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Since the end of the twentieth century, there have been significant changes in the composition of aquafeeds but also advances in manufacturing. These transformations have originated from the need to improve the economic profitability of aquaculture as well as to mitigate its environmental impacts. However, the driving forces behind these changes is the need to decrease the amount of fishmeal (FM) and fish oil (FO) in the feeds, which have traditionally constituted the largest proportion of the feeds, especially for carnivorous fish and shrimp. Partly because of overfishing but especially due to the continuous increase in global aquaculture volume, there is an increasing need for alternative proteins and oils to replace FM and FO in aquafeeds.
Fig. 13.2 Fish-in-fish-out ratio (blue line, left y-axis) and the amount of fish oil used (yellow line, right y-axis) for rainbow trout feed in Finland between 1990 and 2013. (Data from www.raisioagro. com)
The composition of fish feeds has changed considerably as the proportion of FM in diets has decreased from >60% in the 1990s to \20% in modern diets for carnivorous fish such as Atlantic salmon (Salmo salar), and FO content has decreased from 24% to 10% (Ytrestøyl et al. 2015). As a consequence the so-called fish-in-fish-out (FIFO) ratio has decreased below 1 for salmon and rainbow trout meaning that the amount of fish needed in the feed to produce 1 kg of fish meat is less than 1 kg (Fig. 13.2). Thus, carnivorous fish culture in the twenty-first century is a net producer of fish. On the other hand, feeds for lower trophic omnivorous fish species (e.g. carp and tilapia) may contain less than 5% FM (Tacon et al. 2011). Farming such low trophic fish species is ecologically more sustainable than for higher trophic species, and FIFO for Tilapia was 0.15 and for cyprinids (carp species) only 0.02 in 2015 (IFFO). It should be noted that total FM replacement in the diets of Tilapia (Koch et al. 2016) and salmon (Davidson et al. 2018) is not possible without significantly affecting production parameters.
Today, the major supply of proteins and lipids in fish feed comes from plants, but also commonly from other sectors, including meals and fats from meat and poultry by-products and blood meal (Tacon and Metian 2008). Additionally, waste and by-products from fish processing (offal and waste trimmings) are commonly used to produce FM and FO. However, due to EU regulations (EC 2009), the use of FM of a species is not allowed as feed for the same species, e.g. salmon cannot be fed FM containing salmon trimmings.
FM and FO replacements with other ingredients can affect the quality of the product that is sold to customers. Fish has the reputation of being a healthy food, especially due to its high content of poly and highly unsaturated fatty acids. Most importantly, seafood is the only source of EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), both of which are omega-3 fatty acids, and essential nutrients for many functions in the human body. If FM and FO are replaced with products from terrestrial origin, this will directly affect the quality of the fish flesh, most of all its fatty acid composition, as the proportion of omega-3 fatty acids (especially EPA and DHA) will decrease whilst the amount of omega-6 fatty acids will increase along with the increase of plant material that is replacing FM and FO (Lazzarotto et al. 2018). As such, the health benefits of fish consumption are partly lost, and the product that ends up on the plate is not necessarily what consumers expected to buy. However, in order to overcome the problem of decreased omega-3 fatty acids in the final product resulting from lower fish ingredients in aquafeeds, fish farmers could employ so-called finishing diets with high FO content during the final stages of cultivation (Suomela et al. 2017).
A new interesting option for replacing FO in fish feeds is the possibility of genetic engineering, i.e. genetically modified plants which can produce EPA and DHA, e.g. oil from genetically modified Camelina sativa (common name of camelina, gold-of-pleasure or false flax which is known to have high levels of omega-3 fatty acids) was successfully used to grow salmon, ending up with very high concentration on EPA and DHA in the fish (Betancor et al. 2017). The use of genetically modified organisms in human food production, however, is subject to regulatory approval and may not be an option in the short term.
Another new possibility to replace FM in aquafeeds is proteins made of insects (Makkar et al. 2014). This new option has become possible within the EU only recently when the EU changed legislation, allowing insect meals in aquafeeds (EU 2017). The species permitted to be used are black soldier fly (Hermetia illucens), common housefly (Musca domestica), yellow mealworm (Tenebrio molitor), lesser mealworm (Alphitobius diaperinus), house cricket (Acheta domesticus), banded cricket (Gryllodes sigillatus) and field cricket (Gryllus assimilis). Insects must be reared on certain permitted substrates. Growth experiments done with different fish species show that replacing FM with meal made of black soldier fly larvae does not necessarily compromise growth and other production parameters (Van Huis and Oonincx 2017). On the other hand, meals made of yellow mealworm could replace FM only partially to avoid decrease in growth (Van Huis and Oonincx 2017). However, FM replacement with insect meal can cause a drop in omega-3 fatty acids, as they are void of EPA and DHA (Makkar and Ankers 2014).
