Effect of Entry Timing and Salinity on Overwinter Growth of Sockeye Salmon Fry in Seawater

Abstract

Groups of sockeye salmon Oncorhynchus nerka (approximately 2 g in weight) were 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 relative effects of seawater entry timing and salinity (20‰ and 30‰) on overwinter growth in seawater. The growth rates 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 equinox and those transferred after the equinox. Fish transferred to water with reduced salinity (20‰) had significantly better initial growth than those transferred directly to full-strength seawater (30‰) at the same time. This difference did not persist past the winter 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‰ salinity is initially used.