The Responsiveness of the Thyroid System of Atlantic Salmon (Salmo Salar L.) Smolts to Increased Water Velocity.
Serum thyroxine (T4) levels in Atlantic salmon (Salmo salar L.) smolts exceeded those in parr over the spring period. Tri-iodothyronine (T3) and T4 levels present in smolts both at dusk and some hours later in darkness varied with date in a complex manner. On every date, levels of T4 in darkness were lower than those present at dusk but levels of T3 remained unchanged. T3 levels in smolts also remained unchanged following challenge by exposure to raised water velocity for some hours after dusk. Before the 26 May sampling date, T4 levels in challenged smolts were not different from those present in control smolts sampled at the same time. After the 19 May sampling date, however, T4 levels were raised by challenge to exceed those present in controls: this effect was still evident on 15 August when the study ceased. Only on 25 June did T4 levels in challenged fish also exceed those in the appropriate dusk controls, as has been the case in wild migrant smolts which have been studied previously.