In contrast to insects, microalgae typically have nutritionally favourable amino acid and fatty acid (including EPA and DHA) profiles but there is also a wide variation between species in this respect. Partial replacement of FM and FO in aquafeeds with certain microalgae have given promising results (CamachoRodríguez et al. 2017; Shah et al. 2018) and in the future the use of microalgae in aquafeeds can be expected to increase (White 2017) even though their use may be limited by price.
This short summary of potential feed ingredients indicates that there is wide range of possibilities to at least partially replace FM and FO in fish feeds. In general, the amino acid profile of FM is optimal for most fish species and FO contains DHA and EPA which are practically impossible to provide from terrestrial oils, albeit genetic engineering may change the situation in the future. However, GMO products need first to be accepted in legislation and then by customers.
Tailoring aquafeeds which are specific to aquaponic systems is more challenging than conventional aquaculture feed development, as the nature of aquaponic systems requires that the aquafeeds not only supply nutrition to the cultured animals but also to the cultured plants and the microbial communities inhabiting the system. Current aquaponic practice utilizes aquafeeds formulated to provide optimal nutrition to the cultured aquatic animals; however, as the major nutrient input into aquaponic systems (Roosta and Hamidpour 2011; Tyson et al. 2011; Junge et al. 2017), the feeds also need to take into account the nutrient requirements of the plant production component. This is especially important for commercial-scale aquaponic systems, where the productivity of the plant production system has a major impact on overall system profitability (Adler et al. 2000; Palm et al. 2014; Love et al. 2015a) and where improved production performance of the plant component can significantly improve overall system profitability.
Thus, the overall aim of developing tailored aquaponic feeds would be to design a feed which strikes a balance between providing additional plant nutrients, whilst maintaining acceptable aquaponic system operation (i.e. sufficient water quality for animal production, biofilter and anaerobic digester performance, and nutrient absorption by plants). In order to achieve this, the final tailored aquaponic feed may not be optimal for either aquatic animal or plant production individually, but would rather be optimal for the aquaponic system as a whole. The optimal point would be determined based on overall system performance parameters, e.g. economic and/or environmental sustainability measures.
One of the major challenges in increasing production output from coupled aquaponic systems is the relatively low concentrations of both macro- and micro plant nutrients (mostly in the inorganic form) in the recirculating water, compared to conventional hydroponic systems. These low levels of nutrients can result in nutrient deficiencies in the plants and suboptimal plant production rates (Graber and Junge 2009; Kloas et al. 2015; Goddek et al. 2015; Bittsanszky et al. 2016; Delaide et al. 2017). A further challenge is the significant amounts of sodium chloride in conventional fishmeal-based aquafeeds and the potential accumulation of sodium in aquaponic systems (Treadwell et al. 2010). Different approaches can be developed to address these challenges such as technological solutions, e.g. decoupled aquaponic systems (Goddek et al. 2016) (also see Chap. 8), direct nutrient supplementation in the plant production system via foliar spray or addition to the recirculating water (Rakocy et al. 2006; Roosta and Hamidpour 2011) , or the culture of better salt-tolerant plant (see Chap. 12). A new approach is the development of tailored aquafeeds specifically for use in aquaponics.
