Seasonal changes of thyroid hormones in field-collected Atlantic cod in relation to condition indices, water temperature and photoperiod

Abstract

Serum T4 and T 3 in wild Atlantic cod Gadus morhua ranged from 1 to 12 ng ml - and from 2 to 27 ng ml -1 respectively over a 3-year period. In general, the concentrations increased from summer (T 3 ) or early autumn (T 4 ) to maxima in mid-winter and declined abruptly during spring. The T 4 /T 3 monthly means were lowest in summer and highest in winter. The seasonal patterns of thyroid hormones were weakly correlated with changes in water temperature. However, both T4 and T 3 co-varied simultaneously with photoperiod. In addition, T 3 was correlated with the hepatosomatic index and condition factor during summer and autumn. It is suggested that the seasonal changes in the release of T4 from the thyroid were photoperioddriven, and that the course of T 3 was regulated by the metabolic state of the fish during the somatic growth period.