Reduced thermal tolerance during salinity acclimation in brook trout (Salvelinus fontinalis) can be rescued by prior treatment with cortisol

Abstract

The aims of this study were to assess whether thermal tolerance of brook trout (Salvelinus fontinalis) is affected during seawater (SW) acclimation and to investigate the role of cortisol in osmoregulation and thermal tolerance duringSWacclimation. Freshwater (FW)-acclimated brook trout at 18°C (Tacc) were exposed to SW for 16 days, whilst maintaining a FW control. Fish were examined for critical thermal maximum (CTmax) 0 (before), 2, 5 and 16 days afterSWexposure, and sampled at Tacc and CTmax for analysis of plasmacortisol, glucose and Cl-, gill Na+/K+-ATPase (NKA) activity and heat shock protein 70 (HSP70) abundance, and whitemusclewater content. At 2 days inSW, CTmax was significantly reduced (from 31 to 26°C), and then recovered by 16 days. This transient decrease in thermal tolerance coincided with a transient increase in plasma Cl- and decrease in muscle moisture content. Salinity itself had no effect on gill HSP70 abundance compared with the large and immediate effects of high temperature exposure during CTmax testing. To examine the role of cortisol in osmoregulation, brook troutwere administered a cortisol implant (5 and 25 μgg-1CORT) prior toSWexposure. BothCORT doses significantly increased their capacity tomaintain plasmaCl- duringSWacclimation. Treatment with the 25 μg g-1 CORT dose was shown to significantly improve CTmax after 2 days in SW, and CTmax was associated with plasma Cl- and muscle moisture content. These findings indicate that brook trout are sensitive to temperature duringSWacclimation and that thermal tolerance is associated with ion and water balance during SW acclimation.