WHAT NEW INGREDIENT SHOULD BE CONSIDERED PROMISING FOR SUSTAINABLE AQUACULTURE FEED PRODUCTION?

The expansion of the aquaculture sector coupled with the increasing demand for fishmeal and fish oil is prompting the use of new feed ingredients that are currently being evaluated for their potential incorporation into aquaculture feeds, towards more sustainable and cost-effective production. This study aims to summarize existing findings on the effects of investigated alternatives to fishmeal and fish oil on marine and freshwater fish species. Alternative protein sources, including macroalgae, krill, insects, terrestrial animal by-products, and single-cell ingredients, are examined for their effectiveness in promoting growth and welfare of fry and adults of farmed fish species. Fish feed reformulation strategies should ensure the recommended daily nutritional requirements and further indicate alternatives from the meta-analysis, such as microalgae, which are deficient in essential amino acids. Sustainable aquaculture expansion is on the horizon, but what new ingredients can be considered promising for its establishment?

1 Introduction

Over the last decades, one of the biggest challenges confronting the aquaculture sector is the necessity for novel aquafeed ingredients. The main eligibility criterion focuses on the suitability as valid and appropriate alternatives to the widely used fishmeal (FM), fish oil (FO) and plant proteins (e.g., soybean meal), in terms of price, availability, sustainability, and nutritional qualities. High-quality FM is considered an excellent ingredient for aquafeeds due to numerous key-features such as optimal amino acid (AA) and lipid profiles, high palatability and nutrient digestibility, balanced micronutrient content, and absence of anti-nutrients. On the other hand, FO, which is commercially available in prerequisite quantities for utilization in aquafeeds, remains the ideal source of the nutritionally essential eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3). However, the rapid and continuous expansion of aquaculture that stems from the constant increase in seafood demand calls for a shift toward a more sustainable management of FM and FO production. Consequently, there is an impellent urgency to experiment new and environment-friendly ingredients for aquafeeds production.

A plethora of studies have been conducted in several fish species regarding the replacement of FM and FO by plant products. Among the most utilized plant proteins in European aquafeed production are oilseeds (such as soybean, rapeseed, sunflower etc.), cereals (such as corn and wheat glutens etc.), and legume seeds (such as those of peas and beans) as well as their by-products. Similarly, commonly preferred plant oils include soybean oil, rapeseed oil, and linseed oil, which are characterized by high levels of α-Linolenic acid (ALA; 18:3n-3) and a more balanced n-3/n-6 polyunsaturated fatty acid (PUFAs) profile [8, 9]. Although the partial replacement of FM and FO with plant products has been relatively successful, several drawbacks limit the suitability of these ingredients, especially for aquatic species at higher trophic levels with substantial protein requirments. For ingredients of plant origin, major disadvantages include:

• (1) the presence of anti-nutritional factors (ANF) (e.g., protease inhibitors, lectins, phytic acid, saponins, phytoestrogens, antivitamins, and allergens) and nondigestible carbohydrates such as non-starch polysaccharides (NPS),
• (2) the lower protein content in regard to fish requirements,
• (3) the lower palatability in comparison to FM and
• (4) the deficiency of essential amino acids (EAA) such as lysine and methionine.

Miscellaneous techniques (e.g., high-pressure processing, microwave, extrusion, heat increment processes, incorporation of exogenous enzymes and supplements, irradiation, and fermentation) have been employed to increase the digestibility and the palatability of plant meals and to overcome the adverse effects of ANF. Concerning plant oils, the major bottle neck is the lack in EPA and DHA considering the limited ability of the higher in trophic levels marine fishes to de novo synthesize these fatty acids from their lower-carbon chain fatty acid precursors. Thus, the reduced EPA and DHA in the body of fish fed plant oil-based diets can be compensated by using finishing FO-based diets. Nevertheless, the ongoing high demand for raw materials of plant-origin for human consumption and the consequent need for more arable land will eventually lead to a considerable price increase, thus making their use in aquafeeds production economically less opportune. Besides, plant production is associated with various environmentally detrimental impacts including water consumption, deforestation and habitat fragmentation, greenhouse gas emissions, biodiversity loss, and agrochemical inputs. Hence, it is imperative to further seek for novel economically and environmentally sustainable ingredients that can complement or substitute FM, FO, and plant products in aquafeeds, especially for less tolerant species of higher trophic levels.

