THE EFFECT OF DIETARY VITAMIN-A AND ASTAXANTHIN ON THE IMMUNOCOMPETENCE OF RAINBOW-TROUT

Rainbow trout (Oncorhynchus mykiss) were maintained for 4 months on di ets formulated with supplementary vitamin A (18 mg/kg dry diet) and as taxanthin (100 mg/kg dry diet) (diet A+ Ax+), astaxanthin alone (diet A- Ax+), vitamin A alone (diet A+ Ax-) and neither vitamin A nor astax anthin (diet A- Ax-) to investigate the immunomodulatory effects of th ese compounds in fish. Serum concentrations of both vitamin A and asta xanthin reflected dietary levels, with vitamin A supplementation resul ting in levels of 32-40 vs. 4 mug/dl in fish fed unsupplemented diets and astaxanthin supplementation resulting in levels of 9-11 mug/ml wit h levels being undetectable in fish fed unsupplemented diets. However, there was no marked effect of diet on group food conversion efficienc ies or growth rates. Of the immune parameters measured, total serum an tiprotease activity was found to be significantly lower in the group f ed diet A- Ax- and classical serum complement activity was also lowest in this group. In addition, a trend for decreased leucocyte migration was observed in the fish fed diets A- Ax+ and A- Ax- compared with th ose fed diets A+ Ax+ and A+ A-. On the other hand, total serum immunog lobulin level, specific immunoglobulin level following immunization wi th Aeromonas salmonicida, serum lysozyme activity and phagocyte respir atory burst activity were unaffected by dietary vitamin A and/or astax anthin intake. Overall, the small immunomodulatory effect of vitamin A and/or astaxanthin supplementation observed in this study suggests th at these compounds have only limited potential as immunostimulatory ag ents in practical rainbow trout diets.