Coded-Wire Tag Data Suggest a Decline in the Proportion of Age-5 Spring Chinook Salmon

An understanding of the processes that drive demographic trends constitutes a fundamental concept for fisheries management and conservation.  In the upper Willamette River, spring Chinook salmon (Oncorhynchus tshawytscha) are produced by four hatcheries that are managed to meet harvest, mitigation and conservation objectives.  Annual numbers of adult returns provide a measure of program effectiveness for harvest and mitigation objectives.  However, conservation objectives are designed to preserve genetic and life history diversity.  In this study, we examined coded-wire tag data from spring Chinook salmon released from upper Willamette River hatcheries over a 15-year period.  Linear regression analyses suggested a significant decline in the proportion of age-5 adults taken in multiple terminal fisheries and recovered from spawning grounds.  We discuss possible mechanisms for this decline and consider the implications of our findings.