Long-Distance Minijack Migrations Confound Estimates of Salmon Cohort Success

Variation in age of maturity is a normal trait of male Chinook Salmon (Oncorhynchus tshawytscha). However, many hatcheries produce large numbers of males that mature in the same year of their release at age 2 (minijacks).  It is generally thought that most minijacks remain in freshwater and do not smolt; however, some migrate downstream and an unknown proportion enter saltwater.  In this study, the maturation status of the juvenile Chinook Salmon captured in ocean surveys along the Pacific Northwest coast (2001-2012) has been directly assessed by measuring androgen levels in the plasma.  We find that minijacks are commonly captured and have stock-specific distribution patterns that have not been previously described.  Minijacks from the Columbia Basin appear to follow typical juvenile patterns of emigration from the river, then reverse their migration within 1-3 months and re-enter the river according to stock-specific adult return timing (spring, summer or fall).  These fish simultaneously undergo two energetically taxing physiological processes (smoltification and sexual maturation), while carrying out the corresponding migration behaviors. We conclude with a discussion of the destinies of ocean minijacks that migrate to the ocean, whether this migration is maladaptive, and how it may confound projections of salmon cohort survival.