Enlargement of the spleen in rainbow trout fasted at different body sizes and water temperatures

Abstract To clarify the process of spleen enlargement in fasting rainbow trout, spleen histology and blood parameters were investigated at different body sizes (30 g and 300 g) and under different water temperatures (14 °C and 20 °C). In fasting trout, the spleen enlarged and contained densely packed erythrocytes regardless of body size or water temperature. None of the trout sampled after feeding started had an enlarged spleen. Histological analysis revealed the presence of melanomacrophages, hemosiderin and erythrocytes containing lipofuscin-like pigments in the spleen of fasting trout. After fasting, mean erythrocyte corpuscular volume was small and mean corpuscular hemoglobin concentration was high in several individuals, suggesting that these erythrocytes were senescent. The results suggest that fasting may disrupt the cycle of erythrocyte production and destruction in the rainbow trout spleen, inducing a change in erythrocyte shape. This change may be a normal regulatory mechanism of rainbow trout in response to fasting; however, the effects of fasting should be considered carefully when using fasting in aquaculture during seawater acclimation or treatment of disease because enlargement of the spleen is a common symptom of diseases.