Aquaculture's unparalleled growth cannot be achieved at the expense of environmental and social responsibilities. Efficient policies and legal frameworks are needed to safeguard sustainable and equitable aquaculture development with generalized and improved socioeconomic benefits to players along the production and value chain.

The need to decrease the dependence on fish meal and fish oil in the formulation of suitable aquafeeds for cultured species (especially marine) has long been recognized by the aquaculture sector. The stagnation of world fisheries, along with the decreasing trend of fish captures destined for non‐food uses (Food and Agriculture Organization, 2018), has prompted the aquafeed sector to explore alternative ingredients, either marine or land based. Although this search has often pointed toward a number of alternative protein sources, the sustainability of using such alternative ingredients has often been questioned. For instance, soy‐based protein products might be associated with superintensive farming on deforested areas, whereas the use of grains can compete with supply for human consumption.

Another well‐known and recurrent threat to aquaculture expansion is related to disease outbreaks and ineffective biosecurity practices. In the last two decades, most top‐farmed species have faced at least one major disease outbreak that has seriously jeopardized production and even shut down some operations worldwide. The need to enhance the resilience of aquaculture practices to disease is paramount, and innovative solutions must be fostered to allow enduring and more sustainable aquaculture.

Functional aquafeeds can be defined as a feed that provides superior performance than that achieved using a conventional feed through incorporation of specialty ingredients that promote growth, health, and survival (Soto, de Jesús Paniagua, Michel, & Ochoa, 2015). Crafted aquafeeds can play a key role on the pathway to ever more sustainable practices, thereby decreasing environmental footprint and improving economic revenues of farmers.

Plant‐based and algal‐based diets already contribute to decreased dependence of aquafeeds on fish meal and fish oil. However, the use of these ingredients in aquafeed formulations commonly requires the use of functional supplements to enhance performance of the target species. The use of supplements ranges from yeast extracts to phytogenics, probiotics, and prebiotics, among others, each of which exhibit different modes of action. This remarkable variety of supplements offers the opportunity to provide unique tailor‐made functional aquafeeds that can boost profitability. Decreasing the incidence of disease means environmental hazards often associated with the abusive use of therapeutics and disinfectants are also significantly reduced. The risks of potential trade sanctions associated with the presence of prohibited substances and/or excessive levels of allowed therapeutic/prophylactic agents in farmed organisms are also low when using functional aquafeeds that feature health‐promoting supplements.

Functional aquafeeds can also enhance feed conversion, decrease nutrient leaching from the feed, and fine‐tune target species excretion, thus minimizing nutrient loadings on aquatic ecosystems. The main goal of this special issue on functional aquafeeds is to provide a collection of studies that showcase the most recent academic and industry‐driven scientific breakthroughs on this key topic for a more sustainable aquaculture.

1 FURTHER REDUCING FISH MEAL LEVELS IN AQUAFEEDS

Ongoing research to replace fish meal and to find alternative protein sources has been followed by efforts to reduce crude protein levels in aquafeeds. Aquafeeds, particularly for shrimp, often have protein levels in excess of nutritional requirements as well as a fraction of indigestible protein. Excess protein promotes a number of problems, such as the buildup of nitrogenous compounds and phosphorous in culture water and soil, ultimately affecting water quality and increasing the risk of disease outbreaks. Nunes, Sabry‐Neto, and Masagounder (2019) showed that negative growth and feed efficiency effects of reducing crude protein in shrimp feeds can be minimized through a carefully balanced supplementation of crystalline amino acids.

Phytogenics have also been used as a promising strategy to counterbalance the negative consequences of replacing fish meal with vegetable ingredients, particularly in fish feeds. Gonçalves et al. (2019) showed that a blend of plant essential oils can be used to supplement low fishmeal diets of European seabass (Dicentrarchus labrax). The tested phytogenics compensated for negative fish performance, nutrient utilization, and health consequences of replacing fish meal with a mix of soy protein, wheat gluten, corn gluten meal, soybean meal, and rapeseed meal.

