DIEL AND SEASONAL-CHANGES IN RESTING LEVELS OF VARIOUS BLOOD PARAMETERS IN BROOK TROUT (SALVELINUS-FONTINALIS)

Plasma osmolality, chloride, glucose, cortisol, thyroid hormones, and blood hematocrit all varied seasonally in freshwater-adapted, age 1+ brook trout (Salvelinus fontinalis) kept under natural photoperiod and water temperatures. In addition, plasma osmolality, cortisol, and thyroxine all displayed diel cycles that differed from month to month. High cortisol levels were related to sexual maturation and possibly to low water temperatures. Seasonal cycles of plasma thyroid hormones did not seem to be influenced by water temperature. Plasma osmolality, chloride, glucose, and thyroxine showed small but significant seasonal changes similar to the variations observed at the time of smoltification in other salmonid species.