Knowledge Gaps Concerning Bioactive Compounds in Fish Feed

Meeting the nutritional demands of aquaculture species is essential for efficient and, thus, sustainable feed production. Fishmeal unquestionably represents an excellent source of quality protein, essential amino acids, polyunsaturated fatty acids, vitamins, minerals, nutraceuticals (phytochemicals antioxidants), and bioactive compounds (peptides, nucleotides, and creatine) [1]. However, the economic and environmental constraints of using fishmeal in fish diets are significant [2]. Therefore, previous and ongoing research has dramatically changed feed formulations, replacing fishmeal with various alternatives, including plant- and animal-based proteins, insect meals, and microbial protein [2]. However, research activities have mainly focused on aligning and optimizing the nutrient composition of fish feeds when integrating fishmeal alternatives, and thus, for instance, recommendations of dietary inclusion rates of bioactive feed compounds still vary dramatically in the literature (Table 2). Further, some fishmeal alternatives lack or are deficient in bioactive compounds (Table 1) that are considered important for fish health and growth [32], and thus need substitution. It is crucial to enrich scientific data about the nutritional implications of bioactive compounds in fish feeds in addition to protein, fat, and energy content, amino acid, or fatty acid profiles to provide valuable recommendations for nutritionists. Consequently, this paper aims to highlight the knowledge gaps for capturing the full potential of bioactive compounds in diets containing fishmeal alternatives. While extensive literature exists for some bioactive compounds like cholesterol and some minerals, our study considered bioactive compounds (Table 1) that are underreported, particularly with regard to their effective dietary inclusion rates, species requirements, life stages, and specific effects (e.g., nucleotides and glycosaminoglycans).

Bioactive compounds are naturally occurring chemical substances found in small quantities in plants, animals, or other living organisms that have biological effects on cells or organisms [33]. The bioactive compounds considered in this paper (Table 1) are found in fishmeal in high amounts and are limited or not present in several fishmeal alternatives [34]. Additionally, they have known effects on fish but lack critical information for their effective and safe application in aquafeed (Table 2). In our analysis, we focused on two key plant-based proteins used in commercial fish feeds (soybean and rapeseed protein), two animal-based protein sources (blood meal and feather meal), and two novel feed ingredients that are discussed for their potential in sustainable feed production (insect and microbial protein).

Trimethylamine oxide is a non-protein nitrogen compound naturally occurring in fish (Table 1). TMAO is sparsely studied but is reported to promote growth, robustness, health, and feed utilization when included in fish diets [4-6]. However, adverse effects on product quality were observed [3], and TMAO has been suspected to lead to chronic diseases in humans [35]. Inclusion rates with positive effects on fish and without adverse effects on consumers must be determined.

Phospholipids are widely recognized as essential fish feed ingredients. However, their significant effects are often neglected for fish beyond their juvenile life stage. Fish in the grow-out phase can synthesize phospholipids endogenously. Hence, no requirement has been noted for this life stage [7]. However, current research suggests that, similar to creatine, supplementation can still improve growth performance in post-juveniles [8, 9]. The composition of dietary phospholipids could be, in addition to their dietary content, a determining factor for their effects [7]. Future research should focus on species-specific and composition-related effects, especially in fish beyond their juvenile life stage, as they are understudied [7, 8].

Nucleotides are naturally present in plants and animals, as well as in their byproducts (Table 1), in the form of free nucleotides and nucleic acids [34]. NucleoforceFish, AccelerAid, and Bioiberica are examples of commercially available products [34]. However, understanding the digestion, absorption, and metabolism of exogenous nucleotides remains limited. Significant knowledge gaps persist regarding optimal dosage, age-specific responses, and timing of administration [34].

Glycosaminoglycans (GAGs) are abundant in fishmeal, microbes, animal byproducts, and insects but are lacking in plant-based proteins [36] (Table 1). Fish and marine fisheries waste represent cheap sources of GAGs [36]. GAG application in aquafeeds is still underexplored, with limited scientific information on dietary inclusion rates and species-specific requirements. More research is needed to ascertain the effects of substituting animal protein with plant protein on farmed fish, considering glucosamine content [15].

Betaine, a non-essential amino acid and choline metabolite, offers potential benefits for aquaculture species. Nonetheless, a standardized dietary inclusion rate remains elusive. A recent review recommended a 0.99 g/kg inclusion rate for farmed aquatic species [37]. However, it fell short of offering species-specific rates crucial for effective nutritional management. Moreover, other study recommendations have focused on individualized or specific health outcomes under peculiar experimental circumstances, potentially overlooking the complex, life-stage-specific needs of the fish.

