Inter-Population and Individual Variation in Migratory Characteristics of Juvenile Sockeye Salmon

Juvenile Sockeye Salmon from British Columbia generally undertake a rapid northward migration after marine entry to reach the Gulf of Alaska by late summer. Individuals that arrive on maturation grounds relatively early in the year are thought to experience greater rates of growth and survival; however, a fraction of juveniles from these populations appear to delay migration and are captured in southern regions through mid-autumn. We used otolith microstructure techniques to examine differences in size at ocean entry, growth, and phenology between these “lingering” individuals captured late in the year and migrants caught in the same regions during the summer. Comparisons across these metrics were used to test three hypotheses. First, juveniles that delay their migration until the autumn are more likely to have entered the marine environment at a smaller size or have lower growth rates – in this case, delaying migration may act as a means of improving en route survival. Second, apparently different migration strategies represent multiple pulses of river out-migration so that the movement rates of early and late migrating groups are relatively similar. Finally, if increasing variation in migratory strategy buffers populations against stochasticity, juveniles captured in autumn may not differ in condition or phenology.