Alternative ingredients often contain antinutritional factors that negatively affect feed palatability. Marine chemoattractants have been used to offset the effects of fish meal replacement, as shown by Nunes, Sabry‐Neto, Oliveira‐Neto, and Burri (2019). This study showed that krill meal enhanced growth in addition to being a feeding effector for juvenile Litopenaeus vannamei in fishmeal‐challenged diets. Positive results were also found for squid meal, although its use as attractant and feeding stimulator supplement in shrimp feeds often faces issues of quality, cost, and fluctuating availability. Zhu et al. (2019) tested the application of a palatability enhancer encompassing selected amino acids and nucleotides, as well as other flavor enhancers of nonanimal origin, to replace squid meal in shrimp feeds. Besides confirming the beneficial use of such a palatability enhancer and growth promotor, it also showed positive effects on hepatopancreas protease activity and intestinal morphology, illustrating that functional aquafeeds can contribute to growth performance as well as improve health and disease resistance.

2 DISEASE AND HEALTH MANAGEMENT

Probiotics have been widely acknowledged as an important feed supplement in aquafeeds and have the potential to improve water and soil quality in ponds. Al‐Hisnawi et al. (2019) showed the potential of probiotics for intestinal microbiological modulation in rainbow trout (Oncorhynchus mykiss). The study also showed that gene expression data can help explain some of the mechanisms behind the benefits of using probiotics in fish feeds, namely on fish intestinal morphology, immunity, and disease resistance. Similarly, do Vale Pereira, Pereira, Soares, Mouriño, and Merrifield (2019) showed that probiotic bacteria can modulate the gut microbiome, morphology, and immune status of Pirarucu (Arapaima gigas). In addition to specific fish species effects, probiotic strain, and culture settings where probiotics are used, the timing of their supplementation is also of importance for the successful use of probiotics. Kesselring, Gruber, Standen, and Wein (2019) showed that continuous application of a multispecies probiotic is needed to delay L. vannamei mortality when exposed to a Vibrio challenge. The beneficial effects of probiotic administration were not as pronounced when an intermittent application was used as compared to a continuous one.

Together with probiotics, prebiotics have also been used in aquafeeds because of their immune modulatory properties. Rawling et al. (2019) showed the effect of feeding a novel multistrain yeast fraction on European seabass (D. labrax) intestinal health and growth performance. Since this was the first study assessing the use of a multistrain yeast fraction to improve intestinal health and fish performance, it is critical that future studies use similar approaches to assess if such a positive response is dose‐dependent and yeast‐strain‐dependent.

Besides pro‐ and prebiotics, additional functional supplements to improve fish health and disease resistance are also reported in this special issue. For instance, Wang, Zhou, Wang, Mai, and He (2019) investigated the effects of dietary silymarin on the growth performance of turbot (Scophthalmus maximus) as well as its antioxidant capacity and intestinal inflammation when a high plant protein‐based diet was provided. This functional supplement is a flavonoid derived from milk thistle commonly used to treat liver failure and fetal diseases. Xu et al. (2019) pinpointed some positive effects of supplementing grass carp (Ctenopharyngodon idella) feeds with quercetin extracted from the traditional herbal medicine Eucommia ulmoides, which is also a flavonoid compound.

3 THE WAY FORWARD

The articles within this special issue advance the state of the art on the use of functional supplements for aquafeeds. Some of the approaches reported were employed for the first time in aquafeeds, such as the supplementation of a multistrain yeast fraction, whereas other approaches focus on minimizing the negative effects of replacing fish meal with alternative protein sources, particularly from vegetable origin. Growth performance, feed conversion, and gut health parameters are commonly monitored in such applied studies. Nevertheless, it is important that future works survey additional parameters associated with environmental footprint and financial benefits to producers. Innovative approaches addressing the supplementation of aquafeeds to boost their functional role must go beyond biological performance and also assess the parameters associated with the dynamics of nutrient excretion of the target species, nutrient loading into the culture water, and potential deleterious effects on animal health. Establishing such a link between functional aquafeeds, animal performance, disease resistance, and nutrient loading is critical to fully embrace the definition of sustainable aquafeeds and improve public perceptions of the aquaculture industry.

Disruptive breakthroughs in the field of functional aquafeeds may be more easily achieved if information was more readily shared among academic and private research centers. Intellectual property issues are commonly cited as the main barriers to more effective sharing of information. However, potential revenues originating from intellectual property will likely be lower than the financial burden caused by wasting financial and human resources on parallel investigations, redundant studies, and scientific “dead‐ends.” Effective information sharing may also allow companies to better inform policy makers on present and future regulatory needs. Lack of suitable legal frameworks that encompass the most recent scientific breakthroughs often postpone their use by the industry. If such regulatory bottlenecks arise for functional aquafeeds, aquaculture may not be able to deliver the world its much needed sustainably farmed protein.