Increases in average water temperatures and decreases in water oxygen levels (hypoxia), and more frequent and extreme warmin g events (i.e., heat waves), are predicted to occur with climate change. Thus, there is an urgent need to understand the effects of prolonged and short-term warming on the physiology of cultured fishes, including Atlantic salmon ( Salmo salar). This is particularly true with regard to Tasmania and Atlantic Canada where water temperatures have approached/exceeded 20°C, in combination with hyp oxia, and negative effects on production and fish health ( including large-scale mortalities) have been reported. However, with respect to the latter, it is unclear what role temperature and hypoxia played , and how the effects of these environmental challenges on Atlantic salmon can be minimized.
Over the past few years, we have used a multilevel (e.g., epigenetic, genomic , biochemical, whole animal, biologging ) approach and several experimental paradigms to understand how high temperatures alone, and when combined with moderate hypoxia (60-70% air sat.), impact salmon production characteristics and key aspects related to this species’ cage-site culture under realistic temperature scenarios (i.e., using an ITMAX test ; a 1°C increase week-1 from 10°C ).
In this presentation, I will show that while stress gene expression in salmon begins to be affected at 16°C, and feeding decreases dramatically as temperatures approach 20°C, there is little/n o evidence that this temperature, even when prolonged or combined with moderate hypoxia, results in mortalities . The salmon’s capacity to mount an innate immune response is not compromised at these temperatures and plasma cortisol levels (indicative of a secondary stress response) do not increase until 21-22°C. Fish in sea-cages do not avoid surface temperatures up to 19-20°C, and show no signs of stress (i.e., abnormally high heart rates o r arrhythmias). Finally, in lab-based experiments, mortalities only begin when the fish reached 21°C , and even at 23°C mortalities were only ~ 30% .
Through this research, we have also been able to identify epigenetic and genomic markers of temperature and hypoxia tolerance in salmon, and have identified populations/ families of salmon that have critical thermal maximum (CTMAX) and ITMAX values of ~28°C and 25°C, respectively. This should allow us to develop genetic and other markers for use in selecting fish that are more tolerant of these conditions, and for evaluating fish health . However, our data also provide convincing evidence that such efforts must be combined with those to mitigate other stressors/factors that negatively impact the health and welfare of salmon. To this end, I will briefly introduce a ‘schematic model’ (a work in progress) on how biotic and abiotic challenges may interact to cause large-scale (‘mass’) mortalities of salmon at sea-cage operations.