One of major factors accounting for estuarine stress is salinity, as osmoregulatory capability develops throughout the larval sequence of stages (Charmantier, 1998), and most newly-hatched stages are regarded as being more sensitive to low salinity (Charmantier et al., 2002). However, in some freshwater species hatching occurs in freshwater with subsequent seaward exportation, and in this case the newly-hatched stage has strong osmoregulation capability (Charmantier et al.,1998).