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Aquaponics fits into the broader definition of integrated agri-aquaculture systems (IAAS). However, IAAS applies many different aquatic animal and plant production technologies in many contexts, whereas aquaponics is far more tightly associated with integrating tank-based fish culture technologies (e.g. recirculating aquaculture systems; RAS) with aquatic or hydroponic plant culture technologies (Lennard 2017). RAS technologies apply conserved and standard methods for the culture of fish in tanks with applied filtration to control and alter the water chemistry to make it suitable for fish (i.e. fast and efficient solid fish wastes removal, efficient, bacteriamediated conversion of potentially toxic dissolved fish waste ammonia to less toxic nitrate and oxygen maintenance via assisted aeration or directly injected oxygen gas) (Timmons et al. 2002). Hydroponics and substrate culture technologies apply conserved and standard methods for the culture of edible terrestrial plants within aquatic environments (i.e. the plants gain access to the nutrients required for growth via a water-based delivery method) (Resh 2013).
The association of aquaponics with standard RAS aquaculture and hydroponics/ substrate culture means that aquaponics is often defined simply as "... the combination of fish production (aquaculture) and soil-less plant cultivation hydroponics under coupled or decoupled water circulation" (Knaus and Palm 2017). This broad definition places an emphasis on the integration of hardware, equipment or technologies and places little, if any, emphasis on any other aspects of the method.
Because aquaponics is a relatively new industrial-scale technology that applies different methods and approaches, the applied definition appears very broad. Some define aquaponics within a recirculating context only (Cerozi and Fitzsimmons 2017), some concentrate on approaches that do not return the water from the plants to the fish (Delaide et al. 2016) and others include both recirculating and decoupled methods (Knaus and Palm 2017). Further still, some researchers are including the use of aquaculture effluents irrigated to soil-based crop production under the aquaponic title (Palm et al. 2018). Historically, aquaponics, as the breakdown of the word (aquaculture and hydroponics) suggests, was defined as only concerning aquaculture and hydroponic plant production (Rakocy and Hargreaves 1993), so current attempts at associations with soil-based culture seem incongruous.
Fig. 5.1 Schematic representation of the nutrient flows within an aquaponic system. Fish feed is the major nutrient entry point. The fish eat the feed, use what nutrients they need, release the rest as waste and this waste is then partitioned between the microbes, plants and system water. (adapted from Lennard 2017)
Whilst aquaponic systems do integrate tank-based aquaculture technologies with hydroponic plant culture technologies, aquaponic systems work by supplying nutrients to, and partitioning nutrients between, the production inhabitants (fish and plants) and the inhabitants that perform biological and chemical services that assist the production inhabitant outcome (microflora) (Fig. 5.1) (Lennard 2017). Therefore, is aquaponics more a system associated with nutrient supply, dynamics and partitioning rather than one associated with the technology, equipment or hardware applied?
Over the past decades, the definition of aquaponics has included a similar theme, with subtle variations. The broadest definition has generally been provided in the scientific publications of Rakocy and his UVI team, for example:
Aquaponics is the combined culture of fish and plants in closed recirculating systems.
— Rakocy et al. (2004a, b)
This early definition was based on the assumption that one-loop, fully recirculating systems, consisting of a recirculating aquaculture component and a hydroponic component, represented all aquaponic systems, which at the time, they did. Graber and Junge (2009) expanded the definition, due to changes and developments in the approach, as follows:
Aquaponic is a special form of recirculating aquaculture systems (RAS), namely a polyculture consisting of fish tanks (aquaculture) and plants that are cultivated in the same water circle (hydroponic).
— Graber and Junge (2009)
Recent developments and methods ask for a reconsideration of this standpoint. In recent years a shifting of the focus of aquaponics towards a production system that tackles both ecological responsibility and economic sustainability has been present. Kloas et al. (2015) and Suhl et al. (2016) were one of the first to address this economic consideration:
[...] a unique and innovative double recirculating aquaponic system was developed as a prerequisite for a high productivity comparable to professional stand-alone fish/plant facilities.
— Suhl et al. (2016)
The definition issue, or clarifying "what can be defined as aquaponics", has been a point of discussion over the past years. One of the main areas of development has been that of multi-loop (or decoupled) aquaponic systems that aim at providing additional fertilisers to the plants in order to expose them to an optimal nutrient concentration (Goddek 2017). There should be no opposition between the ideologies of fully recirculating and multi-loop aquaponic methodologies, both have their respective places and applications within the appropriate industrial context and a single common driving force of both should be that the technology, whilst being nutrient and water efficient, also needs to be economically competitive to establish itself in the market. In order to replace conventional practices, more than an ideology needs to be offered to potential clients/users — i.e. technical and economic feasibility.
The European COST sponsored Aquaponics Hub (COST FA1305 2017) applies the definition "...a production system of aquatic organisms and plants where the majority (> 50%) of nutrients sustaining the optimal plant growth derives from waste originating from feeding the aquatic organisms", which clearly places an emphasis on the nutrient sharing aspect of the technology.
It must also be stated that the proportion of fish to plants should remain at a level that supports a core prospect of aquaponics; that plants are grown using fish wastes. A system containing one fish and several hectares of hydroponic plant cultivation, for example, should not be considered as aquaponics, simply because that one fish effectively contributes nothing to the nutrient requirements of the plants. Since the labelling of aquaponic products plays an increasingly important role in consumer choice, we want to encourage a discussion by redefining aquaponics based on these multiple developments of the technology. Even though we advocate closing the nutrient cycle to the highest possible degree in the context of best practicable means, a potential definition should also take all developments into consideration.
