Three nutritional feeding trials were undertaken to investigate the nutritional role of zinc (Zn) and selenium (Se) in the promotion of health in rainbow trout (Oncorhynchus mykiss Walbaum). Organic and inorganic sources of both elements were assessed by determining bioavailability, effects on growth, feed utilisation, immuno-competence, oxidative status, fillet quality and interactions with other trace elements. A focus was made on the use of practical diets throughout the study. The first experiment assessed the bioavailability of residual Se and Zn from a white fishmeal based practical diet during a 10-week trial in comparison to diets containing either supplemental Se-yeast and Zn proteinate (Zn-pr) or sodium selenite (Na2SeOJ) and Zn sulphate (ZnS04). Se-yeast and Zn-pr were hypothesised to be more digestible and more effective in raising Se and Zn status than inorganic sources. Apparent digestibility of residual Zn and Se was 21.9% and 54.2% respectively. No conclusive difference in overall Zn bioavailability was observed between treatments. Se-yeast significantly increased Se digestibility to 63.7%, Se levels in all tissues, and the activities of hepatic thioredoxin reductase (Trx-R) and glutathione peroxidase (GSH-Px). Selenite supplementation only resulted in increased GSH-Px activity. Se-yeast was found to be a highly bioavailable Se source, effectively raising Se status. The second experiment determined the efficacy and effect on health of residual Zn in comparison to 125, 312 and 781 mg kgˉ¹ supplemented Zn-pr or Zn sulphate in a practical diet. It was hypothesised that Zn-pr would be more efficacious in the promotion of health than Zn sulphate. The retention of both Zn sulphate and Zn-pr followed the same exponential decay (R2 = 0.978) with increasing dietary Zn loading. This was accompanied with a lack of effect of 12-weeks Zn supplementation on standard growth rate (SGR), feed conversion ratio (FCR), Zn enzyme activity (Cu/Zn-superoxide dismutase and alkaline phosphatase), oxidative status (hepatic and pyloric caeca malondialdehyde (MDA), total plasma antioxidant capacity (TAC) and oxidative DNA damage), immuno-competence (superoxide generation, lysozyme and leukocyte counts), hematocrit, nuclear abnormalities (micronuclei, and notched and blebbed nuclei), and fillet quality (drip loss and astaxanthin). Both dietary Zn-pr (R2 = 0.573) and Zn sulphate (R2 = 0.453) interacted with hepatic manganese (Mn) but not with iron (Fe) or copper (Cu). No significant differences were observed between sources and a strict homeostatic regulation of dietary Zn in rainbow trout was observed. Preliminary investigations were carried out to establish the effects of seven days chronic husbandry related stressors on immuno-competence and oxidative status. It was hypothesised that chronic husbandry related stressors would result in oxidative stress and impaired immuno-competence. Alternating between netting and confinement stressors resulted in oxidative stress as determined by decreased TAC, increased oxidative DNA damage and modulated superoxide generation by leukocytes in the whole blood. The final experiments investigated the efficacy and effects on health of residual Se in comparison to 2, 4, and 8 mg kgˉ¹ supplemental sodium selenite or Se-yeast in a commercial trout grower diet. It was hypothesised that supplemental Se would confer benefits to the health of both stressed and un-stressed fish, and Se-yeast would be more efficacious than selenite in its effects. Following a 10-week trial, the implication of seven days husbandry related stressors were also evaluated. Se retention was reduced in fish fed selenite up to 4 mg kgˉ¹ in comparison to fish fed the basal diet; in contrast Se-yeast increased Se retention. SGR, FCR, hepatic GSH-Px and Trx-R, plasma TAC, hepatic MDA, respiratory burst, lysozyme, nuclear abnormalities, and hematocrit were not affected by the supplemented dietary Se. However, in pre-stress fish fillet quality (drip loss) may be improved by Se-yeast supplementation; this was not observed with selenite. Stress decreased whole body Se in all treatments and increased GSH-Px to the greatest extent in Se supplemented diets suggestive of an increased Se utilisation during stress. Se had no protective effects on immuno-competence and oxidative status post stress. On the contrary, 8 mg kgˉ¹ Se from selenite increased hepatic MDA and resulted in the lowest GSH-Px increases amongst supplemented fish, which may be indicative of the sub-lethal effects of Se in fish fed a high level of inorganic Se; this was not observed with Se-yeast. Selenite (R² = 0.6611), but not Se-yeast (R² = 0.073), significantly interacted with whole body Cu. No interactions were found between dietary Se and Fe or Mn. Levels of whole body Fe, Mn, Cu and Zn were not affected by husbandry related stressors. The study concludes that Se-yeast supplemented in fishmeal based diets is more bioavailable and efficacious in the maintenance of health than selenite. The efficacy of Zn-pr was not distinguished from that of Zn sulphate in the supplementation of practical diets. No difference was observed between Zn-pr and Zn sulphate in terms of bioavailability or efficacy. This may be due to the tight homeostatic regulation of dietary Zn above marginal levels. Se utilisation may increase during chronic husbandry related stress. Due to the lack of any measurable toxic effects and increased retention, Se-yeast may be more suitable for the delivery of Se in fish exposed to husbandry related stress, which was shown to result in oxidative stress. Higher quality practical diets may meet requirements for Se in un-stressed fish, but in conditions of stress, maximal GSH-Px and Se status may only be achieved by diets containing supplemental Se.

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