Inferring the Relative Oceanic Distribution of Salmon from Patterns in Age-Specific Arrival Timing

The oceanic distribution of Columbia River springsummer Chinook Salmon Oncorhynchus tshawytscha is essentially unknown, as they are first detected in the fishery at the mouth of the river and then at Bonneville Dam, located 235km upstream of the river mouth. Although the composite run timing to Bonneville Dam varies interannually, earlier studies revealed that the distinct populations comprising this run are ordered in arrival time. Our analysis of returning tagged adults shows that within populations, the oldest fish (3-ocean; i.e., those that spend 3years in the ocean) arrive first, the 2-ocean fish arrive next, and the 1-ocean fish arrive last. To explore possible mechanisms underlying this persistent age-dependent ordering of arrival time, we simulated the fish's return migration using a model based on a geomagnetic homing mechanism with age-specific swimming speeds. To reproduce the observed age-dependent differences in arrival time with the model, the 2-ocean and 3-ocean fish within a population needed to initiate migration from the same location and time, while 1-ocean fish could initiate homeward migration from a more consistent location in the northern Gulf of Alaska with some delay. Arrival timing patterns provide valuable information that can be used to infer the features of homing migrations, which in turn may be useful for managing fish harvest by integrating age information into existing management techniques and for considering potential responses to climate change. Received April 27, 2012; accepted October 25, 2012