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Movement of Resident Trout Transplanted Below a Barrier to Anadromy

Movement of Resident Trout Transplanted Below a Barrier to Anadromy. Peggy Wilzbach Mark Ashenfelter USGS California Cooperative Fish Research Unit, Humboldt State University. Coastal Rainbow Trout ( O. mykiss irideus).

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Movement of Resident Trout Transplanted Below a Barrier to Anadromy

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  1. Movement of Resident TroutTransplanted Below a Barrier to Anadromy Peggy Wilzbach Mark Ashenfelter USGS California Cooperative Fish Research Unit, Humboldt State University

  2. Coastal Rainbow Trout (O. mykiss irideus) genetic v. environmental basis of migratory polymorphism poorly understood Stream-resident form ? = Steelhead Drawings from Behnke 2002

  3. Objective • To determine if resident rainbow trout isolated above a barrier to anadromy will exhibit migratory behavior when transplanted below the barrier

  4. Experiment Location: Freshwater Creek, CA • 5-m waterfall on upper mainstem, 14.5 km from river mouth • Semi-permanent weir close to mouth allows escapement to be tracked • Watershed-scale full-life cycle monitoring (CDFG) increased probability of detecting transplanted individuals

  5. Methods • Above-barrier fish (>100 mm FL) captured & PIT-tagged • ½ of sample transplanted below falls; ½ were released at point of capture • Transplant location ~ 10 km from tidewater Released downstream Released upstream Fall 05: 22 20 Fall 06: 4344 6564 • Incomplete design: below-barrier fish not transplanted above falls

  6. Methods -cont’d • Attempts made to re-sight or capture transplanted individuals • 7 fixed streamwidth antennas, operated yr-round • Mobile PIT tag interrogation system (meter marker precision and live or dead data): basin surveys May – Jun and in Oct • Downstream migrant traps: Mar - Jun • Juvenile abundance survey/ night dives: summer

  7. (Humboldt Bay) tidewater weir Downstream migrant trap, & Stream-width fixed antenna- transplant release waterfall N Freshwater Creek

  8. Above-barrier population • Individuals derived from resident rather than anadromous parents - based on Sr/Ca analyses of primordial & freshwater growth regions of otoliths) • population skewed toward older individuals Size frequency distribution

  9. Allelic frequency at 11 loci differentiating between rainbow trout & cutthroat trout • genotyping of 18 individuals revealed that all showed some degree of introgression with cutthroat trout, with preferential backcrossing to rainbow) Pure cutthroat trout

  10. Growth rates did not appear to differ between upstream & downstream population 2006 2007 p = 0.51 p=0.35

  11. Below the barrier:Where did the transplants go? • 44 of 65 transplanted individuals were re-sighted or re-captured over 2 y study • Movement varied considerably among individuals: • 26 remained within 500 m of release location • 4 moved upstream (2 moving ~5 km to base of waterfalls) • 9 moved downstream but stayed in freshwater • 1 entered tidewater but returned quickly to freshwater • 4 entered tidewater, with presumed seaward migration

  12. Distance from release point to last known location release freshwater tidewater

  13. Presumed migrant: 185mm FL, 82.13g at release

  14. At recapture (183 d later, increased 7 mm in length, lost 12 g)

  15. Distance or direction of movement by transplanted individuals did not vary with fish size • Individuals that displayed seaward movement ranged in length at transplant from 103mm-185mm (average=132.7mm) • Largest transplanted individual (226mm) found ~ 6km downstream

  16. Act of transplantation may have biased movement of transplants, as downstream habitat was “full of fish” Above-barrier residents enjoyed lower than equilibrium density with removal of transplants HOWEVER: 6% of tagged, above-barrier individuals were found in below-barrier reaches, presumably washing over the falls

  17. Conclusion • The smoltification of at least some transplanted individuals, coupled with above-barrier ‘leakage’ of fish downstream, suggests the potential for resident trout to exhibit migratory behavior and to enter breeding populations of steelhead. • Management implications: • should above barrier populations be included in ESU’s? • Can above-barrier populations be used to help recover below-barrier populations? YES

  18. We thank the California Department of Fish and Game, Steelhead Report Card Program for their support of this work

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