Seasonal Variations in the Smolt Characteristics of Juvenile Atlantic Salmon (Salmo salar) from Estuarine and Riverine Environments

Smolt characteristics such as lipid-moisture dynamics, ATPase activity, salinity tolerance, and condition factor were determined for Atlantic salmon parr (Salmo salar) moving to, or residing in, estuaries of two rivers in eastern Canada. Lipid and water content and gill Na+K+-ATPase activity of these parr differed markedly from river-resident parr. ATPase activity, sex ratio, and condition factor indicated more similarity between estuarine parr and smolts. Estuarine parr were unable to tolerate high salinities in spring, summer, or autumn, but there was an indication of an autumnal increase in salinity tolerance which coincided with maximum levels in ATPase activity and a secondary downstream movement of parr and smolt-like salmon in the autumn. The data suggest that the largest estuarine parr are essentially presmolts which utilize the estuary as a rearing environment prior to subsequent emigration seaward when environmental conditions are again amenable to a reopening of the 'smolt window.' These observations also highlight some inadequacies in applying existing criteria for the smoking process and for modelling life histories of salmonids.