Aquatic animal nutrition research began in the mid‐1960s with the development of semipurified diets to investigate and determine both qualitative and quantitative nutritional requirements. This information was then applied to the development of practical feeds composed of highly digestible ingredients selected to satisfy those nutritional requirements. Prompted by the desire to produce economically and environmentally sustainable feeds, focus evolved to the identification of feedstuffs that could serve as alternatives to replace fishmeal and fish oil ingredients and progress and application continue. With mounting emphasis on sustainability and efficiency, focus has naturally progressed into and intensified within the realm of feed additives, that is, probiotics (bacteria) and prebiotics (chemical compounds), added individually or in combination, to manage populations of intestinal bacteria, as well as the inclusion of exogenous enzymes. Ultimately, these feed additives offer the potential of positively contributing to environmental and economic sustainability that is critical to the realization of increases in global aquaculture production, particularly within intensive production systems (NRC 2015).

A brief review of the six issues of volume 48 (2017) of the Journal of the World Aquaculture Society (JWAS) revealed a total of six articles that report research results about the influence of feed additives, particularly probiotics and prebiotics, on certain measured performance variables. JWAS is serving as an important contributor to the understanding of this explosive area of research seeking effective applications of these additives that will yield a positive impact. The principal goal of the use of probiotics and prebiotics in feed is to modulate the microbial community of the intestine to benefit the host organisms so that healthy gut can be established. Healthy is recognized as imparting an increased efficiency of the use of feed combined with an enhanced ability to resist diseases. The ability to naturally confer disease resistance would be a welcome replacement of the chronic use of subtherapeutic levels of antibiotics to manage the incidence of pathogenic bacteria. Success would curtail the development of antibiotic‐resistant strains and possible accumulation in muscle that could adversely affect the health of the consumer.

Use of probiotics and prebiotics in the poultry and swine production industries originated in the 1970s and success is based on the establishment of a proportional balance of microbial species within the intestine. Established mixtures of live pathogenic strains, particularly Lactobacilli and Streptococci, are commonly added to diets. Short‐chain organic acids and oligosaccharides are the prebiotics that have received the most attention. Exogenous enzymes have routinely been used as feed additives in terrestrial animal production. Classes of phytase, carbohydrase, and protease enzymes have effectively increased nutrient availability, reduced environmental impact, and achieved a positive return on investment. In fact, applications have now entered the realm of combining probiotics and enzymes (symbionts) as feed additives to further enhance growth and immunocompetence (Dersjant‐Li et al. 2015). Like any new venture, the underlying challenge was a successful movement from the demonstratively beneficial effects of these additives to strategies for successful application.

Use of additives, particularly probiotics, in the diets of aquaculture species became the subject of investigations during the last decade of the 20th century. By the beginning of the 21st century, reviews of published investigations began to appear. However, achieving success of feed additives in aquaculture presents unique challenges that exceed those that originally confronted the poultry and swine production industries. For the vast number of aquatic animal species that are currently farmed globally, the problem of host specificity introduces greater complexity in achieving the successful application of specific probiotics and prebiotics as feed additives. The characteristically different trophic levels combined with the physical and chemical diversity within different aquatic production systems would seem to present an almost formidable task. The “one size fits all” principle, that is, common application and corresponding response, does not seem highly plausible. The multivariable complexity is an inherent characteristic for aquaculture. The investigative work that has followed has led to the realization of a need to establish criteria for selection and use. However, despite increased sophistication of methodology and extensive research investigations, many gaps in knowledge remain.

The enzyme phytase has been effectively incorporated into poultry and swine (Humer et al. 2015) and aquaculture feeds (Lemos and Tacon 2017) and has notably improved the utilization of phosphorus found in plant‐derived meals that are used as sources of dietary protein. The use of carbohydrases in the feeds of different fish species has yielded mixed results, some of which may be found in differences in digestive physiology or simply differences in methodological approach (Castillo and Gatlin 2015). For exogenous enzymes as additives, a commercial production strategy that is applied to the manufacture of terrestrial animal feeds, that is, pelletization, is not readily transferrable to aquafeeds manufactured by an extrusion process. The solution may reside in the development of thermostable enzyme additives (Bedford 2000), modification of methods of introduction subsequent to the extrusion process, modification of the extrusion process itself, or predigestion of a feed ingredient prior to inclusion in the feed.

