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. Additionally, there were half as many older fish
(>1 year old) in pools downstream of the waterfall (0.05 vs. 0.13
individuals/m2). In a second stream, between-year variation in the
dominance of migratory vs. resident fish allowed us to explore differences
in fish density and size structure through time, and we found a consistent
pattern. In brief, when migratory genotypes dominated, we found higher
densities of young fish and lower densities of older fish, resulting in a
simpler size structure, compared to when resident genotypes dominated.
Moreover, large resident trout had a slightly higher trophic position than
young fish (3.92 vs. 3.42 in one creek and 3.77 vs. 3.17 in the other),
quantified with stable isotope data. The difference in fish size structure
did not generate trophic cascades. Partial migration is widespread among
migratory populations, as is phenotypic divergence between resident and
migratory forms, suggesting the potential for widespread ecological
effects arising from this common form of intraspecific variation.