Influences of stress phenotype, nutrition and genetic background on the upper thermal tolerance of Atlantic salmon (Salmo salar)

Ignatz, Eric Hans (2023) Influences of stress phenotype, nutrition and genetic background on the upper thermal tolerance of Atlantic salmon (Salmo salar). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Anthropogenic climate change threatens the sustainability of the Atlantic salmon (Salmo salar) aquaculture industry. This thesis studied the effects of three factors [i) stress phenotype; ii) supplemental dietary cholesterol; and iii) family/genetic background] on the performance of farmed Atlantic salmon when exposed to an incremental thermal maximum (ITMax, +0.2°C day⁻¹) challenge that mimicked natural summer sea-cage conditions. No differences in the ITMax of male Atlantic salmon characterized as either low (LR) or high stress responders (HR) based on measurements of post-stress cortisol levels at 9°C was found, and interestingly, stress phenotypes were no longer distinguishable from one another when post-stress cortisol levels were analyzed at elevated temperatures. However, some differences in gene expression were found between LR and HR fish in response to bacterial immune stimulation. Notably, HR salmon mounted a greater (p < 0.05) innate antibacterial immune response than LR salmon at 20°C, whereas LRs had a greater (p = 0.057) response among stress-related transcripts relative to HR fish at 12°C. Supplemental dietary cholesterol did not affect the ITMax of female triploid Atlantic salmon. However, ≤ 5% of the salmon (irrespective of diet) died before temperature reached 22°C, and this suggests that the commercial production of triploids is possible in eastern Canada. In addition, inclusion of +1.30 and 1.76% cholesterol in the diet reduced fillet bleaching above 18°C. Finally, family-based differences in ITMax were detected in diploid, mixed sex, Atlantic salmon, with a 1.7°C separation between the least and most tolerant families. This study also showed that ITMax is a more sensitive and relevant indicator of upper thermal tolerance than the more common/convenient approach of assessing a fish’s critical thermal maximum (CTMax). In the final chapter of this thesis, I characterized the four paralogues of serpinh1 [alias heat shock protein 47 (hsp47)] in Atlantic salmon. This is a well-established biomarker of heat stress in salmonids, and the reported data provide important insights into the evolutionary history and regulatory functions of this essential gene. Overall, this thesis provides novel information on how the Atlantic salmon aquaculture industry can most effectively assess upper thermal tolerance, and navigate this era of accelerated climate change.

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