However, which is the proper strategy to adopt in order to evaluate a novel ingredient? Glencross, Booth, and Allan (2007) and Glencross (2020) proposed a series of seven steps that aquafeed formulators should consider in the evaluation of the potential benefits and constraints associated to the examined novel ingredient: Step 1 characterization (supplier, material, origins, processing, storage history, and chemical composition), Step 2 palatability (attractant or repellent, incitant or suppressant, and stimulant or deterrent), Step 3 digestibility, Step 4 utilization (nutritional response, growth, and feed intake), Step 5 immunology (impact on the immune response and general health of the animal), Step 6 processing effects (assessment of the physical characteristics), and Step 7 product quality influences (sensory evaluation studies). Nevertheless, other important aspects that must be considered to expand our knowledge regarding novel ingredients are the scalability (increase availability over time to match rising demand) and the environmental implications of their production as well as the experimental design (e.g., inclusion level, replication and animal age, or size). Though the adoption of this idealistic approach would be essential to avoid mistakes during the evaluation of alternative ingredients, it is scarcely applied mainly due to economic and time-related constraints, and data accessibility issues. Therefore, priori assumptions on the tested ingredient aid scientists to choose which step is more necessary for an adequate and reliable evaluation in relation to the aim of the study. Alternative methods that may be integrated for better assessment include the investigation of the effects of ANF/negative factors (e.g., phytate, simple sugar molecules, and RNA) regardless of the ingredients in which they are contained. This approach would allow acquiring an alternative perspective and more precise information to effectively cope with these substances that often adversely affect aquaculture species.

According to the latest FAO report (2022), European region accounts for nearly 3% (3291.7 1000 tonnes) of the global aquaculture production. High economic value finfish and mollusc species are intensively farmed in several European countries, with Norway, Turkey, United Kingdom, Greece, and Spain representing the major producers. In specific, salmonids (e.g., Salmo salar and Oncorhynchus mykiss), European sea bass (Dicentrarchus labrax), and gilthead sea bream (Sparus aurata) are among the most cultured finfish species in Europe. Currently, the policies launched by the EU propel the aquaculture industry toward sustainable development in which production and environmental protection are balanced. Novel protein and lipid sources for inclusion in aquafeeds are essential to maintain the sustainable and profitable culture of these species. Recent efforts have been made to explore the nutritional value of several ingredients sources such as insects-derived products, terrestrial processed animal products (PAP), bacterial and fungal single cell ingredients (SCI), micro- and macro-algae, and krill meal (KM). The present study aims to provide a detailed overview of the most examined novel ingredients in recent years through summarizing and discussing the suitability and impact on the abovementioned categories of finfish. The order in which novel ingredients are reviewed herein reflects a conceptual organization based on criteria such as the biological classification, the organisms’ habitats and the processing methods recruited to obtain the final product.

