The influence of anthropogenic disturbance on juvenile Chinook salmon life history variability in the Willamette Valley

Background/Question/Methods

Understanding the distribution, abundance and relative performance of life history types is critical for development of passage for juvenile Chinook salmon in the Willamette Valley Basin (WVB), and an important element of the Willamette Valley Biological Opinion. Recent analyses of screw trap data suggest that juvenile Chinook salmon life history strategies are variable within and among WVB populations, including traits that resemble both an ocean-type life history with subyearling emigration in summer or fall as well as a stream-type life history with yearling emigration the following spring.  We reconstructed the juvenile rearing and migratory patterns of a sample of natural origin adult Chinook salmon that returned to the Middle Fork Willamette Basin.  Scale morphometric patterns and otolith isotope ratios of Sr⁸⁷ /Sr⁸⁶ were used to characterize juvenile life histories and estimate juvenile size at freshwater emigration.  We also used alkaline earth elements Sr, Ba, Mn, Mg, and Ca in otoliths and water, combined with otolith morphometric attributes, to discern movement and rearing of natural origin juvenile Chinook salmon in natal rearing (adult out-plant) tributaries and downstream project reservoirs. 

Results/Conclusions

We found that a significant portion of sampled juvenile Chinook salmon reared in project reservoirs and emigrate from freshwater at large sizes, which may be provide a survival advantage to adulthood. Elemental water samples between the North Fork Middle Fork Willamette and Lookout Point reservoir were heterogeneous, and similar trends were found in otoliths from juveniles captured in each habitat. Preliminary results from analysis of otolith microstructure suggest increased growth in project reservoirs relative to natal rearing streams. Determining the juvenile rearing habitat and emigration ecology of these populations will enhance the understanding of the interaction between life history variation and anthropogenic disturbance and assist in developing appropriate management strategies.