THE LOSS OF HYPEROSMOREGULATORY ABILITY IN MIGRATING JUVENILE AMERICAN SHAD, ALOSA-SAPIDISSIMA

Investigations on juvenile American shad (Alosa sapidissima) revealed several physiological changes associated with downstream migration. Pl asma chloride decreased 20% in wild juvenile shad during the autumn mi gration. Migrants had lower condition factor and hematocrit than non-m igrant shad captured by beach seining. Gill Na+,K+-ATPase activity of migrant shad was higher than non-migrant; a 2.5-fold increase was obse rved in 1993, while a 57% increase was observed in 1994. Similar chang es were observed in laboratory studies of shad maintained in fresh wat er under simulated natural temperature and photoperiod. Plasma chlorid e dropped 68% and gill Na+,K+-ATPase activity increased 3-fold over a 3-month period. Decreased plasma chloride was associated with increase d mortality. Increases in gill Na+,K+-ATPase activity decreases in pla sma chloride and osmolality, and incidence of mortality were delayed a nd moderated, but not eliminated, in shad maintained at constant tempe rature (24 degrees C). Shad did not survive in fresh water past Decemb er regardless of temperature regime. In seawater, all shad survived an d showed no perturbation of plasma chloride at 24 degrees C or simulat ed natural temperature (above 4 degrees C). The decline in hyperosmore gulatory ability, as influenced by declining temperatures, may serve a s a proximate cue for autumnal migration.