SMOLTING STATUS OF DOWNSTREAM MIGRATING ATLANTIC SALMON (SALMO-SALAR) PARR

Significant numbers of Atlantic salmon parr have been observed to accompany seaward migrating smolts in some rivers. Downstream migrating parr in Freshwater River, Newfoundland, have lower salinity tolerance and branchial Na+-K+ ATPase activity than smolts, but moisture content in downstream migrating parr was similar to that of smolts; both groups had higher mean moisture content than nonmigratory parr. Elevated moisture levels and slightly depressed plasma sodium concentrations are consistent with the view that seaward movement is associated with osmoregulatory imbalance in both parr and smolts. Brackish water found in estuaries appears important for gradual development of salinity tolerance in these migrants. -from Authors