The arrival timing of adult Chinook to the Columbia River varies by a month or more and there are currently no good methods to predict when this will occur. Relative stock timing (within-run) has been reconciled to a certain extent with radio-tag observations. Keefer et al. (2004) observed that arrival time of individual stocks is relatively constant compared to the variability in over-all run timing. Spawning success and egg to smolt survival are a function of headwater conditions under strong selective pressure so specific stocks have their own optimal time to arrive relative to the run as a whole. One interpretation is that the run as a whole is more under control of external variables than stock-specific genetic/adaptive differences that account for differences between the stocks.
Traditional enumeration methods at Bonneville divide the runs of Chinook salmon into a spring, summer and fall run based on specific calendar days. While convenient for record-keeping purposes, it ignores the ecological basis for variation in run-timing and therefore prediction of the arrival of Chinook. A late-arriving spring run is seen as smaller and/or appears to have a summer component. The arrival of the spring run is quite dramatic in some years. In recent years, daily counts of spring run arrivals increased by 2-3 orders of magnitude in less than two weeks.
Spring Chinook arrival timing at Bonneville dam could be a result of three different factors. First, their location relative to the mouth of the river at the onset of migration determines the total travel distance. Second, movements of the water in the near-shore environment can accelerate or retard their travel speed as they get close to the mouth of the river. Third, in-stream conditions that are sub-optimal (flow or temperature) are known to delay salmonids. In this white paper, we examine the arrival timing of the spring Chinook salmon at Bonneville Dam. We determine arrival timing independently of the ACOE calendar dates and examine ocean conditions, river mechanisms and within-run variables that may be related to arrival timing. The passage at Bonneville is described with a closed-form mathematical function which in turn quantifies specific between-year timing signals. These signals are related univariately to environmental measures that identify specific mechanisms related to timing.