1. Brian Hodge Peggy Wilzbach Walt Duffy James Hobbs Partial Migration in Wild Oncorhynchus mykiss from the Lower Klamath River Basin USGS California Cooperative Fish and Wildlife Research Unit, Humboldt State University

2. Lower Klamath River Basin  Supports the largest and most popular steelhead fishery in California  Steelhead exhibit the rare “half-pounder” life history One study has addressed O. mykiss life history on a basin scale

3. Objectives: To determine the current life history structure of O. mykiss from the lower Klamath River Basin To determine influencing factors and consequences of individual life history trajectories

4. Methods Photo by Matt Peterson

6. To sea or not to sea? Resident: FL > 95 th quantile for smolt length OR age > max smolt age

7. Method I: Analysis of Otolith Sr Isotope Ratios ( )

8. Migratory History and Maternal Origin *Source: Quiñones et al. 0.7092 0.7066 0.7057 0.7062 0.7064 0.7058 0.7047 0.7066 0.7042 0.7075 0.7072 0.7067 0.7083 0.7052 0.7038

9. Migratory History  Tracked movement between freshwater and marine environments Steelhead: Mean otolith Sr isotope ratio exceeded upper bounds of known in-stream values Rainbow trout: Mean otolith Sr isotope ratio never exceeded upper bounds of known in-stream values

10. Maternal Origin  Compared in the core and freshwater regions (1 tailed t-test, α=0.05) Anadromous: Core ratio significantly greater than freshwater ratio Nonanadromous: Core ratio not significantly greater than freshwater ratio

11. Method II: Qualitative Scale Analysis  Determined age, growth, and life history

12. Results: Maternal Origin vs. Migratory History

13.  Maternal origin was indeterminable for 11 fish (9 STH and 2 RBT,) either because the laser transect missed the core, or because the Sr isotope ratio in the freshwater region exceeded the global marine value. Migratory History Maternal OriginAnadromousNonanadromous Anadromous372 Nonanadromous121 n=63 Maternal Origin vs. Migratory History cont’d

14. Results: Scale Analysis  Observed 32 life history categories (30 spawning trajectories)  5 nonanadromous  27 anadromous FL: 268-508 mm FL: 256-800 mm

15.  43 of 655 fish sampled were residents (M:F = 1.0:1.3) Scale Analysis cont’d

16. Error bars represent ± 1 SE

17. Conclusions  Anadromous and nonanadromous forms of O. mykiss occurr sympatrically in the lower Klamath River Basin  Steelhead and rainbow trout may give rise to the alternative life history form  Origin (likely migratory difficulty) and growth appear to influence the probability of residualization  Apparent costs of a resident life history are reduced size and fecundity at age. However, a presumed benefit is increased survival relative to an anadromous life history

18. Management Implications  Will resident rainbow trout contribute towards recovery of steelhead in the Klamath Basin following dam removal?

19. Acknowledgements California Department of Fish and Game, Steelhead Report Card Program