Evaluation of Dietary Additives on Yellowtail Kingfish ( <i>Seriola lalandi</i> ) Gut and Skin Health and Disease Resistance Against <i>Photobacterium damselae</i> subsp. <i>damselae</i>

The present study evaluated the effectiveness of five dietary additives in improving growth performance, gut and skin health and disease resistance against <i>Photobacterium damselae</i> subsp. <i>damselae</i> (Pdd) in yellowtail kingfish (YTK) (<i>Seriola lalandi</i>). The additives were top-coated onto a commercial diet. The five additives evaluated were brewer's yeast (BY), a yeast-based mannan oligosaccharide (MOS), a polyphenol product, a monoglyceride product and a combination of MOS and a superoxide dismutase (SOD) product. The monoglyceride and polyphenol products were trialled at both a low and high inclusion level. Diets were fed to YTK in triplicate tanks for 55 days. The results found that growth, feed utilisation and somatic indices were unaffected by additive inclusion. None of the additives affected the health of YTK liver, skin or the gut as determined by various histological metrics. These histological findings were supported by results of hindgut gene expression (<i>TNF-α</i>, <i>IL-10</i>, <i>Hsp70</i>, <i>mucin 2</i> and <i>i-mucin</i> and <i>SOD</i>), all of which were unchanged across the dietary treatments. Similarly, the additives did not stimulate the skin mucus quantity and skin mucosa barrier measurements (epidermis thickness and mucus cells), consistent with the results of relevant skin health gene expression measurements (<i>TNF-α</i>, <i>IL-8</i>, <i>calreticulin</i>, <i>Hsp70</i>, <i>mucin 2</i>, <i>CAT</i> and <i>SOD</i>). However, the MOS and low inclusion polyphenol treatments increased survival against Pdd challenge relative to control treatment. The immuno-modulating capacity of the polyphenol product was indicated by the absence of <i>Pdd</i> in faeces following a mucosal (bathe) challenge at the high inclusion; however, this did not translate to enhanced survival under systemic infection. However, further studies are needed to understand if optimisation of the inclusion levels of each additive might more strongly influence the health of YTK.