2 Macroalgae

The macroalgae or seaweed represents a viable alternative ingredient for consideration. The potential inclusion of macroalgae in animal feed including aquafeeds has been recently reviewed by Wan et al. (2019) and Morais et al. (2020). The numerous species of macroalgae are encompassed in three divisions: Chlorophyta (green), Rhodophyta (red), and Phaeophyta (brown). Besides the mere nutritional characteristics, these organisms produce metabolites that may have a strong ameliorative impact on farmed fish health and welfare. However, their nutritional composition depends highly not only on division, species, and genus but also on the seasonality (temperature, salinity, and light), and the geographic localization. Many species of high protein content (even 50%) are encompassed in the Rhodophyta division, such as Chondrus crispus, Gracilaria sp., and Pyropya sp. On the contrary, species included among the Phaeophyta (Spatoglossum macrodontum, Dictyota acutiloba, and D. sandvicensis) display the highest content in lipids (up to 20%), including omega 3 and omega 6 fatty acids (EPA, stearidonic acid, a-Linolenic acid, and arachidonic acid). Phycoerythrin, the proteinic pigment responsible for the high protein content of red seaweed, has been reported to possess bioactive properties of antioxidant and anti-inflammatory nature. The algal cell walls of seaweed can affect the digestibility of the cytoplasmic proteins and therefore physical treatments, including enzymatic and chemical hydrolysis, solvent extraction, fermentation and freeze drying, may be required to overcome this issue. Simple carbohydrates and polysaccharides have key-role as energy stores and are pivotal in macroalgae structural function. Some of these polysaccharides, known as phycocolloids (e.g., agar, alginates, and carrageenan) have been widely used in aquafeed formulation to improve feed stability. Furthermore, an increasing interest has emerged in aquaculture for beta-glucans, a polysaccharides group that may act as immune system boosters. Seaweeds are also rich in carotenoids pigments including carotenes (e.g., β-carotene and lycopene) and xanthophylls (e.g., lutein, astaxanthin, and canthaxanthin) that not only can have an important role in fish flesh pigmentation but also possess functional antioxidant, anti-inflammatory, antitumoral, antimicrobial, antiviral, antiseptic, and immune-stimulatory properties. In addition, seaweeds have a superlative content of water-soluble vitamins such as vitamin B2 (riboflavin), B12 (cobalamin) and C (ascorbic acid), and lipid-soluble vitamins such as vitamin E (a, b, c, d tocopherol, and a, b, c, d tocotrienol) and minerals (sodium, magnesium, phosphorous, calcium, and potassium). Nevertheless, a major drawback to take in consideration is the propensity of macroalgae to accumulate trace elements (e.g., copper, mercury, lead, selenium, cadmium, and arsenic) that can produce toxicological effects. Furthermore, due to their high moisture, another obstacle associated with the commercial scale of macroalgal products is the drying process that represents one of the highest production costs. Novel drying techniques such as ultrasonic, freeze-drying, flash drying, microwave drying, and pulse electric field have been assessed to make this process faster and more efficient.

From a more economically and ecologically sustainable perspective, integration of seaweed production with other activities including integrated multi-trophic aquaculture (IMTA) must be considered due to the risks as well as the numerous benefits that may provide to the aquaculture sector, mainly the reduced “ecological footprint,” the economic diversification and the increased social acceptability of finfish culturing systems. The system of IMTA combines the co-culturing of different trophic-level organisms, especially finfish, with inorganic extractive aquaculture species (e.g., seaweeds) and organic particulate extractive aquaculture species (e.g., bivalves). Such an approach may facilitate the mitigation of risks associated with seaweed cultivation, including sedimentation and emergence of invasive species. Hence, IMTA has a potential key future role in the large-scale production of macroalgae due to high productivity and environmental sustainability.

3 Low-Trophic Marine Animal Species

   3.1 Krill

KM is classified among the most promising and appealing ingredients in aquafeeds formulation due to its desirable nutritional attributes. Krill (Order Euphausiacea) is a marine crustacean comprised of over 80 different species including the Antarctic krill (Euphausia superba), the Arctic krill (Thysanoessa inermis), the Pacific Ocean krill (E. pacifica), and the Atlantic Ocean krill (Meganyctiphanes norvegica) that represent together one of the most abundant animal categories on earth in terms of total biomass. It is still a relatively unexploited nutrient source and has the great potential to be sustainable as a result of fisheries that are well managed and regulated. Krill contains approximately 60% protein, with a similar to FM well-balanced AA profile, and 25% lipid, which comprise adequate levels of phospholipids and omega-3 FA (EPA and DHA). Additionally, it is rich in astaxanthin, vitamins, minerals, trimethylamine oxide, and other low-molecular compounds, mainly free AA and nucleotides, which act as feed-attractants and stimulants, thus increasing aquafeed palatability and improving growth performance. Krill is also a source of chitin that has been recently investigated for its immune-stimulatory properties in fish. Several krill-derived products are available in the market such as KM, defatted KM, low-fluoride krill paste, krill oil (KO), frozen krill, and krill hydrolysate. However, the utilization of this nascent aquafeed ingredient in aquafeed is constrained mainly due to its nutritional variability and its substantially higher cost compared to FM.

