Partial migration alters population ecology and food chain length: evidence from a salmonid fish

Many migratory species, from monarch butterflies to wildebeest, express partial migration, where only a subset of a population migrates. This intraspecific variation is likely to have large ecological consequences. We studied the ecological consequences of partial migration in a salmonid fish, Oncorhynchus mykiss, in coastal streams in California, USA. One ecotype, steelhead trout, migrates to the ocean, whereas the other, rainbow trout, completes its lifecycle in freshwater. Migration has a strong genetic basis in O. mykiss. In one stream, we found differences in the frequency of migration-linked genotypes below and above a waterfall barrier (migratory allele frequency of 60% below vs. 31% above). Below the waterfall, in the migratory-dominated region, the density of young fish (<1 year old) was approximately twice that in the resident-dominated region above the waterfall (0.46 vs 0.26 individuals/m2, respectively), presumably reflecting the higher fecundity of migratory females. Ad...