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Astaxanthin (Axn) is the most commonly used carotenoid to enhance the flesh colouration of salmonids, which is one of the key criteria perceived to reflect the freshness and nutritional quality of the product and consequently affecting its price. However, the digestibility of astaxanthin by salmonids is highly variable, ranging between 20% and 60%. Axn digestibility can also be affected by total dietary lipid content and fatty acid (FA) composition (saturated FAs [SFA], mono‐unsaturated FAs [MUFA] or poly‐unsaturated FAs [PUFA]). This experiment contained four diets fed to juvenile Atlantic salmon for 2 weeks. Two diets formulated with either synthetic or algal carotenoids combined with poultry oil and two additional diets containing algal carotenoids with either tallow or canola oil. The aim of this study was to compare the feed intake, as a measure of diet palatability, and apparent digestibility (AD) of Axn between the two diets containing either synthetic or algal carotenoids. Results showed that algal carotenoids had significantly higher AD than synthetic Axn and that high levels of SFA, such as those in tallow, may inhibit the AD of algal carotenoids. Meanwhile, the high levels of MUFA and, in particular 18:1n‐9, in canola oil significantly improved the AD of algal carotenoids for Atlantic salmon. This study demonstrates the utility of algal carotenoids as natural pigment alternatives for farmed salmon, particularly beneficial when combined with different oil sources that contain more 18:1n‐9, such as in canola, and less SFA, such as in tallow, in order to optimize the utilization of dietary carotenoids. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide (H2O2) is a commonly used treatment for a range of parasitic diseases of marine finfish, including amoebic gill disease (AGD). While this treatment is partially effective at reducing parasite load, H2O2 can have detrimental effects on the host under certain conditions. Treatment temperature and dose concentration are two factors that are known to influence the toxicity of H2O2; however, their impact on the outcome of AGD treatment remains unclear. Here, we investigated the effects of treatment temperature (8, 12 or 16°C) and dose concentration (750, 1,000, 1,250 mg/L) on the efficacy of H2O2 to treat AGD. We demonstrated that a 20‐min bath treatment of H2O2 at all doses reduced both parasite load and gross gill score significantly. Parasite load and gross gill score were lowest in the 1,000 mg/L treatment performed at 12°C. At the high dose and temperature combinations, H2O2 caused moderate gill damage and a significant increase in the plasma concentration of electrolytes (sodium, chloride and potassium). Taken together, our study demonstrates that higher H2O2 treatment temperatures can adversely affect the host and do not improve the effectiveness of the treatment. [ABSTRACT FROM AUTHOR]

Freshwater bathing for 2–3 h is the main treatment to control amoebic gill disease of marine-farmed Atlantic salmon. Recent in vitro studies have demonstrated that amoebae (Neoparamoeba perurans) detach when exposed to freshwater and that some eventually reattach to culture plates when returned to seawater. Here, we evaluated the potential for gill-detached N. perurans to survive a commercially relevant treatment and infect AGD-naïve fish and whether holding used bathwater for up to 6 h post treatment would lower infectivity. AGD-affected fish were bathed in freshwater for 2 h. Naïve salmon were exposed to aliquots of the used bathwater after 2, 4, 6 and 8 h. The inoculation was performed at 30 ppt for 2 h, followed by gradual dilution with seawater. Sampling at 20 days post inoculation (dpi) and 40 dpi confirmed rapid AGD development in fish inoculated in 2 h used bathwater, but a slower AGD development following exposure to 4 h bathwater. AGD signs were variable and reduced following longer bathwater holding times. These results suggest that viable amoebae are likely returned to seawater following commercial freshwater treatments, but that the risk of infection can be reduced by retention of bathwater before release. [ABSTRACT FROM AUTHOR]

Branchial surfaces of finfish species contain a microbial layer rich in commensal bacteria which can provide protection through competitive colonization and production of antimicrobial products. Upon disturbance or compromise, pathogenic microbiota may opportunistically infiltrate this protective barrier and initiate disease. Amoebic gill disease (AGD) is a globally significant health condition affecting salmonid mariculture. The current study examined whether altering the diversity and/or abundance of branchial bacteria could influence the development of experimentally induced AGD. Here, we challenged Atlantic salmon (Salmo salar) with Neoparamoeba perurans in a number of scenarios where the bacterial community on the gill was altered or in a state of instability. Administration of oxytetracycline (in-feed) and chloramine-T (immersion bath) significantly altered the bacterial load and diversity of bacterial taxa upon the gill surface, and shifted the community profile appreciably. AGD severity was marginally higher in fish previously subjected to chloramine-T treatment following 21 days post-challenge. This research suggests that AGD progression and severity was not clearly linked to specific bacterial taxa present in these systems. However, we identified AGD associated taxa including known pathogenic genus (Aliivibrio, Tenacibaculum and Pseudomonas) which increased in abundance as AGD progressed. Elucidation of a potential role for these bacterial taxa in AGD development is warranted. [ABSTRACT FROM AUTHOR]