Therefore, the definition should contain as a minimum, the requirement for a majority of aquaculture-derived nutrients for the plants. A new definition may therefore be represented as:
Aquaponics uis/u defined as an integrated multi-trophic, aquatic food production uapproach/u comprising uat least/u a recirculating aquaculture system (RAS) and a connected hydroponic unit, whereby the water for culture is shared in some configuration between the two units. uNot less than 50% of the nutrients provided to the plants should be fish waste derived/u.
Nutrient-based definitions are open and non-judgemental of the applied technology choice, or even the proportions of each component (fish and plants), as long as fish culture and some form of aquatic (hydroponic or substrate culture) plant production technology is utilised. However, it also focuses the definition on the nutrient dynamics and nutrient sharing aspects of the methods applied and therefore ensures, to at least some extent, that the advantages often associated with aquaponics (water saving, nutrient efficiency, lowered environmental impact, sustainability) are present in some proportion.
The nutrient association definition applied to aquaponics will always be a source of further contention among those who practise it. This is supported by the fact that the name aquaponics is applied to a vast array of different technologies with different nutrient supply motivations and usage outcomes: from system designs and methods that expect, if not demand, that the vast majority of the nutrients required to grow the plants arise from the fish wastes (in some cases, greater than 90%; Lennard 2017) to designs that share plant nutrient supplies between fish wastes and more substantial external additions (e.g. approximately 50:50 fish waste to external supplementation — as many modern, European decoupled aquaponic system designs do; COST FA1305 2017) to those designs that add so few fish that no discernible nutrient supply from the fish wastes to the plants is present (Lennard 2017).
The name aquaponics, until relatively recently (i.e. the last 3—5 years), has been universally applied to coupled and fully recirculating system designs that seek to supply as much of the required plant nutrition from the fish wastes as possible (Rakocy and Hargreaves 1993; Lennard 2017) (Fig. 5.2).
Fig. 5.2 Simplified scheme of the main water flows within a coupled aquaponic system. The nutrient concentrations in the process water are equally distributed throughout the whole system
However, decoupled approaches now represent a proportion of the systems being researched or commercially applied, especially in Europe, and in current practice do not supply plant nutrient requirements from the fish wastes to the same extent as fully recirculating systems do (Lennard 2017; Goddek and Keesman 2018). For example, Goddek and Keesman (2018) state that for 3 examples of current European decoupled aquaponic system designs, the relative addition requirements for external hydroponic-derived nutrients are 40—60% (NerBreen), 60% (Tilamur) and 38.1% (IGB Berlin). Because these decoupled designs are based on integrating existing RAS and hydroponic/substrate culture technologies, they are regarded as aquaponic in nature (Delaide et al. 2016) (Fig. 5.3) (see Chap. 8).
The definition of aquaponics is now being expanded beyond ecological, water and nutrient efficiency drivers and optimisation to also include economic drivers (Goddek and Körner 2019; Goddek and Keesman 2018; Goddek 2017; Kloas et al. 2015; Reyes Lastiri et al. 2016; Yogev et al. 2016) (Chap. 8). The benefits of such an approach are that a positive economic outcome from aquaponics technology is as important as its biological, chemical, engineering, ecological and sustainable credentials and therefore, the economic outcome should play a role within the overall definition (Chap. 8).
Many advantages are often associated with aquaponics, especially in terms of its water-use efficiency, its nutrient use efficiency, its sustainable nature, its ability to produce two crops from the one input source (fish feed) and its lowered environmental impact (Timmons, et al., 2002; Buzby and Lian-shin 2014; Wongkiew et al. 2017; Roosta and Hamidpour 2011; Suhl et al. 2016). These advantages are regularly quoted and applied by commercial aquaponic operators and are used as a marketing and price regulation pathway for the products (fish and plants), and therefore, the use of the name "aquaponics" directly and immediately associates that the products labelled as such have been produced with methods that contain or utilise the advantages listed. However, there is no formal regulation of the industry that dictates that the use of the word (aquaponics) only occurs when the advantages are apparent and present within the technology and methods applied. If the above advantages are assigned to aquaponics as a technology, then surely the technology should provide the prescribed advantages, and if the technology does not provide the advantages, then the word should not be applied (Lennard 2017).
Fig. 5.3 Simplified scheme of the main water flows within a decoupled aquaponic system. The nutrient concentrations in each component may be separately tailored to the individual component requirement
Because aquaponics may be defined either in terms of its hardware equipment integration aspect (RAS with hydroponics), its nutrient sharing or partitioning properties or its ability to provide important advantages, there is still a wide spectrum of possible applications of the name to many different technical approaches that utilise different methods and demand different outcomes. Therefore, it appears that the actual definition of aquaponics is still unresolved.
It appears therefore that very important questions are yet to be answered: what is aquaponics and how is it defined?
This would suggest that one very important aspect for the aquaponic industry to consider is the development of a truthful and agreed-upon definition. The broader aquaponics industry will continue to be full of disagreement if a definition is not agreed upon, and more importantly, consumers of the products produced within aquaponic systems will become more and more confused about what aquaponics actually is — a state of affairs that will not assist the growth and evolution of the industry.