In order to address plant nutrient shortages in aquaponics, tailored aquaponic feeds need to increase the amount of plant-available nutrients, either by increasing the concentrations of specific nutrients after excretion by the cultured animals, or by rendering the nutrients more bio-available after excretion and biotransformation, for rapid uptake by the plants. Achieving this increased nutrient excretion is, however, not as simple as supplementing increased amounts of the desired nutrients to the aquaculture diets, as there are many (often conflicting) factors that need to be considered in an integrated aquaponic system. For example, although optimal plant production will require increased concentrations of specific nutrients, certain minerals, e.g. certain forms of iron and selenium, can be toxic to fish even at low concentrations and would therefore have maximum allowable levels in the circulating water (Endut et al. 2011; Tacon 1987). Apart from total nutrient levels, the ratio between nutrients (e.g. the P:N ratio) is also important for plant production (Buzby and Lin 2014), and imbalances in the ratios between nutrients can lead to accumulation of certain nutrients in aquaponic systems (Kloas et al. 2015). Furthermore, even if an aquaponic feed increases plant nutrient levels, the overall system water quality and pH still needs to be maintained within acceptable limits to ensure acceptable animal production, efficient nutrient absorption by plant roots, optimal operation of biofilters and anaerobic digesters (Goddek et al. 2015b; Rakocy et al. 2006) and to avoid precipitation of certain important nutrients like phosphates, as this will render them unavailable to plants (Tyson et al. 2011). To achieve this overall balance is no mean feat, as there are complex interactions between the different forms of nitrogen in the system (NH<sub3/sub, NHsub4/subsup+/sup, NOsub2/subsup-/sup, NOsub3/subsup-/sup), the system pH and the assortment of metals and other ions present in the system (Tyson et al. 2011; Goddek et al. 2015; Bittsanszky et al. 2016).
Common Nutrient Shortages in Aquaponic Systems
Plants require a range of macro- and micronutrients for growth and development. Aquaponic systems are commonly deficient in the plant macronutrients potassium (K), phosphorus (P), iron (Fe), manganese (Mn) and sulphur (S) (Graber and Junge 2009; Roosta and Hamidpour 2011). Nitrogen (N) is present in different forms in aquaponic systems, and is excreted as part of the protein metabolism of the cultured aquatic animals (Rakocy et al. 2006; Roosta and Hamidpour 2011; Tyson et al. 2011) after which it enters the nitrogen cycle in the integrated aquaponic environment. (Nitrogen is discussed in detail in Chap. 9 and is therefore excluded from the present discussion.)
The use of selected specialist aquaculture feed additives can contribute to the development of tailored aquafeeds specifically for aquaponics, by providing additional nutrients to the cultured aquatic animals and/or plants, or by adjusting the ratio of nutrients. Aquaculture feed additives are diverse, with a wide range of functions and mechanisms of working. Functions can be nutritive and non-nutritive, and the additives can be targeted towards action in the feeds or towards the physiological processes of the cultured aquatic animals (Encarnação 2016). For the purposes of this chapter, emphasis is on three specific types of additives which could assist the tailoring of aquaponic diets: (1) mineral supplements added directly to the feeds, (2) minerals that are added co-incidentally as part of additives that serve a non-mineral purpose and (3) additives which render minerals, which are already present in the feeds, more available to the cultured aquatic animals and/or plants in aquaponic systems.
Supplementing minerals directly in aquaculture diets used in aquaponic systems is one potential method to either increase the amount of minerals excreted by the cultured animals or to add specific minerals required by the plants in aquaponic systems. Minerals are routinely added in the form of mineral premixes to aquaculture diets, to supply the cultured aquatic animals with the essential elements required for growth and development (Ng et al. 2001; NRC 2011). Any minerals not absorbed by the fish during digestion are excreted, and if these are in the soluble (mostly ionic) form in the aquaponic system, these are available for plant uptake (Tyson et al. 2011; Goddek et al. 2015). It is unclear how feasible such an approach would be, as there is scant information about the efficacy of adding mineral supplements to aquafeeds for the purpose of enhancing aquaponic plant production. In general, mineral requirements and metabolism in aquaculture are poorly understood compared to terrestrial animal production, and the feasibility of this approach is therefore not well described. Potential advantages to this approach would be that it could prove to be a fairly simple intervention to improve overall system performance, it could allow supplementation of a wide range of nutrients, and it is likely to be relatively low cost. However, substantial research is still required to avoid any major potential pitfalls that may arise. One of these centres on the fact that the supplemented minerals destined for the plants first need to pass through the digestive tract of the cultured aquatic animals and these could either be absorbed fully or partially during this passage. This could lead to unwanted accumulation of minerals in the aquatic animals, or interference in normal intestinal nutrient and/or mineral absorption and physiological processes (Oliva-Teles 2012). Significant interactions can occur between dietary minerals in aquaculture diets (Davis and Gatlin 1996), and these need to be determined before direct mineral supplementation in aquaponic diets can be employed. Other potential effects may include altered physical structure and chemosensory characteristics of the feeds, which in turn could affect feed palatability. Clearly, there is still substantial research required before this method of tailoring aquaponic feeds can be adopted.
