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'-triiodo-L-thyronine (T-3), and free (protein-unbound) T-3 concentrations were examined in relation to the extensive (up to 650 km) seasonal migrations undertaken by southern Gulf of St. Lawrence cod Gadus morhua L.. Testosterone levels 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 spring. E-2 in females peaked near the onset of the spring migration, and remained elevated until this migration was completed in early summer. However, we suggest that the changes in E-2 levels are functionally tied with egg development and not with spring migratory behaviour. Free T-3 showed no consistent pattern in relation to seasonal migrations, whereas T-4 and T-3 levels increased at the start of the autumn migration. We conclude that thyroid hormones may facilitate the onset of the autumn migration by enhancing metabolism, sensory physiology and swimming capacity.