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#           Research, monitoring, and evaluation of emerging issues and measures to recover the Snake River Fall Chinook salmon ESU      

      August 1, 2021    

The portion of the Snake River fall Chinook salmon Oncorhynchus tshawytscha evolutionary significant unit (ESU) that spawns upstream of Lower Granite Dam transitioned from low to high abundance during 1992–2020 in response to U.S. Endangered Species Act recovery efforts and other federally mandated actions. This annual report focuses on changes in population abundance and habitat use by natural- and hatchery-origin spawners. Typically, we also report on population attributes of natural-origin juveniles, but data on juveniles were not collected in 2020 due to Covid-19. Spawners have located and used most of the available spawning habitat and that habitat is gradually approaching the point that no more redds can be supported. Timing of spawning and fry emergence have been relatively stable, but effects of density dependence are evident in juvenile life stages. Apparent abundance of juvenile fall Chinook salmon has increased and we noted the following changes: parr dispersal from riverine rearing habitat into Lower Granite Reservoir has become earlier; growth rate (g/d) and dispersal size of parr has declined; and passage timing of smolts from the two Snake River reaches has become earlier and downstream movement rate has increased. These findings coupled with stock-recruitment analyses presented in this report provide evidence for density-dependence in the Snake River reaches and in Lower Granite Reservoir resulting from the expansion of the recovery program. The long-term goal is to use this information in a comprehensive modeling effort to conduct action-effectiveness and uncertainty research and to inform Fish Population, Hydrosystem, Harvest, Hatchery, and Predation and Invasive Species Management Research, Monitoring, and Evaluation (RM&E) programs. 

In 2020, the U.S. Geological Survey (USGS) focused survey efforts in the Snake River on deepwater redd searches and fish collection for parentage-based tagging (PBT) analyses. We use a boat-mounted underwater video camera to count 170 deepwater redds at 19 of the 28 sites surveyed. Redd depths averaged 4.2 m. We collected genetic samples from 297 live fall Chinook salmon and 16 carcasses at 44 unique geographic locations that spanned 89 river kilometers. Seventy-two fish were recovered at Eureka Bar (rkm 307.1) and Corral Creek (rkm 349.7), which accounted for 23% of all collected fish in 2020. Most (238 fish) post-spawned salmon were collected from early to mid-November just after peak spawning. A summary of 2019 PBT results can be found in Appendix A.1. 

In 2020, we PIT tagged subyearling fall Chinook salmon in the Clearwater River to obtain population and growth data. In the Clearwater River, we tagged 2,192 subyearlings and recaptured 79 (3.6%) fish in the river and 187 fish (78 tagged by the U.S. Geological Survey, 109 tagged by the Nez Perce Tribe) at Lower Granite Dam during October which provided information for growth estimation. Within riverine habitats, growth in both length and mass were higher for fish tagged with 8-mm tags than with 9- and 12-mm tags. Estimated growth in length and mass of subyearlings was generally lower in Lower Granite Reservoir than in riverine habitats. 

Information on prey resources and juvenile fall Chinook salmon prey consumption was collected to better understand the growth opportunity of late-migrating fish in Lower Granite Reservoir. Zooplankton and surface drifting prey were collected from three reservoir locations from July through October during 2019 and 2020. Fall Chinook salmon diet data were collected from angled fish using gastric lavage. Cladocera and Copepoda were the most abundant zooplankton taxa collected while Diptera was the most common invertebrate taxon collected in surface drift samples. Totals of 49 and 94 juvenile fall Chinook salmon were captured in 2019 and 2020, respectively, and most fish were caught in the lower reach of the reservoir in October of each year. Juvenile fall Chinook salmon consumed mainly dipterans in July and October 2019 and mainly Daphnia during August–October in 2020. Fish showed selection mainly for dipterans in 2019 but strong selection for Daphnia during October 2020. Stomach fullness values were relatively low (<1.1%) during both years. Results show that prey resources are adequate in Lower Granite Reservoir to support positive fish growth during late summer and early autumn. 

## Citation Information

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#### Kenneth Tiffan

##### Research Fish Biologist

Western Fisheries Research Center

ktiffan@usgs.gov

509-538-2972

#### Russell Perry, Ph.D.

##### Research Fish Biologist

Western Fisheries Research Center

rperry@usgs.gov

509-538-2942

#### Dalton Hance

##### Research Statistician (Biology)

Western Fisheries Research Center

dhance@usgs.gov

509-538-2299

#### John Plumb, Ph.D.

##### Research Fish Biologist

Western Fisheries Research Center

jplumb@usgs.gov

509-538-2903

#### Bradley Bickford

##### Fish Biologist

Western Fisheries Research Center

bbickford@usgs.gov

509-758-3311

#### Tobyn Rhodes

##### Fish Biologist

Western Fisheries Research Center

trhodes@usgs.gov

509-758-3311

## Related Content

#### Kenneth Tiffan

##### Research Fish Biologist

Western Fisheries Research Center

ktiffan@usgs.gov

509-538-2972

#### Russell Perry, Ph.D.

##### Research Fish Biologist

Western Fisheries Research Center

rperry@usgs.gov

509-538-2942

#### Dalton Hance

##### Research Statistician (Biology)

Western Fisheries Research Center

dhance@usgs.gov

509-538-2299

#### John Plumb, Ph.D.

##### Research Fish Biologist

Western Fisheries Research Center

jplumb@usgs.gov

509-538-2903

#### Bradley Bickford

##### Fish Biologist

Western Fisheries Research Center

bbickford@usgs.gov

509-758-3311

#### Tobyn Rhodes

##### Fish Biologist

Western Fisheries Research Center

trhodes@usgs.gov

509-758-3311

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6505 NE 65th StreetSeattle, WA98115-5016United States

(206) 526-6282

(206) 526-6654

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