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In larger scale commercial aquaponics fish and plant production need to meet market demands. Fish production allows species variation, according to the respective system design and local markets. Fish choice also depends on their impact onto the system. Problematic coupled aquaponics fish production due to inadequate nutrient concentrations, negatively affecting fish health, can be avoided. If coupled aquaponic systems have balanced fish to plant ratios, toxic nutrients will be absorbed by the plants that are cleaning the water. Since acceptance of toxic substances is species dependent, fish species choice has a decisive influence on the economic success. Therefore, it is important to find the right combination and ratio between the fish and the plants, especially of those fish species with less water polluting activities and plants with high nutrient retention capacity.
The benefits of having a particular fish family in coupled aquaponic systems are not clearly understood with respect to their specific needs in terms of water quality and acceptable nutrient loads. Naegel (1977) found there was no notable negative impact on the fish and fish growth in his use of Tilapia (Tilapia mossambica) and common carp (Cyprinus carpio). The channel catfish (Ictalurus punctatus) was also used by Lewis et al. (1978) and Sutton and Lewis (1982) in the USA. It was demonstrated that the quality of the aquaponics water readily met the demands of the different fish species, especially through the use of 'easy-to-produce'fish species such as the blue Tilapia (Oreochromis aureus, formerly Sarotherodon aurea) in Watten and Busch (1984); Nile tilapia (Oreochromis niloticus), which was often used in studies with different plant species as a model fish species (Rakocy 1989; Rakocy et al. 2003, 2004; Al-Hafedh et al. 2008; Rakocy 2012; Villarroel et al. 2011; Simeonidou et al. 2012; Palm et al. 2014a, 2014b; Diem et al. 2017); and also Tilapia hybrids-red strain (Oreochromis niloticus x blue tilapia O. aureus hybrids), that were investigated in arid desert environments (Kotzen and Appelbaum 2010; Appelbaum and Kotzen 2016).
There has been an expansion in the types of fish species used in aquaponics, at least in Europe, which is based on the use of indigenous fish species as well as those that have a higher consumer acceptance. This includes African catfish (Clarias gariepinus) which was grown successfully under coupled aquaponic conditions by Palm et al. (2014b), Knaus and Palm (2017a) and Baßmann et al. (2017) in northern Germany. The advantage of C. gariepinus is a higher acceptance of adverse water parameters such as ammonium and nitrate, as well as there is no need for additional oxygen supply due to their special air breathing physiology. Good growth rates of C. gariepinus under coupled aquaponic conditions were further described in Italy by Pantanella (2012) and in Malaysia by Endut et al. (2009). An expansion of African catfish production under coupled aquaponics can be expected, due to unproblematic production and management, high product quality and increasing market demand in many parts of the world.
In Europe, other fish species with high market potential and economic value have recently become the focus in aquaponic production, with particular emphasis on piscivorous species such as the European pikeperch 'zander' (Sander lucioperca). Pikeperch production, a fish species that is relatively sensitive to water parameters, was tested in Romania in coupled aquaponics. Blidariu et al. (2013a, b) showed significantly higher Psub2/subOsub5/sub (phosphorous pentoxide) and nitrate levels in lettuce (Lactuca sativ) using pikeperch compared to the conventional production, suggesting that the production of pikeperch in coupled aquaponics is possible without negative effects on fish growth by nutrient toxicity. The Cyprinidae (Cypriniformes) such as carp have been commonly used in coupled aquaponics and have generally shown better growth with reduced stocking densities and minimal aquaponic process water flow rates (efficient water use) during experiments in India. The optimal stocking density of koi carp (Cyprinus carpio var. koi) was at 1.4 kg/m (Hussain et al. 2014), and the best weight gain and yield of Beta vulgaris var. bengalensis (spinach) was found with a water flow rate of 1.5 L/min (Hussain et al. 2015). Good fish growth and plant yield of water spinach (Ipomoea aquatica) with a maximum percentage of nutrient removal (NOsub3/sub-N, POsub4/sub-P, and K) was reported at a minimum water flow rate of 0.8 L/min with polycultured koi carp (Cyprinus carpio var. koi) and gold fish (Carassius auratus) by Nuwansi et al. (2016). It is interesting to note that plant growth and nutrient removal in koi (Cyprinus carpio var. koi) and gold fish (Carassius auratus) production (Hussain et al. 2014, 2015) with Beta vulgaris var. bengalensis (spinach) and water spinach (Ipomoea aquatica) increased linearly with a decrease in process water flow between 0.8 L/min and 1.5 L/min. These results suggest that for cyprinid fish culture, lower water flow is recommended as this has no negative impacts on fish growth. In contrast, however, Shete et al. (2016) described a higher flow rate of 500 L hsup-1/sup (approx. 8 L/min) for common carp and mint (Mentha arvensis) production, indicating the need for different water flow rates for different plant species. Another cyprinid, the tench (Tinca tinca), was successfully tested by Lobillo et al. (2014) in Spain and showed high fish survival rates (99.32%) at low stocking densities of 0.68 kg msup-3/sup without solids removal devices and good lettuce survival rates (98%). Overall, members of the Cyprinidae family highly contribute to the worldwide aquaculture production (FAO 2017); most likely this would also be true under aquaponic conditions and productivity, but the economic situation should be tested for each country separately.
