Timing of Atlantic cod Gadus morhua seasonal migrations in relation to serum levels of gonadal and thyroidal hormones

Serum testosterone, estradiol-17 beta (E-2) L-thyroxine (T-4), 3,5,3'-triio do-L-thyronine (T-3), and free (protein-unbound) T-3 concentrations were ex amined in relation to the extensive (up to 650 km) seasonal migrations unde rtaken by southern Gulf of St. Lawrence cod Gadus morhua L.. Testosterone l evels were low as the stock moved out of the Gulf of St. Lawrence in autumn , and increased only during the final stages of the return migration in spr ing. E-2 in females peaked near the onset of the spring migration, and rema ined elevated until this migration was completed in early summer. However, we suggest that the changes in E-2 levels are functionally tied with egg de velopment and not with spring migratory behaviour. Free T-3 showed no consi stent pattern in relation to seasonal migrations, whereas T-4 and T-3 level s increased at the start of the autumn migration. We conclude that thyroid hormones may facilitate the onset of the autumn migration by enhancing meta bolism, sensory physiology and swimming capacity.