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The following discussion reveals a number of key pros and challenges of coupled aquaponics as follows:
Pro: Coupled aquaponic systems have many food production benefits, especially saving resources under different production scales and over a wide range of geographical regions. The main purpose of this production principle is the most efficient and sustainable use of scarce resources such as feed, water, phosphorous as a limited plant nutrient and energy. Whilst, aquaculture and hydroponics (as stand-alone), in comparison to aquaponics are more competitive, coupled aquaponics may have the edge in terms of sustainability and thus a justification of these systems especially when seen in the context of, for example, climate change, diminishing resources, scenarios that might change our vision of sustainable agriculture in future.
Pro: Small-scale and backyard-coupled aquaponics are meant to support local and community-based food production by households and farmers. They are not able to stem high investment costs and require simple and efficient technologies. This applies for tested fish and plant combinations in coupled aquaponics.
Fig. 7.15 Development of coupled aquaponic systems from (a) domestic waste constructed wetlands (CW) and (b) CW in combination with recirculating aquaculture systems (RAS) to (c) hydroponic units in coupled aquaponic systems
Pro: The plants in contemporary coupled aquaponics have the similar role in treating waste as constructed wetlands do in the removal of waste from water (Fig. 7.15). The plants in the hydroponic unit in coupled aquaponics therefore fulfil the task of purifying the water and can be considered a 'biological advanced unit of water purification' in order to reduce the environmental impact of aquaculture.
Challenge: It has been widely accepted that using only fish feed as the input for plant nutrition is often qualitatively and quantitatively insufficient in comparison to conventional agriculture production systems (e.g. N-P-K hydroponics manure) (Goddek et al. 2016), limiting the growth of certain crops in coupled aquaponics.
Pro: Coupled aquaponic systems have a positive influence on fish welfare. Most recent studies demonstrate that in combination with cucumber and basil, the agonistic behaviour of African catfish (C. gariepinus) was reduced (Baßmann et al. 2017, 2018). More importantly, comparing injuries and behavioural patterns with the control, aquaponics with high basil density influenced African catfish even more positively. Plants release substances into the process water like phosphatases (Tarafdar and Claassen 1988; Tarafdar et al. 2001) that are able to hydrolyse biochemical phosphate compounds around the root area and exude organic acids (Bais et al. 2004). Additionally, microorganisms on the root surfaces play an important role through the excretion of organic substances increasing the solubilization of minerals making them available for plant nutrition. It is evident that the environment of the rhizosphere, the 'root exudate', consists of many organic compounds such as organic acid anions, phytosiderophores, sugars, vitamins, amino acids, purines, nucleosides, inorganic ions, gaseous molecules, enzymes and root border cells (Dakora and Phillips 2002), which may influence the health of aquatic organisms in coupled aquaponic systems. This symbiotic relationship is not available in either pure aquaculture or decoupled aquaponics. However, considerable research still needs to be undertaken to understand the responsible factors for better fish welfare.
Pro: Aquaponics can be considered as an optimized form of the conventional agricultural production especially in those areas where production factors caused by the environmental conditions are particularly challenging, e.g. in deserts or highly populated urban areas (cities). Coupled aquaponic systems can be easily adjusted to the local conditions, in terms of system design and scale of operation.
Challenge: Coupled aquaponic also show disadvantages, due to often unsuitable component ratio conditions of the fish and plant production. In order to avoid consequences for fish welfare, coupled aquaponic systems must balance the feed input, stocking density as well as size of the water treatment units and hydroponics. So far knowledge of component ratios in coupled aquaponics is still limited, and modelling to overcome this problem is at the beginning. Rakocy (2012) suggested 57 g of feed/day per square meter of lettuce growing area and a composite ratio of 1 msup3/sup of fish-rearing tank to 2 msup3/sup of pea gravel that allows a production of 60 kg / msup3/sup tilapia. Based on the UVI-system, the size ratios themselves were perceived as a disadvantage since a relatively large ratio of plant growing area to fish surface area of at least 7:3 must be achieved for adequate plant production. On the other hand, system designs of coupled systems are highly variable, often not comparable, and the experiences made cannot be easily transferred to another system or location. Consequently, far more research data is needed in order to identify the best possible production ratios finally also enabling upscaling of coupled aquaponic systems through multiplying optimal designed basic modules (also see Chap. 11).
Challenge: Adverse water quality parameters have been stated to negatively affect fish health. As Yavuzcan Yildiz et al. (2017) pointed out, nutrient retention of plants should be maximized to avoid negative effects of water quality on fish welfare. It is important to select adequate fish species that can accept higher nutrient loads, such as the African catfish (C. gariepinus) or the Nile Tilapia (O. niloticus,). More sensible species such as the Zander or pikeperch (Sander lucioperca) might be also applied in aquaponics because they prefer nutrient enriched or eutrophic water bodies with higher turbidity (Jeppesen et al. 2000; Keskinen and Marjomäki 2003; [see Sect. 7.7.1. Fish production]). So far, there is scant data allowing precise statements on fish welfare impairments. With plants generally needing high potassium concentrations between 230 and 400 mg/L inside the process water, 200—400 mg/L potassium showed no negative influence on African catfish welfare (Presas Basalo 2017). Similarly, 40 and 80 mg/L orthoP in the rearing water had no negative impact on growth performance, feed efficiency and welfare traits of juvenile African catfish (Strauch et al. 2019).
Challenge: Another issue is the potential transmission of diseases in terms of food safety, to people through the consumption of plants that have been in contact with fish waste. In general, the occurrence of zoonoses is minor because closed aquaponics are fully controlled systems. However, germs can accumulate in the process water of the system components or in the fish gut. Escherichia coli and Salmonella spp. (zoonotic enteric bacteria) were identified as indicators of faecal contamination and microbial water quality, however, they were detected in aquaponics only in very small quantities (Munguia-Fragozo et al. 2015). Another comparison of smooth-textured leafy greens between aquaponics, hydroponics and soil-based production showed no significant differences in aerobic plate counts (APC, aerobic bacteria), Enterobacteriaceae, non-pathogenic E. coli and Listeria, suggesting a comparable contamination level with pathogens (Barnhart et al. 2015). Listeria spp. was most frequent (40%) in hydroponics with de-rooted plants (aquaponic plants with roots 0%, aquaponic plants without roots <10%), but not necessarily the harmful L. monocytogenes species. It was suggested that the source of the bacteria may be due to the lack of hygiene management, with little relevance to aquaponics as such. Another infectious bacterium, Fusobacteria (Cetobacterium), was detected by Schmautz et al. (2017) in the fish faeces with a high prevalence of up to 75%. Representatives of Fusobacteria are responsible for human diseases (hospital germ, abscesses, infections), reproducing in biofilms or as part of the fish intestines. Human infections with Fusobacteria from aquaponics have not yet been recorded but may be possible by neglecting the required hygiene protocols.
In general, there is rather little information about diseases caused by the consumption of fish and plants originating from coupled aquaponic systems. In Wilson (2005), Dr. J.E. Rakocy stated that there was no recorded human disease outbreak in 25 years of coupled aquaponic production. However, a washing procedure of the plant products should be used to reduce the number of bacteria as a precaution. A chlorine bath (100 ppm) followed by a potable water rinse was recommended by Chalmers (2004). If this methodology is used and the contact of the plants or plant products with the recirculating process water is avoided, the likelihood of contamination with human pathogenic bacteria can be strongly reduced. This is a necessary precaution not only for coupled but also for all other forms of aquaponics.