Qualitative and quantitative effects of constant light photoperiod on rainbow trout (Oncorhynchus mykiss) peripheral blood erythrocytes

Immature erythrocytes or polychromatophils are commonly found in low percentages and at different maturational stages in fish peripheral blood. The concentration of these cells is indicative of the erythropoietic status, which can be modulated by environmental factors such as oxygen availability and temperature. To assess the consequences of constant artificial light on erythropoiesis, polychromatophils were quantified in two groups of rainbow trout. At the end of the experiment, fish exposed to two months of constant light showed an increase of immature erythrocytes from 0.07 × 1012 to 0.11 × 1012 cells/l along with a marked decrease in their polychromatophilic intensity, which almost disappeared 5 months after resuming natural photoperiod. In this group after an initial increase, a decrease in immature erythrocyte numbers was observed, suggesting that application of constant light causes a late depression of erythropoiesis in trout. The transient increase of polychromatophils in the same maturity state may be an indicator of a deviation towards other metabolic needs, such as initiation of reproduction.