Effect of entry timing and salinity on overwinter growth of sockeye salmonfry in seawater

Groups of sockeye salmon Oncorhynchus nerka (approximately 2 g in weight) w ere introduced into seawater tanks at three distinct times, 3 weeks before the fall equinox, at the equinox, and 3 weeks after the equinox. All groups were reared for 8 months (September-May) in those tanks to determine the r elative effects of seawater entry timing and salinity (20 parts per thousan d and 30 parts per thousand) on overwinter growth in seawater. The growth r ates in all groups declined from more than 2% of body weight per day (BWD) to less than 0.5% of BWD following the equinox. Growth rates did not differ significantly between fish transferred to seawater before the autumnal equ inox and those transferred after the equinox. Fish transferred to water wit h reduced salinity (20 parts per thousand) had significantly better initial growth than those transferred directly to full-strength seawater (30 parts per thousand) at the same time. This difference did not persist past the w inter solstice, however. These data suggest that managers can successfully rear sockeye salmon fry as small as 1.5 g in seawater if water with 20 part s per thousand salinity is initially used.