| Summary: | Atlantic salmon juveniles reared in a hatchery reached peak smolt condition during May as 15 mo. old fish. Gill Na$\sp+$/K$\sp+$ ATPase activity increased in these fish from 10 $\mu$M Pi/mg protein/h in February to 60 $\mu$M Pi/mg protein/h in March and peaked near 70 $\mu$M Pi/mg protein/h in early May. The fish also survived best in a salinity tolerance test and their condition factor dropped to $<$1.00 during early May. Gill Na$\sp+$/K$\sp+$ ATPase activity in lower mode fish peaked near 60 $\mu$M Pi/mg protein/h and approximately 33% of these fish were able to survive a salinity tolerance test in early May. Atlantic salmon that were introduced into a stream as 8 mo. old parr during October began to migrate downstream around the beginning of April. The average gill Na$\sp+$/K$\sp+$ ATPase activity of fish beginning their migration was 24 $\mu$M Pi/mg protein/h, their condition factor was generally $<$0.95 and they averaged between 130 and 135 mm in fork length. This is in contrast to non-migrating fish whose gill Na$\sp+$/K$\sp+$ ATPase activity was near 8 $\mu$M Pi/mg protein/h, condition factor was $>$1.05 and fork length was generally $<$110 mm. Salmon were also introduced into a stream as 14 mo. old fish during April and some of these fish seemed to migrate as smolts. Those moving downstream had gill Na$\sp+$/K$\sp+$ ATPase activity that was approximately 1.5-fold higher than that of fish that did not move downstream. In hatchery-reared Atlantic salmon plasma thyroxine levels ranged from 3 to 33 ng/mL, but were only elevated during smoltification at one hatchery during the first year of the study. However, plasma triiodothyronine in hatchery-reared smolts increased from near 5 ng/mL to 7-10 ng/mL in early March and remained elevated until May. Triiodothyronine levels in parr increased to 6-7 ng/mL but only for a brief period in May. In feral smolts at the beginning of their downstream migration thyroid hormone levels ranged from 4-10 ng/ml and were not higher than the levels in non-migrating fish. In conclusion, it appeared that Atlantic salmon needed to reach two size thresholds, near 110 and 135 mm in fork length, and experience an increased rate of growth before the end of the smoltification period in May in order to smoltify.
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