Changes in haematology during upstream migration to American shad

Heart mass of American shad Alosa sapidissima did not change during migration in the Connecticut River. Spleen mass decreased and there was an increase in available blood haemoglobin (+22%) and haematocrit (+9%). The decreases in spleen somatic index (-29%) and spleen haemoglobin content (-15%) were dependent upon distance travelled upriver and not seasonal migration timing or short-term exercise events such as passage up a fish ladder. There was no effect of migration timing on any of the blood parameters measured, suggesting that any physiological responses during migration were based on distance travelled rather than seasonally variable conditions such as temperature, although blood haemoglobin (+24%) and haematocrit (+21%) increased after passage up a fish ladder. These changes in haematological physiology occurring during upstream migration may increase swimming performance and migratory success in American shad.