The spatial scale and ecological drivers of homing in a migratory salmon population

Background/Question/Methods

The resilience of organisms to large-scale environmental and climatic change depends, in part, upon the ability to colonize and occupy new habitats. However, a fundamental challenge to understanding dispersal and migratory processes for populations is our inability to follow the behavior of individuals over large spatial scales with fine scale resolution. While previous efforts to describe homing, or natal site fidelity, in migratory organisms have been hindered by the confounding effects of fragmented landscapes or population supplementation strategies, realistic conservation efforts must include considerations of behavioral diversity that drive animal movements and dispersal. To date, no studies of wild populations have been able to combine sampling over the required temporal and spatial scales to develop individual based models of homing. Herein, we quantify homing in a wild, threatened Chinook salmon (Oncorhynchus tshawytscha) population in a pristine wilderness basin within the Frank Church River-of-No-Return Wilderness of central Idaho.

Results/Conclusions

Using natural isotopic signatures (⁸⁷Sr/⁸⁶Sr) in otoliths (earstones) of spawned and dead individuals, we reconstruct the migratory behaviors and habitat transitions of previously unhandled salmon in Big Creek, a major tributary of the Middle Fork Salmon River.  We quantify the frequency and spatial extent of straying and place the propensity to stray in the context of ecological and behavioral factors that span the life cycle of individual fish. Our approach uniquely describes a critical component of life history diversity for salmon, quantifies a mechanism for maintaining metapopulation stability, and establishes scales over which salmon may respond to climate-based range shifts. Our results indicate that homing is scale-dependent, ranging from 55% (when considering spatial scales exceeding 10km from natal site) to 87% (when considering finer spatial ranges). Movements as juveniles (expressed as pre-migratory dispersal from natal areas) and sex highly influence straying occurrence, with males straying at higher rates than females; fish age and spawner density had no effect on straying across 4 years of study. We relate these findings to our current understanding of juvenile movements in the basin and the genetic structure of the Middle Fork Salmon Chinook population. Our study provides support for the consideration and conservation of behavioral diversity for population persistence and establishes the spatial scale at which this may be important for threatened and endangered salmon populations.