Other aquatic organisms such as shrimp and crayfish have been introduced into coupled aquaponic production. Mariscal-Lagarda et al. (2012) investigated the influence of white shrimp process water (Litopenaeus vannamei) on the growth of tomatoes (Lycopersicon esculentum) and found good yields in aquaponics with a twofold water sparing effect under integrated production. Another study compared the combined semi-intensive aquaponic production of freshwater prawns (Macrobrachium rosenbergii — the Malaysian shrimp) with basil (Ocimum basilicum) versus traditional hydroponic plant cultivation with a nutrient solution (Ronzón-Ortega et al. 2012). However, basil production in aquaponics was initially less effective (25% survival), but with increasing biomass of the prawns, the plant biomass also increased so that the authors came to a positive conclusion with the production of basil with M. rosenbergii. Sace and Fitzsimmons (2013) reported a better plant growth in lettuce (Lactuca sativa), Chinese cabbage (Brassica rapa pekinensis) and pakchoi (Brassica rapa) with M. rosenbergii in polyculture with the Nile Tilapia (O. niloticus). The cultivation with prawns stabilized the system in terms of the chemical-physical parameters, which in turn improved plant growth, although due to an increased pH, nutrient deficiencies occurred in the Chinese cabbage and lettuce. In general, these studies demonstrate that shrimp production under aquaponic conditions is possible and can even exert a stabilizing effect on the closed loop — or coupled aquaponic principle.
The cultivation of many species of plants, herbs, fruiting crops and leafy vegetables have been described in coupled aquaponics. In many cases, the nutrient content of the aquaponics process water was sufficient for good plant growth. A review by Thorarinsdottir et al. (2015) summarized information on plant production under aquaponic production conditions from various sources. Lettuce (Lactuca sativa) was the main cultivated plant in aquaponics and was often used in different variations such as crisphead lettuce (iceberg), butterhead lettuce (bibb in the USA), romaine lettuce and loose leaf lettuce under lower night (3—12 ˚C) and higher day temperatures (17—28 ˚C) (Somerville et al. 2014). Many experiments were carried out with lettuce in aquaponics (e.g. Rakocy 1989) or as a comparison of lettuce growth between aquaponics, hydroponics and complemented aquaponics (Delaide et al. 2016). Romaine lettuce (Lactuca sativa longifolia cv. Jericho) was also investigated by Seawright et al. (1998) with good growth results similar to standalone hydroponics and an increasing accumulation of K, Mg, Mn, P, Na and Zn with increasing fish biomass of Nile Tilapia (Oreochromis niloticus). Fe and Cu concentrations were not affected. Lettuce yield was insignificant with different stocking densities of fish (151 g, 377 g, 902 g, 1804 g) and plant biomass between 3040 g (151 g fish) and 3780 g (902 g fish). Lettuce was also cultivated, e.g. by Lennard and Leonard (2006) with Murray Cod (Maccullochella peelii peelii), and by Lorena et al. (2008) with the sturgeon 'bester' (hybrid of Huso huso female and Acipenser ruthenus male) and by Pantanella (2012) with Nile tilapia (O. niloticus). As a warm water crop, basil (Ocimum basilicum) was reported as a good herb for cultivation under coupled aquaponics and was reported as the most planted crop by 81% of respondents in findings of an international survey (Love et al. 2015). Rakocy et al. (2003) investigated basil with comparable yields under batch and staggered production (2.0; 1.8 kg/msup2/sup) in contrast to field cultivation with a comparatively low yield (0.6 kg/msup2/sup). Somerville et al. (2014) described basil as one of the most popular herbs for aquaponics, especially in large-scale systems due to its relatively fast growth and good economic value. Different cultivars of basil can be grown under higher temperatures between 20 and 25 ˚C in media beds, NFT (nutrient film technique) and DWC (deep water culture) hydroponic systems. Basil grown in gravel media beds can reach 2.5-fold higher yield combined with tilapia juveniles (O. niloticus, 0.30 g) in contrast to C. gariepinus (0.12 g) (Knaus and Palm 2017a).
