Early Marine Residence of Spring-Run Chinook Salmon: a Comparison of Growth and Migration in Two Interior Columbia River Populations

Correlative evidence indicates that factors during early marine residence influence subsequent salmon survival although specific mechanisms of mortality remain elusive. It is also suggested that the timing of marine entrance influences growth and survival although there are few empirical data to robustly evaluate this hypothesis. We examined variation in otolith structure and chemistry to determine (1) the size and timing of freshwater emigration; (2) early marine growth; and (3) migration rate of juvenile Chinook salmon from interior Columbia and Snake River populations that were collected off the coasts of Oregon and Washington (1999, 2000, 2002-2004, 2006-2008). A broad range of brackish/ocean residence times (<5 to 80 d) and migration rates (1.3 to 20 km/d) were observed. Patterns of spatial distribution were distinct between the two stocks and juveniles from the Columbia River spring population entered marine waters earlier than those from the Snake River spring/summer population. Although Individuals from both populations resided in coastal waters for extended periods (~30 d) prior to capture, there was only weak evidence that timing of marine entrance was related to subsequent growth. Furthermore, interannual variation in size at emigration from freshwaters displayed only weak positive relationships with subsequent adult abundance. However, size at capture and indicators of initial marine growth were positively related to indices of ocean productivity and future abundance. For the Snake River population, interannual variation in migration rate was positively correlated with the presence of subtropical zooplankton species (r² = 0.70) and negatively correlated with the length of the upwelling season (r² = 0.64), indicating that migration behavior varied in response to local conditions. Within a year, individuals entering marine waters later migrated northward at faster rates, implying seasonal variation in migration patterns. Overall, the evidence indicates that local processes influence growth and migration behavior and that very early marine growth is positively related to survival.