PBT improves abundance estimates of returning hatchery Chinook Salmon

Please Read: DOI: 10.1002/nafm.10823

Improving abundance estimates of spring‐summer Snake River Chinook Salmon for fisheries management

August 2022 North American Journal of Fisheries Management

DOI: 10.1002/nafm.10823

D. Katharine Coykendall, Thomas A. Delomas, Matthew Belnap, and Matthew R. Campbell

The Columbia River basin is home to a spring-summer Chinook Salmon run that returns to the Snake River drainage of Idaho, Oregon and Washington in the Pacific Northwest. Historically, the run was one of the more productive throughout the Columbia River Basin. However, Snake River spring-summer Chinook Salmon have experienced declines in abundance due to over-fishing, habitat degradation, and dams. Several stocks are listed as threatened under the United States Endangered Species Act and supported by mitigation hatcheries funded by Idaho Power Company, the Lower Snake River Compensation Plan and the Bonneville Power Administration. To maximize tribal and state harvest of returning hatchery adults, minimize impacts on wild fish, and ensure that enough hatchery fish return to meet broodstock needs, careful fisheries management is required. Since 2008, managers have used hatchery adults, Passive Integrated Transponder (PIT) tagged as juveniles and detected at Lower Granite Dam (LGD), to generate adult abundance estimates. In-season, these estimates inform state and tribal harvest shares and ensure broodstock needs are met. Post-season, they provide smolt-to-adult survival and return rates. Since 2012, parentage-based tagging (PBT) has provided an alternative method to estimate stock- and age- specific returns at LGD, since returning hatchery adults sampled at LGD can be assigned to their parents. We compared stock-specific abundance estimates between PIT- and PBT-derived methodologies for return years 2016 - 2019. Across all years, PIT tag estimates accounted for 65% of the PBT-based estimates at LGD across all age groups and release sites combined. This underrepresentation across all groups equated to 49,833 fish that were not accounted for in PIT tag abundance estimates. It is clear that PBT-based estimates should aide in-season harvest management and post-season run reconstruction to avoid the known bias of estimates from PIT tags, especially during years of low returns when increased accuracy is critical.

• We provide evidence that Parentage Based Tagging based abundance estimates are more accurate than PIT tag-based estimates, likely due to shedding or mortality that occurs with PIT tags. This is especially topical since Chinook Salmon Oncorhynchus

• We provide evidence that Parentage Based Tagging based abundance estimates are more accurate than PIT tag-based estimates, likely due to shedding or mortality that occurs with PIT tags. This is especially topical since Chinook Salmon Oncorhynchus tshawytscha abundances have decreased dramatically in recent years and managers have struggled to maintain small fisheries while ensuring hatchery brood stock needs are met.

• Abundance estimates of hatchery fish are crucial for maintaining small fisheries while meeting brood stock needs.

• We compare abundance estimates from Passive Integrated Transponders (PIT) tag detection versus Parentage Based Tagging (PBT).

• PBT based abundance estimates provide greater accuracy, because it is genetics-based, there is no tag shedding and no tag related mortality.

• PBT based abundance estimates provide greater precision due to higher tag rates.