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This study examined the in¯uence of different carotenoids on growth and some immune indices in rainbow trout. Six semipuri®ed casein-based diets were formulated to contain one of three different carotenoids: astaxanthin, canthaxanthin and b-carotene, at 100 mg kg ±1 , each of them with vitamins A, C and E either added or omitted. The two control diets contained no carotenoids and were either with or without the vitamins. Rainbow trout weighing about 140 g were fed the diets for 9 weeks. Speci®c growth rate, feed:gain ratio and nonspeci®c immune parameters were determined. Growth and feed conversion were similar among the groups. Immune parameters like production of reactive oxygen species by head kidney leukocytes and plasma total immunoglobulin levels did not vary with the treatment. Serum complement activity in both b-carotene groups and the vitamincontaining astaxanthin group were signi®cantly higher than both the control ®sh. Serum lysozyme activity in the vitamin-containing bcarotene and astaxanthin groups were signi®cantly different from both control groups. Phagocytic activity was also high in the vitamin-containing b-carotene and astaxanthin groups compared with the controls. For phagocytic index, in addition to the foregoing groups, the vitamin-containing canthaxanthin group gave better results compared with the controls. The vitamin-containing astaxanthin and b-carotene groups also exhibited better nonspeci®c cytotoxicity for the peripheral blood lymphocytes at all effector-to-target ratios. Thus, among the carotenoids studied, b-carotene and astaxanthin elevated humoral factors such as serum complement and lysozyme activity, as well as cellular factors such as phagocytosis and nonspeci®c cytotoxicity. In the presence of the vitamins the carotenoids exerted a greater in¯uence on the bio-defense mechanisms of rainbow trout.

SUMMARY: We attempt to assess the impact of different levels of dietary β‐carotene on immune function in rainbow trout. Semi‐purified diets containing 0, 40, 200, and 400 mg β‐carotene/kg dry diet were fed for 12 weeks to fish with average weight of 45 g. In addition to the humoral and cellular immune parameters, growth and feed utilization were examined. There were no marked differences in growth and feed utilization showing that β‐carotene was not particularly efficient in enhancing growth of rainbow trout. Of the immune parameters measured, total immunoglobulin was significantly highest for the 200 mg β‐carotene fed group. Serum complement activity (alternate pathway) at 200 and 400 mg β‐carotene supplementation was significantly higher than that of the unsupplemented group. An increasing trend in lysozyme activity was observed, however, the differences among the groups were not significant. Phagocytic activity was similar among diet groups except at the highest level of supplementation where it was the maximum. Oxygen radical production by peripheral blood leukocytes appeared to be lower at higher levels of carotenoid supplementation. Overall, dietary β‐carotene clearly enhanced immune response parameters in rainbow trout such as serum complement activity and total plasma immunoglobulin but did not show a definite influence for the other factors examined in the present study.

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