Changes in haematology during upstream migration in American shad

Heart mass of American shad Alosa sapidissima did not change during migrati on in the Connecticut River. Spleen mass decreased and there was an increas e in available blood haemoglobin (+22%) and haematocrit (+9%). The decrease s in spleen somatic index (-29%) and spleen haemoglobin content (-15%) were dependent upon distance travelled upriver and not seasonal migration timin g 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, sug gesting that any physiological responses during migration were based on dis tance travelled rather than seasonally variable conditions such as temperat ure, although blood haemoglobin (+24%) and haematocrit (+21%) increased aft er passage up a fish ladder. These changes in haematological physiology occ urring during upstream migration may increase swimming performance and migr atory success in American shad. (C) 1999 The Fisheries Society of the Briti sh Isles.