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Because there are two separate, existing, analogous technologies that produce fish and plants at high rates (RAS fish culture and hydroponic/substrate culture plant production), a reason for their integration seems pertinent. RAS produces fish at productive rates in terms of individual biomass gain, for the feed weight added, that rivals, if not betters, other aquaculture methods (Lennard 2017). In addition, the high fish densities that RAS allows lead to higher collective biomass gains (Rakocy et al. 2006; Lennard 2017). Hydroponics and substrate culture possess, within a controlled environment context, advanced production rates of plants that better most other agriculture and horticulture methods (Resh 2013). Therefore, initially, there is a requirement for aquaponics to produce fish and plants at rates that equal these two separate productive technologies; if not, then any loss of productive effort counts against any integration argument. If the productive rate of the fish and plants in an aquaponic system can equal, or better, the RAS and hydroponic industries, then a further case may be made for other advantages that may occur due to the integration process.
Standard hydroponics or substrate culture has been directly compared with aquaponics in terms of the plant growth rates of the two technologies. Lennard (2005) compared aquaponic system lettuce production to a hydroponic control in several replicated laboratory experiments. He demonstrated that aquaponic lettuce production was statistically lower in aquaponics (4.10 kg/msup2/sup) when compared to hydroponics (6.52 kg/msup2/sup) when a standard approach to media bed aquaponic system design and management was applied. However, he then performed a series of experiments that isolated specific parameters of the design (e.g. reciprocal vs constant hydroponic subunit water delivery, applied water flow rate to the hydroponic subunit and comparing different hydroponic subunits) or comparing specific management drivers (e.g. buffering methodologies and species and the overall starting nutrient concentrations) to achieve optimisation and then demonstrated that aquaponics (5.77 kg/msup2/sup) was statistically identical to hydroponic lettuce production (5.46 kg/msup2/sup) after optimisation of the aquaponic system based on the improvements suggested by his earlier experiments, the result suggesting that improvements to coupled or fully recirculating aquaponic designs can equal standard hydroponic plant production rates. Lennard (2005) also demonstrated fish survival, SGR, FCR and growth rates equal to those exhibited in standard RAS and extensive pond aquaculture for the fish species tested (Australian Murray Cod).
Pantanella et al. (2010) also demonstrated statistically similar lettuce production results within high fish density (5.7 kg/msup2/sup lettuce production) and low fish density (5.6 kg/msup2/sup lettuce production) aquaponic systems compared to a standard hydroponic control (6.0 kg/msup2/sup).
Lennard (Nichols and Lennard 2010) demonstrated statistically equal or better results for all lettuce varieties and almost all herb variety production tested in a nutrient film technique (NFT) aquaponic system when compared to a hydroponic NFT system within the same greenhouse.
Delaide et al. (2016) compared nutrient-complemented RAS production water (nutrients were added to match one water nutrient mixture and strength example by Rakocy — denoted as an aquaponic analogue), fully nutrient-complemented RAS production water (RAS production water with added hydroponic nutrient salts to meet a water nutrient mixture and strength as used for standard hydroponics — denoted as a decoupled analogue) and a hydroponic control (standard hydroponic nutrient solution) in terms of plant growth rate and showed the aquaponic water analogue equalled the hydroponic control and the decoupled analogue water bettered the hydroponic control. However, it must be noted that these were not fully operating aquaponic systems containing fish (and the associated, full and active microbial content) that were compared, but simply water removed from an operating RAS and complemented, then compared to a hydroponic control water.
Rakocy and his UVI team have demonstrated with several studies that Tilapia spp. fish growth rates equal industry standards set by standard aquaculture production practices (Rakocy and Hargreaves 1993; Rakocy et al. 2004a, b, 2006, 2011).
These and other studies have demonstrated that aquaponics, no matter the configuration (coupled and decoupled), has the potential to produce plant production rates equal to, or better than, standard hydroponics and fish production rates of a similar standard to RAS. Therefore, the above discussed requirement for aquaponics to equal its industry analogues (RAS and hydroponics) seems to have been adequately proven, and therefore, the other advantages of aquaponics should be considered.
Efficient water use is regularly attributed to aquaponics. Lennard (2005) stated that the water savings associated with an optimised aquaponic test system (laboratory) were 90% or greater when compared to a standard RAS aquaculture control system where water was exchanged to control nitrate accumulations, whereas the plants in the aquaponics performed the same requirement. Therefore, he demonstrated that aquaponics provides a substantial water-saving benefit compared to standard RAS aquaculture. Interestingly, this 90% water-saving figure has subsequently been stated broadly within the global aquaponics community in a plant use context (e.g. aquaponics uses 90% less water than soil-based plant production (Graber and Junge 2009)) — an example of how scientific argument can be incorrectly adopted by nonscientific industry participants.
McMurtry (1990) demonstrated a water consumption rate in his aquaponic system of approximately 1% of that required in a similar pond culture system. Rakocy (1989) has demonstrated similar 1% water consumption rates compared to pond-based aquaculture. Rakocy and Hargreaves (1993) stated that the daily water replacement rate for the UVI aquaponic system was approximately 1.5% of the total system volume and Love et al. (2015a, b) stated an approximate 1% of system volume water loss rate per day for their aquaponic research system.
