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.

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.

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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.

This study investigated short-term effects of temperature on survival, body weight and the fatty acid profile of farmed marine polychaete Perinereis helleri-an excellent live feed maturation diet for penaeid shrimp broodstock. Two consecutive experiments were conducted to evaluate the effects of low temperature (4, 8, 12, 16 and 20 • C) and high temperature (20, 24, 28, 32 and 36 • C) on a 7-month-old population of P. helleri farmed at Bribie Island Research Centre, Australia. The experimental polychaetes were purged for 24 h at 20 • C before being transferred directly into 500 ml glass chambers, sealed and submersed in different target temperature water baths. The jars were then kept for 3 or 6 h in the low-temperature experiment and 1 or 3 h in the hightemperature experiment, respectively. Results showed that mortality occurred only at 4 • C and 8 • C (p < 0.01). Weight loss occurred across all treatments of the lowtemperature experiment, but only at 36 • C in the high-temperature one. Analysis of lipid classes and fatty acids show that farmed P. helleri is rich in polyunsaturated fatty acids (PUFA, i.e. 37% of total fatty acids), especially arachidonic acid (ARA,.The total value of essential fatty acids was 15.8% of total fatty acids or 16.8 mg/g dry matter. The n-3:n-6 ratio was 1.07 ± 0.04. This study suggests that P. helleri can be transported at ambient temperature of 16-20 • C and can be farmed in a tropical climate.