The mucosal surfaces of fish are considered highly active immunological sites that work to maintain homeostasis and play a key role in the defence against pathogens (Yu et al., 2020). While disease represents a major constraint to the aquaculture industry, the potential influence of dietary additives upon the mucosal barriers and defences of fish is the subject of increasing amounts of research. RT-q-PCR gene expression analysis and histological examinations of experiment one jointly indicate the potential of a dietary prebiotic mixture, a parabiotic and their synbiotic to positively influence the mucosal barrier defences of the Gill-(GIALT), Skin-(SALT) and Gut- associated lymphoid tissues (GALT) of rainbow trout. Significant increases in goblet cell area fraction (CAF) were observed in the SALT, in addition to significantly elevated expressions of tight junction-, mucosal- and immunity-associated genes, within the GIALT, SALT and GALT. Similar results were observed for Atlantic salmon fed prebiotic- and probiotic-supplemented diets in experiment two: differences in goblet cell abundance and CAF were observed between treatments, while RT-q-PCR analysis revealed transcriptional regulation of mucosal- and immune-related genes. Additionally, following pathogen challenge, significantly fewer sea-lice (Lepeophtheirus salmonis) were observed on fish fed the additive supplemented diets. Investigations of experiment three were conducted on Atlantic salmon housed in open sea pens, reflective of commercial aquaculture practices. Significant reductions in sea-lice prevalence and changes to epidermal mucous secretions were observed between treatments. Furthermore, Microfluidic q-PCR analysis allowed for the evaluation of 59 gene targets within the GIALT, SALT, and GALT at two different time-points, providing a holistic view of the expression alterations occurring in response to the prebiotic and probiotic additives investigated, and the latent copepod infection. Through the implementation of in vivo feeding trials, results of the experiments conducted during this research collectively support the proposed interconnectivity of the MALT tissues in both rainbow trout and Atlantic salmon, and serve to highlight the potential of dietary additives to enhance the mucosal barrier defences and reduce sea-lice prevalence.

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