One of the debates hampering the rapid propagation of KM as an aquafeed ingredient was the implication of the high fluoride content on fish fillet. Following a feeding trial with KM, Julshamn et al. (2004) detected no accumulation of fluoride in the tissues of Atlantic salmon, thus suggesting a potential high tolerance of this species to fluoride. On the contrary, Yoshitomi et al. (2006) observed high levels of fluoride accumulation in vertebral bones and consequent reduction in growth of rainbow trout fed a KM diet. However, in a similar study, KM with low fluoride levels as total FM replacement was feasible without any detrimental effects. The fluoride content in krill-based diets has been reported to increase with the KM inclusion level. Hence, preprocessing removal of the krill exoskeleton, where the major level of fluoride is accumulated, may be required to eliminate the negative consequences of dietary fluoride. Considering the aforementioned variation in the nutrient content, it is of great interest to highlight that preparation of KM may interfere with the outcome. Differences among the various studies may be due to the employed processing methods such as heating, boiling, pressing and drying, or the mechanical exoskeleton removal. Deshelled KM, for instance, is characterized by reduced chitin levels, which, in turn, are expected to exert different effects on finfish species, as indicated by Hansen et al. (2010). Moreover, improper processing may enable active nutrients degradation and reduce their bioavailability, thus limiting the nutritional value and adversely affect the feed intake. Future studies on the utilization of krill in aquafeeds should optimize the preparation protocols to ensure the maintenance of high quality nutrients.

   3.2 Tunicates

Tunicates, widely distributed in shallow ocean waters, are a group of filter feeding invertebrate organisms (ascidians, normally called sea squirts). Although tunicates have been previously overlooked as a potential nutrient source, a considerable interest has emerged in recent years. The ascidians’ body is embedded by tunic (45% of total animal weight), a skeletal structure that constitutes mainly of cellulose, protein, and ash, whereas the inner body part (55% of total animal weight) consists mainly of protein. The tunic has an excellent cellulosic composition, whereas the inner part contains proteins and ω-3 fatty acids and may be used as feed ingredients. In addition to the high-quality protein content, these marine bioresources showcase low lipid content and a plethora of bioactive compounds, including sphiningomyelins and tunichromes, with antibacterial, antifungal, antioxidant, and anti-inflammatory properties. For instance, host defense peptides with antimicrobial activities have been isolated from the hemocytes of both Halocynthia papillosa and Microcosmus sabatieri. Moreover, tunicates exhibit rapid growth rates with a short period of maturation, essential features for an easy and efficient mass production, which is also benefited economically by the nonfeeding transient larval stage. Hence, through proper techniques and processing methods, tunicates production may provide a sustainable and scalable source of nutrients for aquafeeds. Recently, Kousoulaki et al. (2022) through the evaluation of various low trophic ingredients as potential partial FM replacements suggested that tunicate (Ciona intestinalis) meal may be considered for inclusion in Atlantic salmon diet given the obtained high growth rate and low FCR. However, processing procedures require immense attention for the successful removal of toxicants, including heavy metals, which tend to accumulate in tunicate tissues.

   3.3 Polychaetes

Among the low-trophic marine animals that have currently emerged as potential alternative ingredients in aquafeed industry are polychaete worms, common inhabitants of estuarine and marine ecosystems. As deposit feeders of particulate organic matter, polychaetes are able to retain wasted essential fatty acids such as EPA and DHA, which contribute to their high content of omega-3 PUFA. In addition, the nutritional profile of polychaetes is characterized by high levels of protein, energy, AA (e.g., arginine, glycine, and cysteine), and minerals (e.g., Na, Ca, Cl, and K). To further emphasize the importance inherent in the potential inclusion of polychaetes in aquafeed, recent studies have demonstrated that species such as Nereis diversicolor and Hediste diversicolor can efficiently convert organic aquaculture waste into high-quality proteins and lipids. Therefore, the introduction of polychaetes in the aquaculture sector may dually serve as a valuable environmentally sustainable feedstuff and as a circular economy promoter.

