The effects of temperature and ploidy on the metabolism and energetics of Atlantic salmon (Salmo salar) infected with amoebic gill disease

Atlantic salmon (Salmo salar) aquaculture is an important industry from the global down to local markets. In Tasmania, the industry faces a serious health risk in the form of amoebic gill disease (AGD). The disease attributes 14 to 20% of production costs through control measures and mortalities. The warmer summer months result in proliferation of AGD suggesting that the 1.3 to 3vÄv¿C temperature increases predicted by the end of the century could detrimentally impact the Atlantic salmon aquaculture industry. This thesis investigates current and future temperature scenarios on chronic and acute thermal tolerance of aquaculture-relevant species and disease status. Specifically, the focus is on respiratory physiology under potentially stressful environmental conditions. The production of triploids can be advantageous to the aquaculture industry due to their inherent sterility allowing them to reach market size without the stress of maturation. In addition, triploids present a unique experimental model to investigate physiological processes due to their altered genome (e.g. larger but fewer cells). Despite observations of reduced thermal tolerance in triploids compared to their diploid counterparts, negligible differences in metabolism or thermal tolerance were found between ploidies in Chapter 2. Diploid and triploid Atlantic salmon were acclimated to three temperatures (10, 14, and 18¬¨‚àûC) at which their metabolic rates (resting and maximum) and acute thermal tolerance was determined. The experiment was conducted over 9 weeks with measurements occurring at weeks 0 (mass), 3 (mass and metabolic rates), 7 (mass and metabolic rates), and 9 (mass, metabolic rates, and critical thermal maximum [CTmax]). While mass, specific growth rate (SGR), and resting metabolic rate (¬¿œÄvÑO2rest) were significantly different in the beginning weeks of the experiment, all three converged by week 7 of the experiment. Maximum metabolic rate (¬¿œÄvÑO2max), and aerobic scope (¬¿œÄvÑO2max- ¬¿œÄvÑO2rest) remained stable across acclimation temperatures, measuring time points and ploidy. Furthermore, CTmax was found to be independent of ploidy. This study suggests that triploidy does not inhibit thermal tolerance in juvenile Atlantic salmon, so therefore diploids were utilized in subsequent chapters. Amoebic gill disease attaches solely to the gills and causes proliferation of the gill epithelium resulting in fusion of the secondary lamellae. This potentially reduces the functional surface area for oxygen uptake. Furthermore, the disease could have adverse effects on the host during periods of poor environmental conditions such as elevated temperatures or hypoxia. Across two chapters, the thermal tolerance and metabolism of AGD-infected diploid Atlantic salmon was investigated. Severely infected Atlantic salmon had impaired acute thermal tolerance as evidenced by a decreased CTmax temperature in Chapter 3. In Chapter 4, na‚àövòve and AGD-infected Atlantic salmon were acclimated to 15 and 19¬¨‚àûC and ¬¿œÄvÑO2rest, ¬¿œÄvÑO2max, aerobic scope, excess post-exercise oxygen consumption (EPOC), and hypoxia tolerance (Pcrit) were assessed. Increasing infection level was positively correlated with ¬¿œÄvÑO2rest at both acclimation temperatures while ¬¿œÄvÑO2max remained stable. The increase in ¬¿œÄvÑO2rest without a concurrent increase in ¬¿œÄvÑO2max caused aerobic scope to decrease with increasing infection level. Furthermore, evidence was found for impaired hypoxia tolerance. These findings suggest that heatwaves and periods of hypoxia could be detrimental to AGD-infected salmon. This thesis demonstrates that future climate change scenarios could have an impact on the Atlantic salmon aquaculture industry. It concludes that the effects of AGD on Atlantic salmon impairs acute thermal tolerance which could be detrimental with the projected increase in prevalence of heatwaves with climate change. However, given the chance for acclimation (i.e. an increase in average temperatures), infected salmon at higher temperatures (e.g. 19¬¨‚àûC) could cope as well as those at lower acclimation temperatures (15¬¨‚àûC).