SURVIVAL AND GROWTH OF TWO STOCKS OF PRESMOLT CHINOOK SALMON HELD OVERWINTER IN MARINE NET-PENS WITH AN ARTIFICIAL FRESHWATER LENS

Abstract Two hatchery stocks of juvenile spring chinook salmon Oncorhynchus tshawytscha originating from two southeast Alaska rivers were reared in freshwater to an average size of 8–10 g and placed in marine net-pens with an artificial freshwater lens system (AFLS) at four entry times from mid-September to early November 1987. Overwinter survival to May 1988 averaged 83.6% for Unuk River stock and 76.0% for Chickamin River stock and did not differ significantly in the AFLS among the four entry dates. Survival over 24 h differed significantly when entry was directly into 30‰ seawater. The 24-h plasma sodium levels in the juveniles challenged to either full-strength seawater or marine net-pens with an AFLS were not useful predictors of overwinter survival. Over winter, both stocks grew significantly better in the AFLS than in freshwater. The AFLS appears to be a cost-effective means of substantially increasing smolt production while minimizing freshwater use.