The ability to control the sexual maturation and spawning process to produce large numbers of high-quality seeds is a primary requirement for the successful development of aquaculture [1]. This matter relies on optimal broodstock management strategies in terms of nutrition, physical and chemical parameters of water, genetics, and the age of fish in captivity [[1], [2], [3]]. Among these factors, broodstock nutrition still needs to be given priority to assist the continued development and improvement of aquaculture [4].

Reproduction is an energy-costly process that can be successfully faced only when sufficient energy stores are available [5]. It is well-accepted in the literature that broodstock nutrition can significantly impact gonadal development and reproductive performance in fish [4,6]. Accordingly, optimal broodstock nutrition is essential for economically producing a healthy, high-quality egg and sperm [2].

Fish nutrition has advanced in recent years with the development of balanced eco-friendly functional feeds that promote optimal growth, egg and sperm quality, and fish health [7,8]. As one of the main functional feed additives, probiotics have been described as live microorganisms that increase growth performance and appetite [9,10], enhance food conversion by an enzymatic contribution to digestion [10,11], improve feed value (macro-and micronutrients made available by the probiotic) [8], inhibition of adherence and colonization of pathogenic microorganisms [12,13], stimulation of the immune system [12,14], and increase stress resistance [15,16]. Furthermore, higher reproductive output and reduced malformations have been reported as the favorable nutritional effects of probiotics [5,11]. Consequently, the modulation of the gut microbiota with dietary administration of probiotics represents a potential strategy to improve the reproduction status of fish.

Up to now, the effects and mechanisms of the actions of probiotics on the reproductive cycle of male fish have been poorly studied [[17], [18], [19], [20], [21], [22], [23]]. This is while many kinds of research in recent years have been performed on the effect of probiotics on female fish reproductive function [17,20,[24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]]. In this regard, future studies need to focus on the effect of probiotics on reproductive process of male breeders in different fish species. In general, studies in broodstock nutrition indicate that probiotics can improve reproductive performance by triggering the production of regulatory hormones responsible for sexual maturation [5,31]. It is well established that certain probiotics not only promote ovulation and fecundity but can also affect semen quality and may support the hatching and survival of embryos and larvae after fertilization [5,17,22]. Accordingly, since the control of semen quality is an essential issue for the aquaculture industry, the use of dietary supplementation of probiotics for male breeders could be a practical method for improving the reproductive status and semen quality for the production of commercial species [7].

Although aquaculture production is expanding to a wide diversity of cultured finfish, salmonids remain an important contributor to fish production in many countries [9]. Rainbow trout (Oncorhynchus mykiss) is the most popular salmonid fish species and has received great attention in terms of importance to aquaculture and remains a core of inland fish production in many countries around the world [9,35]. A recent study suggested that multi-strain probiotics administration can be a suitable strategy to increase feed absorption and high-quality egg production for female rainbow trout breeders [24]. However, despite the special importance of this fish, no study has been done on male breeders so far. Thus, in order to improve reproduction status and increase sperm quality in rainbow trout, it is necessary to find the potential effect of dietary supplementation of probiotics on male breeders. Consequently, the present study aimed to evaluate the influence of multi-strain probiotics as a functional feed additive on reproductive aspects of male rainbow trout broodstock, including semen quality, seminal plasma compositions, and fertilization ability of spermatozoa.