The effect of diet and short duration hyperoxia exposure on seawater transfer in coho salmon smolts ( Oncorhynchus kisutch)

Abstract

Coho salmon smolts ( Oncorhynchus kisutch) were fed one of three casein-based diets containing different levels of vitamin E (Vit. E) and Selenium (Se). Diet 1 contained no supplemental Vit. E or Se, diet 2 contained moderate levels (Vit. E=30 IU kg −1 and Se=0.2 mg kg −1) and diet 3 contained high levels (Vit. E=900 IU kg −1 and Se=1.0 mg kg −1). Smolts were reared for 6 weeks on their respective diets and then exposed to hyperoxia (364±10% atmospheric levels) for 0 h, 6 h, 24 h or 1 week (wk) and then subject to a 24-h normoxic seawater (27 ppt) challenge (0 h-SWC, 6 h-SWC, 24 h-SWC and 1 wk-SWC respectively). In diet 1, no significant increase in plasma [Cl −] was observed in the 0 h-SWC treatment relative to freshwater values, however, plasma [Cl −] was significantly elevated in seawater following 6 h, 24 h and 1 wk exposure to hyperoxia. In diets 2 and 3, plasma [Cl −] levels were significantly elevated over freshwater levels except for the 0 h-SWC at the end of the experiment. In diets 2 and 3, 2 of 12 fish died following seawater transfer in each group after being exposed to hyperoxia for 1 wk (1 wk-SWC). No other mortalities were observed during this study. Thus, exposure to hyperoxia for as little as 6 h impaired hypoosmoregulatory ability in coho salmon smolts and elevated levels of Vit. E and Se were not protective. Furthermore, plasma [Cl −] following seawater transfer was significantly greater in diet 3 than diets 1 and 2 throughout the data set, indicating that high levels of Vit. E and Se may impair hypoosmoregulatory ability during seawater transfer.