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

Ž . Coho salmon smolts Oncorhynchus kisutch were fed one of three casein-based diets Ž . Ž . containing different levels of vitamin E Vit. E and Selenium Se . Diet 1 contained no Ž y1 supplemental Vit. E or Se, diet 2 contained moderate levels Vit. Es30 IU kg and Ses0.2 y1. Ž y1 y1. mg kg and diet 3 contained high levels Vit. Es900 IU kg and Ses1.0 mg kg . Smolts Ž were reared for 6 weeks on their respective diets and then exposed to hyperoxia 364"10% . Ž . atmospheric levels for 0 h, 6 h, 24 h or 1 week wk and then subject to a 24-h normoxic Ž . Ž . seawater 27 ppt challenge 0 h-SWC, 6 h-SWC, 24 h-SWC and 1 wk-SWC respectively . In diet w yx 1, no significant increase in plasma Cl was observed in the 0 h-SWC treatment relative to w yx freshwater values, however, plasma Cl was significantly elevated in seawater following 6 h, 24 w yx 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 Ž . 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 w yx 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. q 1999 Elsevier Science B.V. All rights reserved.