Tomatoes (Lycopersicon esculentum) were described by Somerville et al. (2014) as an 'excellent summer fruiting vegetable' in aquaponics and can cope with full sun exposure and temperatures below 40 ˚C depending on tomato type. However, economic sustainability in coupled aquaponics is disputed due to the reduced competitiveness of aquaponics tomato production compared to high-engineered conventional hydroponic production in greenhouses in, e.g. the Netherlands Improvement Centre of DLV GreenQ in Bleiswijk with tomato yield of 100.6 kg msup-2/sup (Hortidaily 2015), or even higher (Heuvelink 2018). Earlier investigations focused on the cultivation of this plant mostly compared to field production. Lewis et al. (1978) reported nearly double the crop of tomatoes under aquaponics compared to field production and the iron deficiency which occurred was fixed by using ethylene diamine tetra-acetic acid. Tomatoes were also produced in different aquaponic systems over the last decades, by Sutton and Lewis (1982) with good plant yields at water temperatures up to 28 ˚C combined with Channel catfish (Ictalurus punctatus), by Watten and Busch (1984) combined with tilapia (Sarotherodon aurea) and a calculated total marketable tomato fruit yield of 9.6 kg/msup2/sup, approximately 20% of recorded yields for decoupled aquaponics (47 kg/msup2/sup/y, Geelen 2016). McMurtry et al. (1993) combined hybrid tilapia (Oreochromis mossambicus x Oreochromis niloticus) with tomatoes in associated sand biofilters which showed optimal 'plant yield/high total plant yield' of 1:1.5 tank/biofilter ratio (sand filter bed) and McMurtry et al. (1997) with increasing total plant fruit yield with increasing biofilter/tank ratio. It must be stated that the production of tomatoes is possible under coupled aquaponics. Following the principle of soilless plant cultivation in aquaponics sensu stricto after Palm et al. (2018), it is advantageous to partially fertilize certain nutrients such as phosphorous, potassium or magnesium to increase yields (see challenges below).
The cultivation of further plant species is also possible and testing of new crops is continuously being reported. In the UK, Kotzen and Khandaker have tested exotic Asian vegetables, with particular success with bitter gourd, otherwise known as kerala or bitter melon (Momordica charantia) (Kotzen pers. comm.). Taro (Colocasia esculenta) is another species which is readily grown with reported success both for its large 'elephant ear' like leaves as well as its roots (Kotzen pers. comm.). Somerville et al. (2014) noted that crops such as cauliflower, eggplant, peppers, beans, peas, cabbage, broccoli, Swiss chard and parsley have the potential for cultivation under aquaponics. But there are many more (e.g. celery, broccoli, kohlrabi, chillies, etc.) including plants that prefer to have wet root conditions, including water spinach (Ipomoea aquatica) and mint (Menta sp.) as well as some halophytic plants, such as marsh samphire (Salicornia europaea).
Ornamental plants can also be cultivated, alone or together with other crops (intercropping), e.g. Hedera helix (common ivy) grown at the University of Rostock by Palm & Knaus in a coupled aquaponic system. The trials used 50% less nutrients that would be normally supplied to the plants under normal nursery conditions with a 94.3% success rate (Fig. 7.10).
Fig. 7.10 Three quality categories of ivy (Hedera helix), grown in a coupled aquaponic system indicating the quality that the nursery trade requires (a) very good and directly marketable, (b) good and marketable and (c) not of high enough quality
Besides the chosen plant and variant, there are two major obstacles that concern aquaponics plant production under the two suggested states of fish production, extensive and intensive. Under extensive conditions, nutrient availability inside the process water is much lower than under commercial plant production, nutrients such as K, P and Fe are deficient, and the conductivity is between 1000 and 1500 μS / cm, which is much less than applied under regular hydroponic production of commercial plants regularly between 3000 and 4000 μS / cm. Plants that are deficient in some nutrients can show signs of leaf necroses and have less chlorophyll compared with optimally fertilized plants. Consequently, selective addition of some nutrients increases plant quality that is required to produce competitive products.
In conclusion, commercial plant production of coupled aquaponics under intensive fish production has the difficulty to compete with regular plant production and commercial hydroponics at a large scale. The non-optimal and according to Palm et al. (2019) unpredictable composition of nutrients caused by the fish production process must compete against optimal nutrient conditions found in hydroponic systems. There is no doubt that solutions need to be developed allowing optimal plant growth whilst at the same time providing the water quality required for the fish.
