How Current Global Aqua-Nutrition Research Matches the New Industry and Market Demands

As a result of decades of rapid development, aquaculture has become the largest contributor to human seafood consumption [1]. Consequently, research activity in aquaculture has prospered in the past decades. Specifically, aqua-nutrition research has made significant contributions to the development of the modern aqua-feed industry, and, as a section constituting the major cost (˃60%) of aquaculture activities, aqua-feed has come to form the base material of the aquaculture industry. However, considering the diverse aquatic animal species (˃730 species globally) [1], various ingredient sources, and different aquaculture modes, obtaining a global and comprehensive understanding of the current status of aqua-nutrition research is challenging, and these challenges prevent us from better understanding the entire research scenario in this area.

To address this, a year-round (April 2023 to March 2024) survey about publications concerning topics related to aqua-nutrition was conducted across 50 journals that were the main target journals for the publication of research articles in aqua-nutrition (Supplementary Material). After manual screening, a total of 1094 articles on topics concerning aqua-nutrition were collected, and the principal features of these articles, such as target nutrient, target species, animal development stage, water type, and author information, were also collected (Supplementary Table S1).

The variety of the target nutrients described in the collected articles constituted key information, with this helping to better understand the research interest in this area, as well as the new demands of the industry (Figure 1). In general, the proportion of the different nutrient categories targeted in the collected articles ranked as follows: feed additives (50.6%) > proteins (29.5%) > lipids (8.3%) > minerals (5.8%) > vitamins (3.5%) > carbohydrates (2.2%).

Throughout aqua-nutrition research, the major purpose has been to investigate the optimal requirement of key nutrients in aquaculture animals. Numerous studies have been conducted to determine the optimal requirements of crude proteins, crude lipids, key amino acids, key fatty acids, key vitamins, and key minerals in a number of economically important aquatic species [2]. This has fundamentally supported the formulation of commercial aqua-feeds. Nevertheless, as the aqua-feed industry develops, new demands appear, and research interests must adjust accordingly.

The results of the aforementioned survey clearly indicate that the current hottest research area in aqua-nutrition is feed additives. This is associated with a transformation in the ways the aquaculture and aqua-feed industries have developed. For instance, antibiotics have been globally used for disease prevention and treatment; however, it has been made clear that the use of antibiotics results in a series of negative impacts on environmental and food safety, such as drug residue in seafood products and increased drug tolerance in pathogens. Therefore, the use of antibiotics is being prohibited in progressively more countries, and alternative measures—in particular dietary strategies—are highly encouraged as a result.

Among the feed additives (607 articles), the top three most frequently studied nutrient categories were plant extracts (175 articles), microbial agents (92 articles), and polysaccharides (49 articles), followed by chemical compounds (40 articles) and medical herbs (39 articles). All of these feed additives are immunostimulants and health regulators that evidence the urgent need to search for alternatives to antibiotics.

Plant extracts and medical herbs have been a research hotspot for a decade due to the demand for natural and green health regulators in aquaculture. The foremost advantages of these additives are their high levels of safety and broad-spectrum efficacy. Moreover, due to the widespread distribution of candidate plant species all over the world, plant-derived health regulators stem from very diverse and widespread sources. In particular, in some remote places where other immunostimulators may not be available, local plants can be used as convenient health regulators for farmed aquatic animals. In the last year, extracts from over 100 plant species were studied (Figure 2), and, among all the feed additives studied, curcuma-derived compounds and tannins attracted the most research interest. The positive effects of these plant extracts on animal growth, immunity, and disease resistance have been demonstrated in a number of both fish and crustacean species [3,4,5].

Other than plant extracts, microbial agents are another research hotspot due to similar safety and efficacy concerns. Upon investigating the microbial agents studied in the literature, a noteworthy characteristic was that probiotics (74 out of 92 articles in total) still dominated this area; from these, Bacillus received the most research interest (Figure 3). Its mature producing techniques, stable efficacy, and—most importantly—high tolerance to pelleting temperature were the main reasons for its long-lasting popularity [6]. It is interesting to note that the research interest in bifidobacterium, which is popular in terrestrial animals, appears to be decreasing. By contrast, increasingly more aquatic host-derived bacteria were screened and used as probiotics for aquaculture species [7], as these probiotics may have a higher capacity to colonize and proliferate in the intestines of aquatic animals. Another important trend in this category was that postbiotics have emerged as a promising candidate for health maintenance. As non-viable probiotics, postbiotics can be more conveniently produced, stored, and applied in aqua-feeds [8].

Regarding polysaccharide, β-glucan continues to be a classic immunostimulant (Figure 4) that receives the most research interest, which is likely due to its stable efficacy and mature producing processes. However, other than yeast, novel sources such as oat and algae have been used to produce β-glucan [9,10].

After feed additives, the second and third most popular research topic category was proteins (354 articles) and lipids (100 articles), respectively, and were mostly centered on alternative protein (260 articles) and lipid sources (50 articles), respectively (Figure 1). The rapidly growing aquaculture industry has exerted mounting pressure on the availability of feed ingredients, in particular fishmeal and fish oil [1]. This has triggered an overwhelming research interest in searching for economically viable substitutes for fishmeal and fish oil. Among the protein sources studied, animal protein sources (120 articles) and plant protein sources (119 articles) received similar levels of research attention, with insects (52 articles) and terrestrial plants (102 articles) ranked as the hottest subtypes in research, respectively. Insect protein has emerged as a promising solution for the partial or complete replacement of fishmeal and fish oil. Insect protein production uses low-value, renewable food by-products as substrates and, thus, has many sustainability and environmental benefits, although in some cases, the nutrient composition of insect meal was unstable and varied depending on the type of substrate used [11]. Mycoprotein has also been used as a novel alternative protein source for aqua-feeds. Mycoproteins from two bacterial species, Clostridium autoethanogenum and Methylcoccus capsulatus, which use industrial waste gas and methane as substrates, respectively, have been approved for application in aqua-feeds in key global markets [12,13]. Large-scale applications of these novel mycoproteins could significantly contribute to efforts to lower carbon discharge in the aquaculture industry.

