“…The gut microbiota of endothermic animals as the case with fish is classified as autochthonous or indigenous, when they are able to adhere and colonize the host's mucus layer or gut epithelial surface, or allochthonous, when they are incidental visitors in the GI tract, and are expelled after some time without colonizing (Kim, Brunt, & Austin, ; Ringø & Birkbeck, ; Ringø, Olsen, Mayhew, & Myklebust, ). The adherent bacteria may play an important role in health and disease control of fish, and in this regard, it is of importance to bear in mind that the GI tract of fish is one of the major infection routes for some pathogens (Birkbeck & Ringø, ; Bøgwald & Dalmo, ; Groff & LaPatra, ; Harikrishnan & Balasundaram, ; Ringø, Myklebust, Mayhew, & Olsen, ; Ringø et al., ; Ringø, Løvmo et al., ).…”