Smolting Status of Downstream Migrating Atlantic Salmon (Salmo salar) Parr

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Abstract
Significant numbers of Atlantic salmon (Salmo salar) 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. However, moisture content in downstream migrating parr was similar to that of smolts; both groups had higher mean moisture content than nonmigratory parr. Mean plasma sodium concentration was significantly higher than in smolts, but somewhat lower than in 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.