Compared to protein sources, the target lipid sources studied in the collected articles were distributed more homogeneously, ranking as follows (Figure 1): vegetable oil (28 articles) > microalgae (12 articles) > poultry oil (7 articles) > insect oil (3 articles). Vegetable oil continues to be a basic lipid source for aqua-feeds. However, microalgae oil and insect oil have emerged as important novel lipid sources. Microalgae oils can provide long-chain polyunsaturated fatty acids (LC-PUFAs), which had mainly been supplied by fish oil, and insect oil such as black soldier fly (Hermetia illucens) larvae oil contains a high level of lauric acid (12:0), making it a good substrate for energy-oriented β-oxidation. Nevertheless, their high production costs continue to be the major limiting factor for the large-scale application of these novel lipid sources. Therefore, other than searching for alternative lipid sources, maximizing the efficiency of fish oil utilization was studied as another way to spare the use of fish oil. One measure with regard to this concern involved the mixed use of fish oil and other alternative lipid sources in order to use the saturated and monounsaturated fatty acids in terrestrially sourced oils and spare the use of the LC-PUFAs in fish oil. Another measure involved the application of lipid metabolism modulators (37 articles among feed additives, Figure 1) in order to improve the digestion, absorption, and utilization of fish oil in fish and crustacean species.

Concerning the future, the research and development of novel feed ingredients will always be an urgent task and a hot topic for long-term periods, considering the shortage of key ingredients for aqua-feeds. Despite the fact that aquaculture only consumes a small fraction of feed ingredient resources compared to other animal food production systems, aquaculture competes with the livestock industry and direct human consumption for crop resources [14]. This needs to be seriously considered.

Among the minerals studied, selenium (23 articles) received the most attention, followed by zinc (14 articles), phosphorus (9 articles), and iron (6 articles) (Figure 1). Zinc is a component of a series of key metabolic enzymes such as carboxypeptidase, alkaline phosphatase, and lactate dehydrogenase and thus plays an important role in the metabolism of nucleic acids, protein and energy, antioxidant capacity, and the immune response. Minerals in their nano and organic forms are being more frequently used in aqua-feeds. Among the vitamins examined in the literature, vitamin D₃ (10 articles) and E (10 articles) were the most frequently studied, followed by vitamin B (9 articles) and C (8 articles) (Figure 1). The novel functions of vitamin D₃ in the metabolism of phosphorus and lipids have been elucidated. Compared to proteins and lipids, carbohydrates (only 26 articles) have attracted little research interest—much less than minerals and vitamins (Figure 1). This is because, to date, it is still very difficult to improve the ability of aquatic animals to utilize carbohydrates.

Similar to the distribution of target nutrients studied in the collected articles, the distribution of target animal species also matched market demands. The collected articles referred to 187 target animal species (Figure 5), 65.9% reared in freshwater and 34.1% in seawater (Figure 6). The top five species were all global species, including Nile tilapia (Oreochromis niloticus) (127 articles), Pacific white shrimp (Litopenaeus vannamei) (91 articles), rainbow trout (Oncorhynchus mykiss) (70 articles), largemouth bass (Micropterus salmoides) (67 articles), and carp (Cyprinus carpio) (43 articles). The other most frequently studied species, such as grass carp (Ctenopharyngodon idella) (34 articles), European sea bass (Dicentrarchus labrax) (31 articles), Atlantic salmon (Salmo salar) (31 articles), Chinese mitten crab (Eriocheir sinensis) (28 articles), hybrid grouper (Epinephelus fuscoguttatus♀× E. lanceolatus♂) (28 articles), and gilthead seabream (Sparus aurata) (25 articles), are important local species in key markets such as Europe and China. Among the countries studied, China contributed the most articles (more than 500 articles), matching the largest aquaculture production and researcher population there. Similarly, Egypt (62 articles), India (43 articles), and Iran (43 articles) made many contributions to aqua-nutrition research, which also matches the emerging, big aquaculture production, elevated markets, and growing research groups there [1].

Regarding the development stages of the animals studied, most of the articles (92.9%) focused on the juvenile and grow-out stages of commercial aquaculture species (Figure 7), with only a small proportion of these focusing on broodstock (3.5%) and larvae (2.4%). This was mainly due to the long duration, high-cost, and high-level technical requirements needed in the latter studies. However, nutritional research on broodstock and larvae is very important—even more important than those on juvenile and adult animals. Future systematic research in this area is highly recommended.

In conclusion, the survey of aqua-nutrition-related publications conducted over the last year revealed valuable information about the research trends and hotspots in this area. Both research on and the development of natural and green feed additives, novel feeding ingredients replacing fishmeal and fish oil, and new forms of trace elements are urgent needs and future directions in aqua-nutrition research. Additionally, research in aqua-nutrition must always match the demands of the aquaculture industry and the market, as they currently do.