Assessing Migration Success in Altered River Corridors

Migration is a spectacular example of biocomplexity, and migratory species are frequently important ecological, cultural, and economic components of ecosystems.  Migration differs among populations and can be described by differences in migratory systems, a conceptual framework for understanding the multi-scale and hierarchal factors affecting migratory species.  An important component of the migratory system is the migration syndrome—the set of phenotypic and behavioral traits that are expressed by individual organisms during migration events.  Here, I present a conceptual framework of migration syndromes in diadromous fishes and use three case examples from the Columbia River that illustrate how differences in migration syndromes may dramatically affect interpretations of “success” during upstream adult migrations.  The scientific paradigm for the study of fish migration has been strongly influenced by salmonid studies, and a key feature of anadromous salmonid migration is strong natal homing.  High site fidelity is associated with local adaptation and genetic structuring at fine scales (as small as 10s of m).  American shad appear to home at the scale of river basin, suggesting a lower degree of site fidelity and population structure at moderate scales (10-100s of km).  In contrast, current evidence suggests little or no site fidelity to natal drainage during upstream migration by Pacific lamprey.  Lamprey population genetic structure is relatively weak and only evident at scales of 10³ km.  These among-species differences in site fidelity result from differences in breeding site selection behaviors and these differences presumably also influence behavioral decisions during passage of local-scale obstacles (waterfalls and fishways).   For example, a “failed” passage attempt in a taxon with strong homing accurately represents an unsuccessful passage event, whereas the same outcome in a taxon with low natal site fidelity may actually reflect adaptive behavior in a species with a flexible breeding site selection program.  Consequently, evaluations of migration success and development of passage metrics and standards should explicitly consider the underlying migration syndrome of the target species.