4 Insects

Recent interest in the establishment of insect products as viable alternatives to FM has been exponentially growing among the aquaculture nutritionists. In Europe, the use of insect-based meals in aquafeeds was approved in July 2017 according to the Commission Regulation (EU) 2017/893, which authorized products derived from the following insect species: (i) Hermetia illucens (HI, Black Soldier Fly) and Musca domestica (MD, common housefly); (ii) Tenebrio molitor (TM, Yellow Mealworm) and Alphitobius diaperinus (Lesser Mealworm); and (iii) Acheta domesticus (House cricket), Gryllodes sigillatus (Banded cricket), and Gryllus assimilis (Field cricket).

Insects, main component of the natural diet of many finfish and crustacean species, are excellent qualitatively and quantitatively sources of proteins. Insect meals (IM) are considered an ideal representation of a sustainable feed ingredient with low ecological impact. For instance, mass production of insects emits lower greenhouse gases and requires much less water and land compared to the livestock production, although the high energy consumption for insect rearing is a drawback. Insects are also able to revalorize organic and waste materials, converting them into valuable biomass rich in protein and lipids. Overall, the chemical and nutritional composition of an insect-based product is related to the species, the life stage, the processing treatment (e.g., defatting, drying), and the rearing substrate. In addition to the protein content that can reach up to 70%, many species of insects have a good EAA profile close to that of FM. However, it is worth noticing that deficiencies in methionine and lysine contents have been recorded in many IM. Furthermore, insects tend to be rich sources of several vitamins and minerals including iron, zinc, copper, and manganese. Contrary, one of the major drawbacks of IM utilization is related to its FA profile that is rich in n-6 and insufficient in n-3 FA. Hence, the final FA profile of an IM-fed fish, which generally reflects that of the consumed diet, is low in PUFA especially of EPA and DHA. A fish fillet with low levels of n-3 FA, which are beneficial for human health (reviewed in Hossain, 2011), is considered of lower nutritional quality for consumption. Several methods have been developed to tackle this issue such as defatting processes for the production of a final high-protein IM with the remaining lipid potentially revalorized as biofuel, and the nutrient enrichment of insects’ rearing substrate with omega-3 rich products. Another concern regarding the debate on the use of IM stems from its high content of chitin, a polysaccharide-component in insects’ cuticles that is bounded to the proteins and thereby may affect their digestibility. Experiments on inclusion of chitin in aquafeeds are controversial. Although it is considered an anti-nutritional factor due to its hypothesized negative implication in fish digestibility, the closely related shrimp or crab chitin has been recognised for its immune-stimulatory role. The potentially positive or negative effects of insect chitin on finfish growth or health remain to be investigated.

Insect meal certainly represents one of the most promising candidates for the replacement of FM in the near future. However, to render IM as a reliable FM alternative, it is vital to drastically augment their mass production for the constantly increasing demand, which in turn may facilitate the reduction of their currenlty noncompetitive price. In addition, rearing strategies must be implemented to ensure the stable proximate composition of insect products, which may often vary due to several factors (e.g., substrate).

Because of their richness in chitin, short-chain fatty acids, and antibacterial molecules, insects may not only be seen as valuable ingredients to replace FM but also as potential fish heath promoters when used at low dietary doses  although future studies are needed to ascertain this hypothesis.

5 Terrestrial Processed Animal Proteins (PAPs)

Terrestrial processed animal proteins (PAPs) derive from the conversion of rendered animal by-products into valuable sources of protein (45%–65% crude protein [CP]), rich in most EAA except lysine and methionine. PAPs have been widely adopted in aquaculture by many countries all over the world as a more cost-effective and available protein source than FM. However, in Europe and UK, the inclusion of PAP in aquafeeds was restricted due to public and political concerns related to the bovine spongiform encephalopathy crisis (EC regulations 999/2001 and 1234/2003). In 2013, the European Food Safety Authority [EFSA Panel on Animal Health and Welfare (AHAW) 2013] lifted some of the bans for specific category III sources, that is, raw material derived from nonruminant animals specially treated to fit for human consumption, including blood meal (BM), meat and bone meal (MBM) from porcine origin, hydrolysed feather meal (HFM), and other rendered poultry by-products (PBM). The alleviation of the previously in force restrictions represents a game-changer for aquafeeds production in Europe. The use of PAP is indeed a valid and sustainable venue to reduce the overall cost production of high trophic level species, providing high digestible ingredients rich in protein. Nevertheless, the recently introduced constraints on animal by-products processing (e.g., heat and pressure) may negatively affect the nutritional value of the final product. Ergo, the experimentation of these new ingredients has become imperative before their inclusion in aquafeeds.

