Artificial photoperiod influence on the immune system of juvenile rainbow trout (Oncorhynchus mykiss) in the Southern Hemisphere

Abstract

Abstract The influence of artificial photoperiods on the immune system of juvenile rainbow trout ( Oncorhynchus mykiss ) was analyzed in the Southern Hemisphere. Experimental fish were exposed to different photoperiod regimes during 140 days: 14 acclimatizing days at LD 10:14, 60 days at LD 24:0, 30 days at LD 10:14 and 30 days at LD 12:12. Control fish were kept at a “natural” photoperiod of 14 acclimatizing days at LD 10:14, 92 days at LD 10:14 and 30 days at LD 12:12. Our results indicate that in experimental fish, as compared to controls, immunosuppression occurs as measured by a decrease of mitogen-induced polyclonal expansion of T lymphocytes. This suppression was evident during the second month of the artificial photoperiod regime (LD 24:0 for 60 days, starting during late autumn) and rapidly normalized after finishing this protocol. Furthermore, a significant enhancement of this type of response could be observed in B lymphocyte populations of experimental fish when the natural photoperiod regime was resumed. Significantly elevated plasma cortisol levels corroborate that stress was present in the experimental fish. This indicates that a prolonged change in the natural photoperiod adversely affects the rainbow trout immune system, and that in spite of this, fish are rapidly capable of resuming normal function. On the other hand, hematological observations show that only thrombocyte numbers are significantly lowered by artificial photoperiod regimes, whereas the numbers of lymphocytes and erythrocytes did not show significant changes.