**1. Introduction**

World capture fishery production reached a peak of approximately 96 million tons in 2018; the most recent estimates suggest that 52% of marine stocks are fully exploited, 17% are overexploited, and 7% are totally depleted, while human population and the demand for marine and other aquatic resources continue to increase [1]. Global aquaculture would make a considerable contribution toward bridging the gap between supply and demand. Unfortunately, its development is hampered by an inadequate supply of feed, particularly fishmeal, which is scarce and expensive [2]. This has stimulated the evaluation of a variety of alternative dietary protein sources with the objective of partially, or totally, replacing fishmeal protein in aquafeeds [3]. The use of food industry waste as animal feed is an alternative of high interest because it stands to bring both environmental and public benefits, besides reducing the costs of animal production [4–6]. In particular, with reference to the fishing industry, the cost of waste management for aquaculture is typically in the range of USD 0.05 to USD 0.065 per pound of fish produced, representing concerns from both an economic and an environmental point of view. Waste management, in fact, contributes to the overall costs of production and reduces farmers' net income. Moreover, the improper management of fish wastes could have a negative environmental impact, such as eutrophication effects, on natural aquatic ecosystems [7].

**Citation:** Tropea, A.; Potortì, A.G.; Lo Turco, V.; Russo, E.; Vadalà, R.; Rando, R.; Di Bella, G. Aquafeed Production from Fermented Fish Waste and Lemon Peel. *Fermentation* **2021**, *7*, 272. https://doi.org/ 10.3390/fermentation7040272

Academic Editor: Diomi Mamma

Received: 28 September 2021 Accepted: 19 November 2021 Published: 21 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

By-products coming from the fishing industry, such as viscera, skin, scales and bones, representing up to 30–80% of the fish body weight, are discarded as solid wastes by industrial fish-processing operations [8] but, due to their composition, they have great potential to be used as protein supplements in aquaculture feeds [9,10]. Their conversion to aquafeed is also encouraged by the significant advantage that they do not require any thermal–chemical and/or enzymatic hydrolysis pretreatment steps. Since the pretreatment step is neither economically favorable nor environment-friendly, its elimination from the process makes the utilization of fish waste economic and more environmentally friendly [7].

Biotechnological methods like fermentation with microbe cultures are gaining more popularity for the treatment of waste [11,12].

Among the different microbes used, especially for the fermentation of animal/fish processing wastes, lactic bacteria have advantages over other microbes, as they are generally recognized as safe (GRAS) [13]. In addition, the products obtained upon fermentation with *Lactobacillus* are also reported to have additional beneficial effects on various aquatic animal intestines (anti-microbial properties, antioxidative properties), making them suitable for food/feed applications. In fact, they easily adapt to the intestinal environment of both aquatic and domestic animals, making them favorable for use in probiotic aquaculture feeds [14–18].

Among the microorganisms applied, yeasts have also been used as inoculum, along with lactic bacteria, to ferment fish waste [19] for converting it to a useful product that can be used as an ingredient to balance the food rations of animals. Yeast has many different immunostimulatory compounds, e.g., nucleic acid, b-glucans, and mannan oligosaccharides [20,21]. These compounds may enhance the growth of different fish species and therefore can be considered as the best health promoters for fish culture [22].

Feed nutritional composition is important; the major growth-promoting factors are proteins and lipids, since they are known to influence the growth and the body composition of fish [23,24].

Fermented fish waste is a liquid product, obtained by the liquefaction of tissues carried out by the enzymes already present in the fish and accelerated by an acid pH [10]. Natural fillers, such as agricultural by-products, can also be added to the substrate [25].

Citrus peel can be used as filler [26] during fermentation, playing at the same time an important role as a prebiotic source [27–29]. Among its beneficial effects, it has been reported that prebiotics can elevate fish resistance to pathogens and improve growth performance, feed utilization and lipid metabolism, as well as stimulating the immune response through the modulation of intestinal microbiota [18,30–32].

The aim of this research was to process non-sterilized fish wastes, supplemented with lemon peel as a filler and prebiotic source, by biological fermentation using combined starter cultures of *Saccharomyces cerevisiae* American Type Culture Collection (ATCC)4126 and *Lactobacillus reuteri* ATCC 53608 for bio-transforming these by-products into a high protein content supplement, rich in healthy microorganisms, for aquaculture feeds.

For this purpose, and to verify the optimum nutritional composition of aquafeed, proteins, crude lipids, ash and lipid content percentages were monitored throughout the process. The influence of the fermentation process on fatty acid concentrations was also evaluated. Finally, microbiological analyses of the starters and total and fecal coliform bacteria quantification were carried out, to evaluate the healthiness of the final product.