6 Single-Cell Ingredient (SCI)

Despite that single-cell ingredients (SCI) have been drawing the attention of aquaculture nutritionists for many years as potential products for aquafeeds these ingredients have only recently achieved a special position among the viable alternatives to FO and FM due to the drastic enhancement of the production technologies. SCI, which encompass the taxonomic groups of bacteria, fungi, and microalgae, are functionally divided in single-cell proteins (SCP) and single-cell oils (SCO) based on the nutritional content of protein and lipid. Although microalgae have arguably been a major focus on a plethora of studies, there has also been a noticeable interest, especially over the last years, on applications with yeast and bacteria as protein, lipid and other bioactive aquafeeds supplements. The major bottleneck in the rapid escalation of SCI utilization has been the difficulty to economically compete with other low-cost ingredients. Thence, the development of methods and strategies for larger-scale and lower-cost production will be essential for their application in aquaculture. To simultaneously maximize cellular growth and mitigate the production cost, several potential feedstocks for the production of SCP are explored, including wastewaters, industrial and agricultural residues, and bioindustry by-products, which require diverse production processes, for example, aerobic, anaerobic, or photosynthetic bioreactor (reviewed in Jones et al. 2020). For instance, an economically viable approach that is currently applied by commercial companies is the utilization of biodegradable agro-industrial residues as nutritional substrates for the cultivation of microorganisms.

    6.1 Bacteria and Fungi

Fungal SCI, which are categorized in filamentous and unicellular fungi (yeast), have gained increased research interest for aquafeed production, especially as supplements, due to their relatively high protein (45%), energy and micronutrient content and high digestibility (average 80%). Among fungal SCI, there has arguably been a predominance of work with yeast. Although the homogenization process has been used to improve the digestibility of yeasts due to the thick cellular walls, this procedure jeopardizes the beneficial prebiotic and immunostimulatory effects of components present in the walls such as mannan-oligosaccharides (MOS), beta-glucans, chitin, and nucleotides.

The experimentation of bacterial products in aquafeed has targeted bacterial SCP that encompass three genera Spirulina, Methylococcus, or Methylophilus. In addition to the high digestibility (average 86%), bacterial SCP also have a high protein content (average 60%) and an EAA and nonEAA suitable profile. It is fundamental to highlight that the nutrient composition of fungal, microbial, and algal SCI vary drastically with the production conditions (e.g., temperature, drying time, organic substrate, harvesting time, and storage). A recent review on potential sources of SCP strains and their respective production processes have been published by Jones et al. (2020).

   6.2 Microalgae

Microalgae, the basis of the trophic chain in the oceans, are the primary alimentary source for the zooplankton, which in turn are the nourishment for fish. Key nutrients, such omega-3 fatty acids, flow from microalgae to fish through the trophic chain. Hence, why not entirely bypass the marine food chain in order to obtain these precious nutrients for direct consumption through algal culture? Microalgae are widely produced as aquafeed supplements and have a privileged position as a potential bulk feedstuff characterized by excellent levels of protein, lipid (in particular EPA, DHA, and ARA), vitamin, and minerals. It is also worth mentioning that several species have a high content of astaxanthin, a pink pigment with immunostimulatory and antioxidant properties. A further motive to spur the delving of sustainable microalgae inclusion in aquafeeds is their potential to provide the basis for a circular aquaculture industry in the framework of a greater bioeconomy. Indeed, microalgae production, through a biorefinery approach, may lead to numerous benefits for the aquaculture sector including the mitigation of its ecological impact.

