Introduction
The successful transfer of post-smolts depends on the fish’ abilities to mount good stress responses, initiate neural adaptations and maintain cognitive properties (neural plasticity) to sustain resilience during the sensitive SW phase. However, current year-round transfer strategies can expose farmed post-smolt Atlantic salmon (Salmo salar ) to a broad range of thermal stressors that may challenge these functions, with negative implications regarding fish allostasis, health and welfare.
Material and methods
To investigate the effect of various transfer strategies , SW adapted post-smolts at 10-13ºC were transferred to low (10ºC, 7ºC, and 4ºC) and high temperatures (13ºC, 16ºC, and 18ºC), with thermal differences consistent with off-(winter) and on-(summer) season transfer strategies. D ifferences in allostatic loads, altered physiological states, and capacities to overcome these thermal challenges were evaluated based on the stress response, neural plasticity and ability to withstand stress: Fish were examined for modulations in primary stress responses (plasma cortisol levels ) and telencephalic neural plasticity genes important for cognition (neurod, bdnf, pcna, c-fos, mr, gr1, gr2 and hsd11b2) following the transfer, acclimation and in response to an acute challenge test (ACT) using confinement stress.
Results and discussion
Our findings show that elevated allostatic loads are present at both ends of the thermal profile, however, post-smolts clearly have a higher ability to withstand low temperatures compared to high ones. Accordingly, post-smolts retained robust abilities to maintain brain plasticity and initiate key neural adaptations against low thermal challenges. In contrast, post-smolt transfer to elevated temperatures adversely inhibited stress responses and neural functions, pointing to allostatic overload and poorer capacities to deal with increasing thermal challenges. In all, these findings demonstrate thermal differences in the production environment is associated with stresses that can alter key neural processes important for good stress management, abilities still conserved at low temperatures, but not at higher ones. This conforms to the natural life-history strategies of Atlantic salmon and highlights the importance of satisfying low thermal requirements of post-smolts during production, but also points to the potential future challenges of climate change.