Combining fish and plants in closed aquaponics can generate better plant growth (Knaus et al. 2018b) combined with benefits for fish welfare (Baßmann et al. 2017). Inside the process water, large variations in micronutrients and macronutrients may occur with negative effects on plant nutritional needs (Palm et al. 2019). A general analysis of coupled aquaponic systems has shown that there are low nutrient levels within the systems (Bittsanszky et al. 2016) in comparison with hydroponic nutrient solutions (Edaroyati et al. 2017). Plants do not tolerate an under or oversupply of nutrients without effects on growth and quality, and the daily feed input of the aquaponic system needs to be adjusted to the plant's nutrient needs. This can be achieved by regulating the stocking density of the fish as well as altering the fish feed. Somerville et al. (2014) categorized plants in aquaponics according to their nutrient requirements as follows:
Plants with low nutrient requirements (e.g. basil, Ocimum basilicum)
Plants with medium nutritional requirements (e.g. cauliflower, Brassica oleracea var. Botrytis)
Plants with high nutrient requirements such as fruiting species (e.g. strawberries, Fragaria spec.).
Not all plants can be cultured in all hydroponic subsystems with the same yield. The plant choice depends on the hydroponic subsystem if conventional soilless aquaponic systems (e.g. DWC, NFT, ebb and flow; aquaponics sensu stricto' — s.s. — in the narrow sense) are used. Under aquaponics farming ('aquaponics sensu lato' — s.l. — in a broader sense, Palm et al. 2018), the use of inert soil or with addition of fertilizer applies gardening techniques from horticulture, increasing the possible range of species.
Under hydroponic conditions, the component structures of the subsystems have a decisive influence on plant growth parameters. According to Love et al. (2015), most aquaponic producers used raft and media bed systems and to a smaller amount NFT and vertical towers. Lennard and Leonard (2006) studied the growth of Green oak lettuce (Lactuca sativa) and recorded the relationship Gravel bed > Floating raft > NFT in terms of biomass development and yield in combination with the Murray Cod (Maccullochella peelii peelii) in Australia. Knaus & Palm (2016—2017, unpublished data) have tested different hydroponic subsystems such as NFT, floating raft and gravel substrate on the growth of different plants in the FishGlassHouse in a decoupled aquaponic experimental design, requiring subsequent testing under coupled conditions. With increasing production density of African catfish (C. gariepinus, approx. 20—168 kg/msup3/sup), most of the cultured crops such as cucumbers (Cucumis sativus), basil (Ocimum basilicum) and pak choi (Brassica rapa chinensis) tended to grow better, in contrast to Lennard and Leonard (2006), in gravel and NFT aquaponics (GRAVEL > NFT > RAFT; Wermter 2016; Pribbernow 2016; Lorenzen 2017), and Moroccan mint 'spearmint' (Mentha spicata) showed the opposite growth performance (RAFT = NFT > GRAVEL) with highest leaf numbers in NFT (Zimmermann 2017). This demonstrates an advantage of gravel conditions and can be used figuratively also in conventional plant pots with soil substrate under coupled aquaponic conditions. This type of aquaponics was designated as 'horticulture — aquaponics (s.l.)' due to the use of substrates from the horticultural sector (soil, coco fibre, peat, etc.) (see Palm et al. 2018). This involves all plant cultivation techniques that allow plants to grow in pots, whereby the substrate in the pot itself may be considered equivalent to a classical gravel substrate for aquaponics. Research by Knaus & Palm (unpublished data) showed variance in the quality of commonly grown vegetables and thus their suitability for growing in this type of aquaponics with soil (Fig. 7.11, Table 7.1). In this type of aquaponics, beans, lambs lettuce and radish did well.