Scientists have endeavored to ameliorate the technology for the large-scale production of microalgae and a myriad of studies have been published regarding the successful inclusion of several microalgae species on aquafeeds for herbivores, omnivores, and carnivores fish. The major bottlenecks limiting the expansion of microalgae biotechnology are the expensive production costs and the scarce number of current large-scale production systems. These issues may be solved in the near future through an increase by several orders of magnitude of the production capacity, thus enhancing the efficiency of the production systems (e.g., photobioreactors and fermentation systems) with a subsequent cost reduction. Another issue in microalgae production that has received particular attention in the last years is related to their indigestible recalcitrant cell walls that prevent the absorption of the nutritional cell content. Mechanical (bead milling, ultrasonication, microwave, pulsed electric field, high-speed homogenization, and high-pressure homogenization) and nonmechanical (enzymatic and chemical) treatments for cell-wall disruption have been developed to overcome this hurdle. However, these practices are still economical inconvenient in relation to their benefits.

Most microalgae species investigated in aquaculture are encompassed in the genera Chlorella, Tetraselmis, Tisochrysis, Nanofrustulum, Scenedesmus, Isochrysis, Arthrospira, Phaeodactylum, Haematococcus, Pavlova, Haematococcus, Nannochloropsis, and Desmodesmus. A multitude of studies have been conducted over the last years to estimate the optimal microalgae inclusion levels for several fish species.

The novel microalgae-based ingredients have the potential to complement or entirely replace FM and FO in aquafeeds. However, major advances are required to substantially dwindle the production cost. The discovery of new cost-effective technologies and nutritious highly-digestible strains and the development of biorefinery approaches to extract multiple co-products from microalgae biomass will allow an exponential diffusion of microalgae utilization in aquaculture.

   6.3 Marine Heterotrophs Thraustochytrids

Despite often mistaken in published literature as microalgae, Thraustochytrids are marine obligate heterotrophic unicellular protists. Because of the lack of a plastid or any vestiges of photosynthetic apparatus, Thraustochytrids are incompetent to photosynthesize and rely on the presence of organic matter to grow and develop. They are classified in five genera Thraustochytrium, Aplanochytrium, Japonochytrium, Ulkenia, and Schizochytrium, while within the group of Shizochytrium sensu lato three further genera have been proposed: Aurantiochytrium, Oblongichytrium, and Schizochytrium sensu stricto. The interest for these microorganisms to be mass-cultivated has increased exponentially due to their high content in LC-PUFAs, mainly DHA, carotenoid pigments, squalene, and exopolysaccharide.

Among all the genera, Schizochytrium sp. contains high lipid levels (55%–75% in dry matter), including up to 49% of total lipids of DHA. Numerous studies on Atlantic salmon and rainbow trout have been conducted to assess the potential inclusion of Schizochytrium (dry whole-cell and extracted oil) as FO substitute. Recent studies have demonstrated that Schizochytrium sp. (T18) oil is adequate to entirely replace FO without compromising Atlantic salmon growth performance and digestibility of dietary nutrients, energy or fatty acids, while simultaneously contributing to DHA levels elevation in the muscle. The latter was also evident in the liver and the muscle of rainbow trout fed diets containing Schizochytrium sp. oil. Dietary Schizochytrium sp. as FO substitute at 20% enhanced the WG, SGR, feed efficiency, and immune response in rainbow trout. Additionally, Schizochytrium sp. supplementation in a plant-based diet sustained the reproductive performance and egg quality in female rainbow trout broodstock and provoked metabolic programming processes in the progeny. The inclusion of the abovementioned Thraustochytrids species has also been successfully tested in marine fish species. However, issues related to the ingestible nature of the recalcitrant walls of these organisms, the presence of NSPs and the low level of EPA need to be further addressed to increase the inclusion of these microorganisms in aquafeeds. As for the latter, a dietary blend of Schizochytrium sp. with EPA-rich microalgae species has been suggested as an effective diet-formulation strategy for FO replacement in gilthead sea bream . Also, the use of Schizochytrium oil from a strain that produces both EPA and DHA has proved an effective source of omega-3 fatty acids for Atlantic salmon post-smolts.

