Molecular and physiological responses predict acclimation limits in juvenile brook trout ( Salvelinus fontinalis )

Abstract

Brook trout (Salvelinus fontinalis) populations are at risk of exposure to high water temperatures in the species native range in eastern North America. We quantified the physiological and molecular responses of juvenile brook trout to six acclimation temperatures that span the thermal distribution of the species (5, 10, 15, 20, 23, and 25{degrees}C). Using quantitative PCR (qPCR), we measured the mRNA transcript abundance of temperature-induced cellular stress genes to identify a potential sub-lethal temperature threshold for brook trout between 20-23{degrees}C. Brook trout exhibited an upregulation of stress-related genes (heat shock protein 90-beta; heat shock cognate 71 kDa protein; glutathione peroxidase 1) and a downregulation of transcription factors and osmoregulation-related genes (Na+/K+/2Cl- co-transporter-1-a; nuclear protein 1) at temperatures [≥]20{degrees}C. We also used respirometry to assess the effects of the acclimation temperatures on oxygen consumption. Standard metabolic rate results indicated that energy expenditure was higher at temperatures [≥]20{degrees}C. We then examined the effects of acclimation temperature on metabolic rate and blood plasma parameters in fish exposed to an acute exhaustive exercise and air exposure stress. Fish acclimated to temperatures [≥]20{degrees}C exhibited elevated levels of plasma cortisol, muscle lactate, and plasma glucose after exposure to the acute stressors. After 24 h of recovery, fish showed longer metabolic recovery times at 15 and 20{degrees}C and cortisol levels remaining elevated at temperatures [≥]20{degrees}C. Our findings suggest that brook trout may have a limited ability to acclimate to temperatures >20{degrees}C and increases in temperatures beyond 20{degrees}C may impact brook trout populations.