Monitoring of populations near the margins of a species’ natural distribution, which often occur in habitats approaching their thermal limits and may be susceptible to even modest temperature increases, can provide insights on the effects of climate stressors expected to affect more northerly populations in the future. Annual monitoring activities track the escapement, document spatial and temporal spawning distribution and estimate juvenile production and outmigration characteristics of fall-run Chinook salmon, Oncorhynchus tshawytscha, in California’s Stanislaus River. Monitoring during the recent record-breaking drought revealed only minor migration- and spawning delays for returning adults, despite water temperature conditions considered detrimental to egg incubation. Results from degree-day analyses to estimate fertilization dates of emigrating juvenile salmon suggests diminished reproductive success of early spawners. While peak timing of reproduction might shift over time resulting from poor fitness of early spawners, this reduces in-river rearing opportunities of juveniles, which must emigrate before thermal connectivity to the marine environment is lost in spring. Long-term air temperature data indicates that cooling below certain temperature thresholds occurs progressively later in fall, while the same thresholds are exceeded earlier in spring, reducing the window for emigration of juvenile fall-run Chinook salmon and, by extension, further diminishing population resilience

1. AFS / TWS 2019
Reno, Nevada
Hellmair, M., T.J. Pilger, M.L. Peterson, D. Demko, & A.N. Fuller
Reproductive failure of Chinook salmon:
effects of limited behavioral plasticity in
warming rivers

2. Introduction and Background
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• Stanislaus River
• Tributary to San Joaquin River
• Nearly 100 km below barrier
• Southern boundary of
distribution
• Intensively monitored system
• Adult upstream migration
• Spawning surveys
• Juvenile Outmigration
• Complementary studies
• Discharge and water temperature

3. www.FISHBIO.com
Goodwin Dam (°C)
RST
Weir
Knights Ferry (°C)

4. Methods: Adult Return Timing
Resistance board weir downstream of spawning grounds*
• Annually since 2003
• Automated infrared and video monitoring 24/7
• Used to calculate passage percentiles
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* Peterson et al. 2017. Environmental Factors […] . NAJFM 37: 78-93.

5. Methods: Reproductive Timing
Redd surveys throughout spawning reach*:
• Biweekly throughout the spawning period
• Counts and location of new redds
• Superimposition
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*Peterson et al. 2019. Stability in reproductive timing […]. Fisheries Management and Ecology, in press.

6. Methods: Juvenile Production and
Outmigration Timing
RST sampling downstream of spawning grounds*:
• Operated annually throughout the outmigration period
• All catch counted, subsample measured
• Mark-recapture efficiency releases
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*Pilger et al. 2017. Evaluation of Long-Term Mark-Recapture Data […]. SFEWS 17(1).

7. Adult Return Timing
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8. Reproductive Timing
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9. Reproductive Timing, cont.
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10. Reproductive Timing, cont.
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11. Reproductive Timing, cont.
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12. Reproductive Timing, cont.
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13. Reproductive Timing, cont.
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14. Juvenile Production
• Focus on fry (<35 mm FL)
• Assumed recently emerged (within days)
• Evaluate predicted vs. observed timing
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15. Juvenile Production, cont.
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16. Juvenile Production, cont.
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17. Juvenile Production, cont.
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18. Summary
• Adults arrive AND spawn without great delay
despite unsuitable temperatures.
• High incubation mortality, reduced production.
• Phenological shift due to directional selection
and heritability of migration and reproductive
timing? Unlikely!
• Window of thermal suitability for outmigration
is shrinking!
• Implications beyond the Stanislaus River.
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19. Implications, cont.
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Figure modified from Herbold et al. 2018. SFEI 16:2.
• Juveniles of all runs
migrate through San
Francisco estuary
• Lower river temperature
cannot be regulated
• Increasing air
temperatures warm water
temperatures

20. Implications, cont.
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21. Implications, cont.
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22. Conclusions
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• Shift in phenology limited by shrinking window
of thermal suitability
• Short term: management of coldwater storage
• Long term: range contraction?

23. Acknowledgements
Funding provided by: Oakdale and South San Joaquin
Irrigation Districts
We thank the following people for their efforts, support,
and advice:
FISHBIO – Shaara Ainsley, Patrick Cuthbert, Jeremy
Pombo, John Montgomery, Jim Inman, Chrissy Sonke,
Mike Kersten, Tara Lamb, Garth Jaehnig, Rob Fuller,
Earl Fuller, Logan Douglas, Scott Stocker, Rick
Biedenweg, Graham Buggs, Ian Herzberger, Ben Griffith,
Steve Clark, Dee Thao, Erin Loury, Bryce Neal, and
Garret Muniain
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24. FISHBIO
Oakdale, California
Chico, California
Santa Cruz, California
FISHBIO Laos
Vientiane Capital, Lao PDR
FISHBIO CR
Boca del Rio Sierpe
Costa Rica
Questions?
Michael Hellmair
michaelhellmair@fishbio.com
530.892.9686
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