Brian HodgePeggy Wilzbach

Walt DuffyJames Hobbs

Partial Migration in Wild Oncorhynchus mykiss from the

Lower Klamath River Basin

USGS California Cooperative Fish and Wildlife Research Unit, Humboldt State University

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

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

MethodsMethods

Photo by Matt Peterson

To sea or not to sea?

Resident: FL > 95th quantile for smolt length OR age > max smolt age

Method I: Analysis of Otolith Sr Isotope Ratios (

)

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

0.70420.7075

0.7072

0.7067

0.70830.7052

0.7038

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

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

Method II: Qualitative Scale Analysis

Determined age, growth, and life history

Results:Maternal Origin vs. Migratory History

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 HistoryMaternal Origin

Anadromous

Nonanadromous

Anadromous 37 2Nonanadromous 12 1

n=63

Maternal Origin vs. Migratory History cont’d

Results: Scale Analysis

Observed 32 life history categories (30 spawning trajectories) 5 nonanadromous

27 anadromous

FL: 268-508 mm

FL: 256-800 mm

43 of 655 fish sampled were residents (M:F = 1.0:1.3)

Scale Analysis cont’d

Scale Analysis cont’d

4301.22128.0 FLFecundity Error bars represent ± 1 SE

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

Management Implications

Will resident rainbow trout contribute towards recovery of steelhead in the Klamath Basin following dam removal?

Acknowledgements

California Department of Fish and Game,Steelhead Report Card Program