PHYSIOLOGICAL AND GROWTH DIFFERENCES IN 3 STOCKS OF UNDERYEARLING SOCKEYE-SALMON (ONCORHYNCHUS-NERKA) ON EARLY ENTRY INTO SEAWATER

We compared the impact of exposure to seawater on three sockeye salmon (Oncorhynchus nerka) stocks: one that normally migrates to sea as und eryearlings (sea-type) and two with the more common life history strat egies of 1 (river-type) or 2 (lake-type) yr of freshwater residence pr ior to seaward migration. Innate differences in survival, ability to r egulate tissue chlorides, and oxygen consumption when first introduced into salt water were more evident in April and May when fish were les s than 50 mm in length. In fish longer than 50 mm, the only significan t differences among the stocks were in saltwater growth. Between June and August, sea-type fish showed faster growth than river-type fish wh ich in turn grew faster than lake-type fish. When introduced into salt water in October, virtually no growth occurred in any stock, regardle ss of fish size. River-type and lake-type sockeye, which normally over winter 1 and 2 yr, respectively, in freshwater, can be reared in seawa ter if underyearlings are raised to a length of 50 mm before release i nto salt water, similar to the normal life history of sea-type underye arlings. Early life history appears to be influenced more by habitat t han by genetics.