No abstract

Photoperiod was the main environmental factor controlling the onset of par r–smolt transformation in steelhead trout (Salmo gairdneri) as evidenced by subsequent changes in migratory behavior and coefficient of condition. Photoperiod appeared to act as a synchronizer of an endogenous rhythm because smolt-like fish were observed in the dark at constant temperature from December through July for fish ≥ 19 cm. Phase and frequency or phase adjustment only of the annual photoperiod cycle advanced or delayed smolting. Increasing day length was the major photoperiod constituent stimulatory to smolting whereas day length, as such, and accumulated hours of exposure were of lesser importance. Temperature did not appear to influence the onset of smolting but fish reared under a variable temperature cycle (6.9° to 18.6 °C) generally migrated in larger numbers than did those reared under a constant temperature (12.3 °C). When the seasonal temperature cycle was out of phase and behind the photoperiod cycle, the migratory period was extended. Conversely, when the temperature cycle was in advance of the photoperiod cycle, the migration period was shortened.

No abstract

The volume of blood in the steelhead trout ( Salmo gairdneri gairdneri) was determined by the simultaneous use of radioactively labeled human serum albumin (HSA-I131), Evans blue dye (T-1824), and radioactively labeled red blood cells (RBC-Cr51). Blood volume derived from the plasma-hematocrit method utilizing Evans blue dye = 3.5 ± 0.9; for labeled albumin = 3.3 ± 0.9; through simultaneous use of both dye and labeled albumin = 2.9 ± 0.9. Blood volume obtained directly through the sum of the plasma and packed red cell volumes as measured with either Evans blue dye or labeled albumin and tagged red blood cells = 2.8 ± 1.0. Values are expressed as per cent of body weight and are in agreement with those that have been reported in the literature for bony fishes (Osteichthyes).

Marked yearling hatchery-reared steelhead smolts were stocked at varying distances from tidewater in two Oregon coastal streams in April of 1964 and 1965 and the numbers of returning adults caught in the sport fishery were determined.Objectives of the study were to determine the preciseness of the homing mechanism in directing the returning adult fish to particular locations in a stream and the contribution to the sport fishery of adult fish of different release origins.Homing imprint was demonstrated to be influenced by stocking site within a stream. A contribution to the fishery of a particular release group was found to be dependent on a delay in upstream migration in the geographic area of stocking and on the fishing effort in the area.