Effect of salinity on survival and growth of Atlantic salmon ( Salmo salar) parr and smolts

The two main findings were that stunting in parr transferred to seawater can be temporary, and the growth rate of smolts is largely independent of salinity. In late-May, large lower-modal group parr ( x wt.=26 g ) including some with partial smolt characteristics, and upper-modal group smolts ( x wt. = 36 g ) were randomly allocated among four tanks at salinities of 10, 20, 31 ppt (seawater), or fresh water (control). Smolts exhibited 0% mortality. Mortality of parr in seawater was size dependent and reached 47% in late-June, with few mortalities thereafter. Mortality was < 10% in salinities ≤ 20 ppt. Survivors in > 20 ppt showed elevated gill Na + K + - ATPase activity, normal plasma osmolality and muscle water content, but retarded (“stunted”) growth. The degree of stunting was dependent on salinity and body size; the smallest parr in seawater remained 11–12 cm fork length from May to September, intermediate sized parr were stunted for 1–2 months, whereas the largest members of the lower-modal group exhibited growth rates similar to smolts. Growth of smolts in 20 ppt and seawater was temporarily inhibited compared to smolts in ≤ 10 ppt, but by September there was no significant difference in body weight between the smolts in the four salinities.