1). FCR was computed as the ratio between consumed feed and total fish weight increment (TFWI). Tench were fed with a trouts’starter feed from the company "Biomar" (Inicio Plus 501) with 54% protein and 18% fat. Animals were fed twice a day applying an amount equivalent to 1.5% of total biomass, therefore modifying the quantity of feed provided after each fish weighting. The following parameters were analysed daily: ambient and water temperature by means of a maximum-minimum thermometer (TFA, Germany) and the volume of water replacement due to evaporation and transpiration water losses. Daily water replenishment rate was calcu- lated as the ratio between the average daily consumption and the total water running in each system. Water samples were collected weekly from the fish tank in order to measure pH, electrical conductivity (EC) and nitrates. The pH and EC were determined with a pH-meter GLP 22 and an EC-Meter BASIC 30 (Crimson instruments, Barcelona, Spain); respectively. When the pH of the water dropped rapidly to values close to 7, sodium and potassium hydroxides were added to the sump until it reached a value above 7 again. Nitrates concentration was obtained by means of an RQflex 10 plus(MERK,Darmstadt,Germany).Dissolvedoxygenlevelswere determined using colorimetric test kits (TETRA test O2). Aquaponic systems usually show low concentration levels of ele- ments such as K, Fe or Ca. Therefore, the plants were periodically examined to observe the occurrence of nutrients insufficiency. To alle- viate these potential deficits, K2SO4at 1.5% was foliarly applied in all aquaponics systems. The applications were performed twice a week (Mondays and Fridays), first thing in the morning with a manual sprayer. Chelated iron solution (1%) was directly added to the water (EDDHA Sequestrene 138 Fe), 0.1 L for each system. This was done on Fridays every fortnight. 2.4.Data analysis The effect of the substrate type used in the aquaponic production of strawberry, bare roots and rockwool, on studied variables was assessed by means of an analysis of variance (ANOVA). Since multiple, repeated measurements were made in an experimental unit in our experimental design, a“repeated measures structure”was considered. In this design, the observations can no longer be considered to be independent, and as a result, we assumed that there were correlations in the residual errors among time periods. A mixed general linear model (GLM), considering substrate type as fixed factor and sampling time (measured as elapsed time since transplanting) and block as random factors, was performed with the STATGRAPHICS Centurion XVIII statistical package (version 18.1.12, StatPoint Technologies Inc, Warrenton, VA). Previously, the normalityandhomoscedasticitywerecheckedwiththeSmirnov- KolmogorovtestandtheLevenetest,respectively.Powertrans- formations of data (Y=Xa) were performed when both or one of these criteriawerenot met.The besttransformation meetingnormality criteria was obtained by means of Box-Cox transformation using the samesoftware.Inallthecases,withthispowertransformation, normality and homocedasticity were met. Post hoc analysis was per- formed using the HSD Tukey test and differences were considered significant when P<0.05. When interaction between factors was found to be significant, the effect of main factors could not be discussed since this means that a substrate has an effect that changes along the sampling period.Inthesecases,theevolutionalongsamplingperiodwas described and compared between the two substrates studied. 3.Results 3.1.Physicochemical water parameters Despite the fact that the air temperature presented important vari- ations within the greenhouse (2–41◦C), water temperature in fish tanks underwentlowerdeviations,rangingbetween17and28.1◦C throughout the period in which this test was performed (Fig. 3). In general terms, the three systems studied were very similar with regards to the studied parameters of water quality. Nitrate content of the water increased from initial values around 20–30 mg L1to values higher than 70 mg L1after 42 days from the start of the test (Fig. 4). In this same period, pH decreased from initial values close to 8, similar to those corresponding to the available water source, to values between 7.2 and 7.6 as a consequence of the activity of nitrifying bacteria. In order to stabilize both parameters, the accumulated amount of water added to the system was increased from 49 days since planting (Fig. 5). At the end of this trial, the daily water replenishment rates for systems 1, 2 and 3 were 2.16%, 2.21% and 2.22%; respectively. In relation to the electrical conductivity of water, the variations observed in Fig. 5 were analogous to those followed by the nitrate content as a consequence of the above referred water replacement operations. Weekly measurements of the dissolved oxygen levels showed stable values (5.02±0.40 mg L1), which are considered adequate values for both fish and plants. 3.2.Tench production Initial fish biomass in each of the three tanks was around 700 g, with 122 animals per tank (Table 1) and average weights per fish were be- tween 5.71 and 5.76 g. After an acclimatization period of 30 days, average fish weights increased to 6.46–6.47 g, reaching 10.93–10.97 g at the end of the trial. Average daily gain ratios (ADGR) ranged from 0.023 and 0.025 g day1at the initial period to 0.074–0.081 at the end, with a global value for the full period of 0.057 g day1. Specific growth ratios(SGR)recordedatthesamedatesvariedfrom0.383to 0.408–0.749–0.833% day1, with global values of 0.707–713% day1. Feed amounts supplied to the animals were increased as they grew, so that during the first month they were given 195 g, in the second 359 g and in the third 420 g, which meant a total consumption of 974 g per tank. According to the feed consumption and the weight gained by the fish, the feed conversion ratios (FCR) were calculated, with values in the range 2.22–2.32 during the first 30 days and 1.48–1.61 at the end of the trial, with global values of 1.53–1.56. The mortality rate of the fish was very low, since only one animal died in one of the three tanks studied in this trial. As happened with the water quality parameters between the three tanks shown above, there was a great uniformity in the results obtained for the different parameters related to the growth of fish in the three tanks. No discordant data were found for average weights at different ages, average daily growth rate, specific growth rate; mortality, or feed conversion ratios. 3.3.Strawberry production Almost all the plants in the trial were able to produce fruits during the study period, as it can be observed in Table 2. Total production per plant varied between 0 and 105 g, where zero values indicate that the plants did not produce fruits at an early stage. Taking into account that thetotalareaoccupiedbythecropwas7.2 m2(includingthat V.M. Fern ́andez-Caban ́as et al.