Probiotic and prebiotic additives that are destined for practical application strategies must be subject to rigorously defined methodology that is characterized by highly controlled experimental conditions, a standardized protocol of selection criteria, an effective method of introduction, and consistent production and reevaluation. Certain response criteria for aquaculture species should be based on universal standards of evaluation. Additionally, factors such as source, strain, dose, and even duration of provision must be recognized as affecting the presence and magnitude or complete absence of a response. An appreciation of limitations based on particular methods of preparation (manufacture), the species (host) under consideration, and testing conditions must also prevail. Field testing is critical because positive results obtained in the laboratory may not be applicable to field conditions. In addition, in many cases, assessment of the efficacy of additives has not been conducted under the actual commercial production system that is characteristic of the species under investigation.

The importance of such a controlled approach of evaluation for aquaculture was recognized early as illustrated by the guidance offered by Verschuere et al. (2000). Monitoring tools need to be developed and established, whereby the composition and function of the gut bacteria biome can be confidently characterized. For probiotics, the ability to produce a consistent response must include a method of dietary inclusion that successfully preserves the quantity and quality (activity) of the additive, whether it be live, with or without a protective barrier, freeze‐dried or heat‐killed bacteria, or even a bacterial extract. The method of delivery may be integral to effecting a response and may even be species specific. Possible adverse side effects on the nutritional quality of a product must also be considered in assessing the overall value of a feed additive.

The positive benefits of the use of feed additives for aquaculture are undeniable. An impressive amount of knowledge has been gained over the past 10–15 yr as evidenced in recent reviews devoted to probiotics (Ringø et al. 2010; Lakshmi et al. 2013), prebiotics (Gatlin 2015; Hoseinifar et al. 2017), and exogenous carbohydrase enzymes (Castillo and Gatlin 2015). We are at a crossroads wherein a notable gap between science‐based knowledge and successful practical application needs to be closed. The lack of such an integration somewhat resides in an incomplete understanding of what the product is actually doing and under what conditions desired beneficial performance responses will occur predictably and consistently. Establishment of rigorous and systematic assessment techniques and attention to detail are essential for confident progression into the realm of cost‐effective commercial‐scale application. We are not there yet! Such a situation has been affirmed by Merrifield et al. (2010) for use of probiotics and prebiotics in salmonid feed.

An understanding and application of the knowledge of the efficacy of probiotics, prebiotics, and exogenous enzymes continue to move forward with the realization that they indeed make a positive difference in the health and digestive efficiency of the host, results that translate into the desired economic benefits. Probiotics and prebiotics are often included as ingredients in commercially produced aquafeeds. The aquaculture feed manufacturing industry is now confronted with a plethora of commercially available probiotic, prebiotic, and enzyme products. Resourceful companies that have developed feed additives have capitalized on the increased efficiency of production and corresponding economic benefit achieved by the swine and poultry industries to entice aquaculture producers and the corresponding feed industry. Defining the role of probiotic, prebiotic, and enzyme additives in aquaculture remains in an emergent state. However, this condition offers a unique opportunity for investigative collaboration of teams composed of public‐ and private‐sector researchers and growers. Results of comprehensive testing must be transparent and communicated forthrightly to the aquaculture community.

Feed additives represent an exciting opportunity to support the sustainability of the aquaculture industry. However, investigative approaches must be found on a prevailing realization of the complexity of multiple species, production systems, and forms of additives, as well as feed manufacture limitations. With this acknowledgment, combined with a recognition of the need for standardized evaluation, a definitive path toward achieving the highest level of success is possible. The JWAS welcomes manuscript submissions of both fundamental and applied research investigations of probiotics, prebiotics, and exogenous enzymes for publication. These contributions will assist in attaining a specific level of understanding that is ultimately needed for the sound and confident use of probiotics, prebiotics, and exogenous enzymes in commercial aquafeeds.

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