Abstract
Abstract During peak smoltification, the interactive effects of cortisol and ambient seawater challenge were compared in two strains of Atlantic salmon ( Salmo salar ) smolts: a domesticated strain, AquaGen and a native River Imsa strain. Tissue and blood samples were taken from untreated fish on 20 May. Fish were then transferred to experimental tanks, allowed to recover for 24 h and cortisol (50 mg kg body mass −1 ), dissolved in vegetable oil, or vegetable oil alone (sham) was implanted. Samples were taken 5 days post implantation. Fish were then exposed to 24 h ambient seawater challenge (FW–SW) or freshwater to freshwater replacement (FW–FW) and sampled as before. Sham implantation had no significant impact on any of the measured parameters. Cortisol implantation significantly elevated plasma cortisol in FW–FW Imsa smolts, while no effect was observed in FW–SW Imsa smolts. Cortisol implantation had no effect on the plasma cortisol levels of AquaGen smolts regardless of FW–SW challenge. Increased plasma cortisol corresponded with significantly higher plasma glucose levels in FW–FW Imsa smolts. Plasma Na + and Cl − levels were not affected by cortisol implantation but were significantly increased in FW–SW smolts of both strains. Gill Na + ,K + -ATPase activity increased in response to cortisol implantation in only FW–FW smolts but not FW–SW smolts. Gill Na + ,K + -ATPase α-subunit mRNA levels were not affected by strain, cortisol injection or transfer protocol, while both CFTR I and CFTR II mRNA levels were significantly higher in AquaGen versus Imsa smolts regardless of treatment. CFTR I mRNA was elevated following cortisol implantation in FW–FW smolts from both strains suggesting CFTR I expression is under the control of cortisol. These findings also suggest that plasma cortisol levels are regulated differently between strains and that cortisol implantation and ambient FW–SW challenge interact, interfering with the individual effects of each of these factors.
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