Creatine is an amino acid derivative found in animal tissues but not in plant proteins (Table 1) that can promote growth in different fish species. Exogenous creatine sources include creatine monohydrate, creatine hydrochloride, creatine anhydrous, and guanidinoacetate. However, Schrama et al. [38] alleged that creatine might cause allergenic concerns in fish, possibly due to unguided inclusion rates in fish diets. This lack of scientific information is especially prevalent regarding the life stages of different fish species [34].

Collagens are abundant in fish, insects, and terrestrial animal byproducts but are entirely lacking in plant-derived proteins such as soybean and rapeseed [23] (Table 1). Low-cost sources of collagen include leather waste and hydrolyzed fish by-products [23]. Collagen supplementation in fish diets is promising (particularly those based on plant proteins), although adverse growth outcomes have been reported [23]. Besides, limited literature exists on optimal collagen dietary inclusion rates for different fish life stages and species-specific requirements.

Dipeptides such as carnosine, balenine, and anserine are water-soluble non-nitrogen protein compounds exclusively found in fish meat and animal tissues [25] (Table 1). It is hypothesized that dietary supplementation with dipeptides in plant-based diets could improve cultured fish growth performance and muscle development [25]; however, research is still limited.

Tripeptides are oligopeptides formed from three amino acids necessary for fish development, growth, metabolism, and immune and antioxidant defenses. Dietary fortification with exogenous tripeptides is essential, especially for plant-based diets, to compensate for potential deficiencies in essential amino acids and may aid in their absorption and metabolism [39]. However, their physiological roles and dietary needs per species require further investigation.

Aquaculture nutritionists must broaden their focus beyond the nutritional content when assessing the suitability of novel and established fishmeal alternatives. Bioactive compounds are vital as they can enhance diet stability, functionality, and palatability to promote fish growth and health. Bioactive compounds improve feed palatability through appetite stimulation and enhanced aroma/flavor, increasing feed intake. Additionally, these compounds support better nutrient absorption and digestibility by modulating the gut microbiota. This is particularly important when addressing nutritional gaps in alternative feed sources. By definition, bioactive compounds have biological effects on organisms that can be beneficial or detrimental, depending on the context and concentration. Nonetheless, multiple factors of these bioactive compounds in aquafeeds are still underrepresented in current literature. They must be addressed so that these compounds can be used to their full potential and that their detrimental effects on fish are limited. The optimal inclusion level, considering species ecotype, specification, life stage, digestibility, palatability, and the microbiome, among other factors, is essential to research in the near future.

Adequate inclusion rates are the limiting factor for the effective and safe use of most bioactive compounds discussed in this paper. Even basic information on their effects on fish is lacking for dipeptides and glycosaminoglycans. Research on the effects of phospholipids on post-juvenile fish needs to be addressed. Additionally, the literature has insufficient information regarding the chemical properties and quantities of the discussed bioactive compounds in several alternative protein sources.

Furthermore, the variability in experimental inclusion levels (Table 2) raises questions about the efficacy and safety of these ingredients, as different studies often report conflicting results. Factors such as species-specific responses, ingredient processing variations, interactive effects with other dietary nutrients, and the base diet's nutritional composition can all contribute to these discrepancies. Deficient inclusion rates can limit their availability for the targeted effect, as they may be diverted from their intended physiological targets to fulfill more critical biological functions. In contrast, excessive inclusion may lead to inefficient utilization or detrimental effects [38]. Understanding the optimal dosage of these compounds is crucial for achieving the desired physiological outcomes.

Aquaculture researchers should prioritize these limiting factors of bioactive compounds to ensure safety for farmed species. By addressing these issues, they can leverage the growth and health benefits of bioactive compounds while managing risks such as adverse interactions and toxicity. This information is needed to establish standardized diet formulation protocols from optimized nutritional safety and aquaculture productivity perspectives. This can pave the way for fishmeal replacement or supplementation strategies that ensure efficient aquafeed production and improve animal health.

Stanley Iheanacho: conceptualization, writing – original draft, visualization, writing – review and editing, validation, data curation, investigation, funding acquisition. Stéphanie Céline Hornburg: writing – review and editing, validation, methodology. Carsten Schulz: conceptualization, supervision, data curation, writing – original draft, writing – review and editing, validation, methodology. Frederik Kaiser: supervision, data curation, conceptualization, investigation, writing – original draft, writing – review and editing, validation, methodology.

The authors declare no conflicts of interest.