The comparison of aquaponics to RAS can realise substantial water savings and aquaponics uses small amounts of replacement water on a daily basis. A welldesigned aquaponic system will seek to use water as efficiently as possible and therefore only replace that water lost via plant evapotranspiration (Lennard 2017). In fact, it has been proposed that water may even be recovered from that lost due to plant evapotranspiration via employing some form of air water content harvesting scheme or technology (Kalantari et al. 2017). Coupled aquaponic systems appear to provide a greater potential to conserve and lower water use (Lennard 2017). If the nutrient dynamics between fish production and plant use can be balanced, the only water loss is via plant evapotranspiration, and because the water is integrally shared between the fish and plant components, daily makeup water volumes simply represent all the water lost from the system plants (Lennard 2017). Decoupled aquaponic designs present a more difficult proposition because the two components are not integrally linked and the daily water use of the fish component does not match the daily water use of the plant component (Goddek et al. 2016; Goddek and Keesman 2018). Therefore, water use and replacement rates for aquaponic systems are not completely resolved and probably never will be due to the broad differences in system design approaches.
Efficient nutrient utilisation is assigned to the aquaponic method and cited as an advantage of the aquaponic approach (Rakocy et al. 2006; Blidariu and Grozea 2011; Suhl et al. 2016; Goddek et al. 2015). This is generally because standard RAS aquaculture utilises the nutrients within the fish feed to grow the fish, with the remainder being sent to waste. Fish metabolise much of the feed they are fed, but only utilise approximately 25—35% of the nutrients added (Timmons et al. 2002; Lennard 2017). This means up to 75% of the nutrients added to fish-only RAS are wasted and not utilised. Aquaponics seeks to use the nutrients wasted in RAS for plant production, and therefore, aquaponics is said to use the nutrients added more efficiently because two crops are produced from the one input source (Rakocy and Hargreaves 1993; Timmons et al. 2002; Rakocy et al. 2006; Lennard 2017). The extent of fish waste nutrient use does differ between the various aquaponic methods. The fully recirculating UVI model does not utilise the majority of the solid fish wastes generated in the fish component and sends them to waste (Rakocy et al. 2006), the fully recirculating Lennard model takes this a step further by using all the wastes generated by the fish component (dissolved wastes directly and solids via external microbial remineralisation with main system replacement) (Lennard 2017). Many decoupled approaches also attempt to utilise all the wastes generated by the fish component, via direct use of dissolved wastes and again, via external microbial remineralisation with main system replacement (Goddek et al. 2016; Goddek and Keesman 2018). All of these methods and approaches demonstrate that a primary driver for the aquaponic method is to utilise as many of the added nutrients as possible and therefore attempt to use the added nutrients as efficiently as possible.
Independence from soil has been cited as an advantage of the aquaponic method (Blidariu and Grozea 2011; Love et al. 2015a, b). The advantage perceived is that because soil is not required, the aquaponic system or facility may be located where the operator chooses, rather than where suitable soil is present (Love et al. 2015a, b). Therefore, the aquaponic method is independent of location based on soil availability, which is an advantage over soil-based agriculture.
It has been argued that aquaponics provides an advantage by mimicking natural systems (Blidariu and Grozea 2011; Love et al. 2014). This is supported by the ecological nature of the aquaponic approach/method, as outlined in Sect. 5.7 above, with the associated advantages related to diverse and dense microfloral communities (Lennard 2017).
Aquaculture has a potential direct environmental impact due to the release of nutrient-rich waste waters to the surrounding environment — generally, aquatic environments (Boyd and Tucker 2012). Some hydroponic methods may also possess this potential. However, aquaponics can exhibit lowered or negated direct environmental impact from nutrient-rich waste streams because the main waste-generating component (i.e. the fish) is integrated with a nutrient use component (i.e. the plants) (Rakocy et al. 2006; Blidariu and Grozea 2011; Goddek et al. 2015; Lennard 2017). However, some aquaponic methods do produce wastes (e.g. the UVI model); but these are generally treated and reused for other agricultural practices at the aquaponic facility site (Timmons et al. 2002; Rakocy et al. 2006). Many aquaponic methods rely on the use of standard aquaculture feeds, which contain varying concentrations of sodium, usually via the use of fish meal or fish oil as an ingredient (Timmons et al. 2002). Sodium is not utilised by plants and therefore may accumulate over time in aquaponic systems, which may lead to a requirement for some form of water replacement so sodium does not accumulate to concentrations that affect the plants (Lennard 2017). However, it has been reported that some species of lettuce had the ability to take up sodium, when being exposed to aquaculture water (Goddek and Vermeulen 2018).
Coupled or fully recirculating aquaponic systems integrally share the water resource between the two main components (fish and plant). Because of this fully connected and recirculating aquatic nature, coupled aquaponic systems exhibit a self-controlling mechanism in terms of an inability to safely apply herbicides and pesticides to the plants; if they are applied, their presence may negatively affect the fish (Blidariu and Grozea 2011). Fully recirculating advocates see this inability to applying pesticides and herbicides as an advantage, the argument being that it guarantees a spray-free product (Blidariu and Grozea 2011). Decoupled aquaponics advocates also seek to not apply herbicides or pesticides; however, due to the fact that the water is not recirculated back to the fish from the plants, the ability to apply pesticides and herbicides to the plants is present (Goddek 2017). Therefore, the application or lack of application of pesticides and herbicides to the plant component of aquaponic designs is seen differently by groups who advocate for different design approaches.
There is a perception that the presence of both fish and plants in the same aquatic system provides positive synergistic effects to fish and plant health (Blidariu and Grozea 2011). This has been indirectly demonstrated by the ability of aquaponics in some studies to produce plant growth rates greater than those seen in standard hydroponics (Nichols and Lennard 2010; Delaide et al. 2016). However, no direct causal link has been established between the presence of both fish and plants and any positive outcome to fish or plant health.