Fig. 7.11 Experiments with a variety of commonly grown vegetables, under winter conditions in winter 2016/2017 in the FishGlassHouse (University of Rostock, Germany)
Table 7.1 Recommendation for the use of gardening plants in aquaponic farming with the use of 50% of the regular fertilizer in pots with soil
table thead tr class="header" thName/th thLat. Name/th thPossible for aquaponics/th thMark/th thNutrient regime/th /tr /thead tbody tr class="odd" tdBeans/td td iPhaseolus vulgaris/i /td tdYes/td td1/td tdExtensive/td /tr tr class="even" tdPeas/td td iPisum sativum/i /td tdNo/td td2/td tdIntensive/td /tr tr class="odd" tdBeet/td td iBeta vulgaris/i /td tdNo/td td2/td tdBoth/td /tr tr class="even" tdTomatoes/td td iSolanum lycopersicum/i /td tdNo/td td2.3/td tdBoth/td /tr tr class="odd" tdLamb’s lettuce/td td iValerianella locusta/i /td tdYes/td td1/td tdBoth/td /tr tr class="even" tdRadish/td td iRaphanus sativus/i /td tdYes/td td1/td tdBoth/td /tr tr class="odd" tdWheat/td td iTriticum aestivum/i /td tdNo/td td2/td tdBoth/td /tr tr class="even" tdLettuce/td td iLactuca sativa/i /td tdYes/td td1/td tdIntensive/td /tr /tbody /table
The plant choice (species and strain) and especially the hydroponic subsystem and/or substrate, including peat, peat substitutes, coco fibre, composts, clay, etc. or a mix of them (see Somerville et al. 2014), has a significant impact on the economic success of the venture. The efficiency of some substrates must be tested in media bed hydroponic sub-units (e.g. the use of sand (McMurtry et al. 1990, 1997), gravel (Lennard and Leonard 2004) and perlite (Tyson et al. 2008). The use of other media bed substrates such as volcanic gravels or rock (tuff/tufa), limestone gravel, river bed gravel, pumice stone, recycled plastics, organic substrates such as coconut fibre, sawdust, peat moss and rice trunk have been described by Somerville et al. (2014). Qualitative comparative studies with recommendations, however, are very rare and subject of future research.
The combination of different aquatic organisms in a single aquaponic system can increase total yields. First applied by Naegel (1977), this multispecies production principle was coined from the term polyculture combined with aquaponics in coupled systems as 'polyponic' (polyculture + aquaponics) by Knaus and Palm (2017b). Like IMTA (integrated multitrophic aquaculture), polyponics expands the diversity of the production systems. Using multiple species in one system has both advantages and disadvantages as (a) diversification allows the producer to respond to local market demands but (b) on the other hand, focus is spread across a number of products, which requires greater skill and better management. Published information on polyponics is scarce. However, Sace and Fitzsimmons (2013) reported better plant growth of lettuce, Chinese cabbage and pakchoi in polyculture with freshwater shrimp (Macrobrachium rosenbergii) and Nile tilapia (O. niloticus) in coupled aquaponics. Alberts-Hubatsch et al. (2017) described the cultivation of noble crayfish (Astacus astacus), hybrid striped bass (Morone saxatilis x M. chrysops), microalgae (Nannochloropsis limnetica) and watercress (Nasturtium officinale), where crayfish growth was higher than expected, feeding on watercress roots, fish faeces and a pikeperch-designed diet.
Initial investigations at the University of Rostock showed differences in plant growth in two identical 25msup2/sup backyard-coupled aquaponic units with the production of African catfish (Clarias gariepinus) and Nile Tilapia (Oreochromis niloticus, Palm et al. 2014b). The plant yields of lettuce (Lactuca sativa) and cucumber fruits (Cucumis sativus) were significantly better in combination with O. niloticus. This effect was also seen by Knaus and Palm (2017a) with a 2.5-fold higher yield in basil (Ocimum basilicum) and two times more biomass of parsley (Petroselinum crispum) combined with O. niloticus. Another comparison between O. niloticus and common carp (Cyprinus carpio) showed a twofold higher gross biomass per plant (g plantsup-1/sup) of tomatoes (Solanum lycopersicum) with tilapia and a slightly increased gross biomass of cucumbers (Cucumis sativus) with carp, however, with higher cucumber fruit weight in the O. niloticus aquaponic unit (Knaus and Palm 2017b). The yield of mint (Mentha x piperita) was approximately 1.8 times higher in the tilapia unit, but parsley was 2.4 times higher combined with the carp (Knaus et al. 2018a). The results of these experiments followed the order of plant growth: O. niloticus > C. carpio > C. gariepinus, whilst fish growth showed a reverse order with: C. gariepinus > O. niloticus > C. carpio.
According to these results, the fish choice influences the plant yield and a combination of different fish species and their respective growth performance allows adjustment of a coupled aquaponics to optimal fish and plant yields. During consecutive experiments (O. niloticus only, C. gariepinus only), a higher basil (O. basilicum) biomass yield of 20.44% (Plant Growth Difference — PGD) was observed for O. niloticus in contrast to the basil yield with C. gariepinus (Knaus et al. 2018b). Thus, O. niloticus can be used to increase the plant yield in a general C. gariepinus system. This so-called boost effect by Tilapia enhances the overall system production output and compensates i) poorer plant growth with high fish growth of C. gariepinus as well as ii) poorer fish growth in O. niloticus with a boost to the plant yield. A first commercial polyponic farm has opened in Bali, Indonesia, producing tilapia combined with Asian catfish (Clarias batrachus) and conventional farm products.