THE RELATIONSHIP BETWEEN STREAM OR RIVER DISCHARGE AND THYROID-HORMONE LEVELS IN WILD ADULT ATLANTIC SALMON (SALMO-SALAR L)

Thyroxine and tri-iodothyronine levels were determined in groups of ad ult Atlantic salmon (Salmo salar L.) of both sexes at river entry in s ummer or as they entered a spawning tributary in late autumn. Hormone levels were considered in relation to river or stream discharge rate a t capture. For tri-iodothyronine, there was a quadratic or linear rela tionship with discharge rate for each sex at both sites. For thyroxine , there was a quadratic relationship with discharge rate at tributary entry but no relationship with discharge rate at river entry. It is ar gued that raised hormone levels at higher discharge rates are a probab le consequence of increased motor activity associated with greater wat er velocity. The quadratic nature of most of the models is attributed to physiological or behavioural limitation of the response.