Bimodal length distributions of cultured chinook salmon and the relationship of length modes to adult survival

Groups of juvenile spring chinook salmon, Oncorhynchus tshawytscha, reared in 14 experimental raceways at Willamette Hatchery, Oregon, showed bimodal distributions of lengths before their release from the hatchery. Estimates of lengths of juveniles at ocean entrance were made from analyses of scales of returning adults. From these estimates, survival to adulthood was not associated with either the upper or lower length modes. For the 1991 and 1992 brood fish, there were no significant differences between the average lengths of juveniles measured at the hatchery and those at ocean entrance, as estimated by scale analysis. For the 1989 and 1990 brood fish, the average length of fish measured at the hatchery was significantly greater than that at ocean entrance, as measured by scale analysis, suggesting that larger fish had poorer survival in these years. Adult salmon returning to the hatchery were sorted into year classes by analyzing coded wire tags. Older age classes resulted from smaller fish at ocean entrance, as estimated by scale analyses. However, within age classes of 4- or 5-year-old fish, there was no relationship between the size of the adult at return to the hatchery and the size at which the juvenile entered seawater.