Certain classes of feed additives are added to aquafeeds in the form of ionic compounds and where only one of the ions contributes towards the intended activity. The other ion is viewed as a co-incidental and unavoidable addition to the aquafeed and is often not considered in any aquaculture research. One specific example of such a class of often-used feed additives are the organic acid salts, where the intended active ingredient in the aquafeed is the anion of an organic acid (e.g. formate, acetate, butyrate or lactate) and the accompanying cation is often ignored in the cultured animals' nutrition. Thus, if the accompanying cation is chosen purposefully to be an important macro- or micro plant nutrient, there is the potential that it could be excreted by the cultured animals into the system water and be available for uptake by the plants.
Short-chain organic acids and their salts have become well known and often used in feed additives in both terrestrial animal nutrition and in aquaculture, where the compounds are employed as performance enhancers and agents to improve disease resistance. These compounds can have different mechanisms of functioning, including acting as antimicrobials, antibiotics or growth promoters, enhancing nutrient digestibility and utilization and acting as directly metabolizable energy source (Partanen and Mroz 1999; Lückstädt 2008; Ng and Koh 2017). Either the native organic acids or their salts can be utilized in aquaculture diets, but the salt forms of the compounds are often preferred by manufacturers as they are less corrosive to feed manufacturing equipment, are less pungent and are available in solid (powder) form, which simplifies addition to formulated feed during manufacturing (Encarnação 2016; Ng and Koh 2017). For a comprehensive review on the use of organic acids and their salts in aquaculture, readers are referred to the work of Ng and Koh (2017).
Employing organic acid salts in aquaponics has the potential to have dual benefits in the system, where the anion could enhance the performance and disease resistance of the cultured aquatic animals, whilst the cation (e.g. potassium) could increase the amount of essential plant nutrients excreted. The potential advantage of this approach is that dietary inclusion levels of organic acid salts can be relatively high for a feed additive, and research regularly reports total organic acid salt inclusion of up to 2% by weight (Encarnação 2016), although commercial manufacturers tend to recommend lower levels of approximately 0.15—0.5% (Ng and Koh 2017). The cation of organic acid salts could constitute a significant proportion of the overall weight of the salt, and as these are fed daily to the cultured animals, they could contribute a significant amount of nutrients to the plants in an aquaponic system over the course of a growing season. No published research is currently available that reports findings for this line of enquiry and as with direct mineral supplementation to aquaponic feeds, this approach needs to be validated through future research to determine the fate of the cations added as part of the organic acid salts (whether they are excreted or absorbed by the aquatic animals), and whether there are any interactions with minerals or nutrients. It remains, however, an exciting future avenue of investigation.
Increasing amounts of plant ingredients are used in formulated aquafeeds, yet minerals from plant raw materials are less bio-available to cultured aquatic animals, mainly due to the presence of anti-nutritional factors in the plant-based dietary ingredients (Naylor et al. 2009; Kumar et al. 2012; Prabhu et al. 2016). This means that a higher proportion of minerals are excreted in the faeces in bound form, requiring 'liberation' before being available for plant uptake. One typical example is organic phosphorus occurring as phytate, which can bind to other minerals to form insoluble compounds, where microbial action in the environment is required before the phosphorus is released as plant-available, soluble phosphate (Kumar et al. 2012).
The use of exogenous enzymes in tailored aquaponic diets could potentially contribute towards releasing increased amounts of nutrients from high-plant content aquafeeds for both animal and plant nutrition in aquaponic systems. The most often employed enzymes in aquafeeds are proteases, carbohydrates and phytases, both to improve nutrient digestion and to degrade anti-nutritional compounds like phytate (Encarnação 2016), which can result in additional nutrients being released from aquafeeds. Although it is known that exogenous enzyme supplementation leads to improved nutrient utilization in the cultured animals, it is unclear whether additional nutrients would be excreted in plant-available form, therefore avoiding a separate remineralization step in aquaponic systems (see Chap. 10). Additionally, interactions between exogenous enzymes and nutrients in different parts of the digestive tract of fish are possible (Kumar et al. 2012), which will have further implications for the amounts of nutrients excreted for plant growth. Further research is therefore also required to determine the utility of exogenous enzymes specifically for use in aquaponic feeds.