CHANGES IN LIFE-HISTORY PARAMETERS IN A NATURALLY SPAWNING POPULATIONOF CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA) ASSOCIATED WITH RELEASESOF HATCHERY-REARED FISH

Chinook salmon (Oncorhynchus tshawytscha) spawning runs in Glenariffe Stream, New Zealand, exhibited significant changes in life history tra its following supplementation releases of hatchery-reared juveniles. T otal run strength did not change but the proportion of naturally produ ced fish declined to 34%. Attempts to separate spawners of natural and hatchery origin were unsuccessful, and 31-48% of natural spawners are now of hatchery origin. Hatchery males were smaller at age 2 and 3 th an males of natural origin, and more often matured as jacks, producing an 86-mm decrease in mean fork length over 28 years. There was no cha nge in length at age or age at maturity for female spawners. The propo rtion of jacks entering Glenariffe Stream each year was positively cor related with the proportion of jacks in the ensuing cohort. Most diffe rences between fish of natural and hatchery origin were related to hat chery rearing practices, but the decline in age at maturity among natu rally produced males appears to reflect traits inherited from parent s tock of hatchery origin. Hatchery releases may also favour the surviva l of ocean-type fry over stream-type fry, possibly reversing a tendenc y for stream-type behaviour to evolve in response to the lack of estua ries on most New Zealand chinook salmon rivers.