Effects of dietary ascorbic acid on oxygen stress (hypoxia or hyperoxia), growth and tissue vitamin concentrations in juvenile rainbow trout ( Oncorhynchus mykiss)

Abstract

We examined the effects of three different dietary levels of ascorbic acid (AA) on growth and tissue concentrations in rainbow trout reared in water with three different saturation levels of dissolved oxygen; hypoxia, normoxia, and hyperoxia. Juvenile rainbow trout (initial weight, 1.87±0.04 g) were distributed into 27 fiberglass, conical tanks with a 3×3 factorial design (3 vitamin C levels×3 dissolved oxygen levels). Three replicate groups of fish (23 fish/tank) were assigned to one of three levels of oxygen and one of three diets containing different levels of ascorbic acid (10, 100 and 1000 mg AA equivalent/kg diet) designated as marginal, required and high-dose vitamin in the diets, respectively. For the oxygen treatments, hypoxia (50% oxygen saturation), normoxia (100%), and hyperoxia (180%) were set up using systems equipped with oxygen enriching (O 2 injected) or stripping (with N 2) columns. The growth rates of fish fed experimental diets during 18 weeks are provided and a two-way ANOVA test showed that the growth rates were significantly affected by dissolved oxygen ( P<0.001) and dietary ascorbic acids levels ( P=0.006). No interaction, however, was found between the parameters ( P=0.251). Liver AA concentrations were significantly affected by dietary AA levels ( P<0.001), but not by dissolved oxygen level ( P=0.142). There was no interaction between the two parameters for liver AA concentration after 18 weeks. Dissolved oxygen and dietary AA levels also significantly affected gill AA concentrations, and an interaction was observed between the two parameters after 18 weeks. Hematocrit values increased and the antioxidant index (ferric reducing ability of plasma, FRAP) decreased with a decrease in oxygen saturation levels. These findings suggest that higher levels of dietary AA have beneficial effects on growth in the hypoxia and normoxia conditions and there seems to be a trend in an increased rate of tissue AA degradation in hyperoxia.