Development of seawater adaptation in juvenile steelhead trout ( Salmo gairdneri) and domesticated rainbow trout ( Salmo gairdneri)—Effects of size, temperature and photoperiod

Abstract

As shown by repeated 96-h LC 50 tests, salinity tolerance increased continuously with increasing size in juvenile steelhead trout and domesticated rainbow trout. For both races salinity tolerance was highest at 11°C, lowest at 17°C and intermediate at 5°C. Small (7–15 g) steelhead and rainbow trout grew equally well at 14‰ and 19‰ salinity, while growth rate was reduced at 24‰ salinity. In steelhead trout exposure to constant 14.5-h daylength in January accelerated several changes associated with smoltification such as silvering, decrease in condition factor and increase in salinity tolerance compared to fish reared under 9.5-h daylength. Growth rate in fresh water and 29‰ sea water was not affected by photoperiod. In rainbow trout there was no significant photoperiodic effect on silvering, condition factor, salinity tolerance and seawater growth. Growth rate in fresh water was ca. 10% higher in the 9.5-h group than in the 14.5-h group.