Changes in plasma thyroxine levels during smoltification in hatchery-reared one-year and two-year Atlantic salmon (Salmo salar)

Plasma thyroxine (T₄) levels were measured in 1-year (S1) and 2-year (S2) Atlantic salmon (Salmo salar), in County Mayo, Ireland, for two consecutive years (1983 and 1984). The two year-classes of Atlantic salmon for this locale in Ireland had different plasma T₄ profiles during the usual smoltification period. The S2 group showed significantly increased plasma T₄ levels (P<0.05) in mid-April of 1983. The corresponding elevation in mid-April of 1984 was not significant. Plasma T₄ levels of the S1 group remained low throughout the entire period of measurement (February through June/July) in both years. The increase in plasma T₄ in the S2 group in 1983 and 1984 does not appear to be new moon-related; nor did the mid-April increase in plasma T₄ of the S2 fish correlate with rainfall or water temperature. An increase in plasma T₄ during April 1984 coincided with increased stream flow, suggesting that a change in water quality may have affected plasma T₄ at that time. In June of both years, an increase in plasma T₄ in both the S1 and S2 fish occurred as water temperature increased. No significant difference was found in triiodothyronine (T₃) between the year-classes or within each year-class during the smoltification process.