Physiological and immunological biomarkers of chronic thermal stress in post-smolt Atlantic salmon, Salmo salar

Abstract

The increasing prevalence of marine heat waves is a challenge facing global aquaculture, especially for temperate species like Atlantic salmon which, in Australia, are farmed in relatively warm waters. However, we do not understand the mechanistic basis for fish underperformance at temperatures above their thermal optimum and this limits our capacity to develop interventions to offset the effects of warming on production efficiencies. In this study, we compared a diverse suite of physiological and immunological biomarkers in post-smolt Tasmanian Atlantic salmon (∼60–100 g) following 6 weeks of exposure to elevated (19 °C) or optimal rearing temperatures (15 °C). Fish held at 19 °C experienced a significant increase in standard metabolic rate, but no change in maximum metabolic rate which resulted in a significant reduction in factorial aerobic scope. There was a significant increase in upper thermal tolerance limits, but a reduction in hypoxia tolerance in 19 °C-exposed fish. CTmax was also more oxygen-dependent in 19 °C fish suggesting that hypoxia can compound the effects of heat stress on Atlantic salmon. Fish held at 19 °C experienced osmoregulatory disturbances with increase in plasma osmotic pressure, but this was not due to reduced performance of branchial Na⁺K⁺ATPase activity. Innate (respiratory burst and white blood cell counts) and adaptive immune responses were also impaired in 19 °C-acclimated salmon. These data demonstrate that the loss of production efficiency in salmon at high temperatures is complex given that multiple physiological systems are impacted simultaneously. Whole animal performance could be improved through the provision of targeted nutritional support to key physiological and/or immune parameters that may be disrupted during periods of thermal stress.