7 Relationship Between the Novel Ingredients and the Nutritional Requirements of Finfish Species

The EAA requirements are quantitatively differentiated among finfish species as well as in relation to the stage of development and body size (e.g., fry and broodstock). To a large extent, fry tend to require higher levels of most EAA compared to broodstock. Despite the species–specific differences, finfish, similarly to most terrestrial vertebrates, presents a common core in terms of the 10 EAA required in the diet: arginine (Arg), histidine (His), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), threonine (Thr), tryptophan (Try), and valine (Val). Among the alternative ingredients reviewed herein, insect meal (HI and TM), microalgal meal (Chlorella vulgaris), PMM (Poultry meat meal), and SDHM (Spray-dried haemaglobin meal) showcase the most optimal EAA content, similar to that of FM, and seem to meet most of finfish requirements. For instance, IM EAA profile is adequate for both salmonids and marine finfish despite the lower content in arginine and lysine compared to FM. However, EAA deficiencies are expected to occur especially in higher FM substitution levels. High histidine and methionine deficiencies are apparent in microalgae meal, while SDHM is deficient in isoleucine, phenylalanine, and valine.

Similarly, according to the fatty acid requirements of finfish, Krill oil (KO) is an optimal ingredient for aquafeed with higher levels of EPA, DHA, linoleic, and arachidonic acid compared to Fish Oil (FO). Additionally, Isochrysis biomass fulfills the fatty acid requirement of both salmonids and marine finfish and could also be consider as an alternative to FO despite the lower EPA levels. On the contrary, the applicability of Schizochytrium sp. (T18) oil may be limited by the low linoleic and arachidonic acid content and thereby should only be utilized as FO substitute in the diets of gilthead sea bream. Among the alternative ingredients, microalgae, and macroalgae seem to not fully meet the aforementioned requirement, while PAP, KM, and bacteria are sufficient in CP content.

With a meta-analysis of the scientific literature to identify new ingredients that effectively meet the protein (amino acids) and lipid (EPA and DHA) needs of fish. The study highlights the importance of careful reformulation strategies in aquaculture feeds, although microalgae are promising, they may require additional sources of essential amino acids to achieve optimal nutrition.

8 Conclusions

The rapid expansion of the aquaculture industry entails an increased demand for aquafeeds, which must be in compliance with both the nutritional requirements of the cultured species and the latest sustainable development strategies. Current industry practices and strategies might indicate or support recommendations that are more imminent and immediate, even reflect a transition to the ideal situation. Recommendations with a longer timeframe might shift the emphasis to sustainability in a changing world, decarbonisation, biotechnology, and the need to separate feed and human food ingredients. Underpinning the recommendation would be the nutrient profile and the suitability for aquaculture species. The novel feed ingredients reviewed herein acquire several desirable nutritional and health-promoting characteristics, while simultaneously serve the turn of the sector toward sustainability. A number of the discussed novel ingredients are already utilized in the commercial production of aquafeeds, while others may potentially substitute FM in the diets of salmonids and marine fish species. However, numerous bottlenecks and drawbacks deter the efficient total replacement of FM and FO with alternative ingredients without negative implications in feed characteristics, fish health and growth, or production cost. Single cell ingredients and biotechnology-driven solutions alongside the unexplored potential for product modification and the role in decarbonisation may be considered a robust recommendation. Although an inherently attractive proposition due to the essential nutritional profile and the waste reduction through the circular economy, insects may not be as advanced as required to be at this time. Collectively, new ingredients need to be tested in a more integrative, holistic and multifactorial fashion evaluating their consistency, price, availability, nutritional qualities, and absence of contaminants or anti-nutritional factors. Nutritional requirements are not a monolith but rather display diversification depending on miscellaneous factors such as the species-specificity, the developmental stage, the digestive and metabolic adaptations, the genotype, and the environment. Thus, nutrition approaches in regard to dietary ingredients and additives ought to be driven by precise knowledge. Furthermore, nutritionists should not focus their efforts in finding a “magical” ingredient with extraordinary nutrient qualities but to produce the appropriate combination of complementary products, including sustainable produced FM and FO, which along with the improved commercial-scale production technologies and strategies may generate a low-price highly nutritious diet.

Source : Panteli, N., Kousoulaki, K., Antonopoulou, E., Carter, C., Nengas, I., Henry, M., Karapanagiotidis, I. and Mente, E. (2024), Which Novel Ingredient Should be Considered the “Holy Grail” for Sustainable Production of Finfish Aquafeeds?. Rev Aquac. https://doi.org/10.1111/raq.12969

Aquaculture Feed magazine Africa Volume 1, Issue 3. 2024