|Abstract or Summary
- Partial migration is a common migratory behavior where some individuals in a population migrate and others do not. Patterns of partial migration can vary dramatically, especially for species that inhabit a wide range of environments. I described and predicted spatial variation in marine migration (anadromy) of female Oncorhynchus mykiss (steelhead and rainbow trout) in the John Day River catchment basin, Oregon. This large catchment (20,500 km2), where O. mykiss is known to exhibit partial migration, encompasses a broad range of environmental conditions. I collected 149 juvenile O. mykiss across 72 sites and identified locations used by anadromous females by assigning maternal origin (anadromous versus non-anadromous) to each juvenile. These assignments used comparisons of strontium to calcium ratios in otolith primordia and freshwater growth regions to indicate maternal origin. Individuals within sites were more likely to have the same maternal origin than expected at random (χ2 test, P < 0.001). I used logistic regression to predict probability of anadromy among sites in relation to stream size (as indicated by mean annual runoff). Stream size and maternal origin for
single fish collected from 69 sites were used to construct a predictive model. I examined the ability of this model to predict new individuals at 47 of these sites (where a second fish was collected) using a variety of diagnostics, including kappa statistics and receiver operating characteristic curves. The model predicted anadromy in this second set of individuals with a moderate level of accuracy (e.g. 68% correctly predicted with a 0.5 classification threshold). Residuals from the models were not spatially autocorrelated, as indicated by Mantel tests. This result also suggested that remaining variability in the expression of anadromy was due to localized influences, as opposed to broad-scale gradients unrelated to stream size. The importance of stream size implies that occurrence of migratory females was related to environmental variability, but stream size is an indicator of many potential processes, and I was unable to identify a specific likely mechanism. However, I was able to demonstrate 1) that it is possible to predict the probability of anadromy across broad environmental gradients, and 2) the validity of a sampling approach that minimizes the need for sacrificial sampling of individuals. These results are important for the management of O. mykiss because anadromous individuals (steelhead) within the John Day River watershed are listed as a threatened species, and it is difficult to discern steelhead from non-anadromous individuals (rainbow trout). My results provide managers with the first broad-scale description and prediction of locations supporting anadromy, and provide a broad template that can guide future habitat restoration, monitoring, and research to better manage and understand the expression of anadromy in O. mykiss.