Determining Optimum Size at Release for Hatchery‐Origin Tucannon River Spring Chinook Salmon Using PIT Tags

Abstract

The Tucannon River spring Chinook Salmon Oncorhynchus tshawytscha hatchery program has consistently failed to reach the program target adult (age 4 and older) smolt‐to‐adult return (SAR) rate of 0.87%. This has resulted in the hatchery program falling short of meeting its hatchery adult return goal of 1,152 fish. To determine whether hatchery fish released at a larger size would increase the number of returning adults, we released 95,256 PIT‐tagged, hatchery‐origin spring Chinook Salmon with known FLs (range = 73–212 mm) categorized into five length‐classes (<120, 120–139, 140–159, 160–179, and ≥180 mm FL) over eight brood years (2006–2013) to examine how size at release affected the smolt‐to‐adult survival (SAS) rate to the Columbia–Snake River system and the SAR rate to the Tucannon River. We used this information to determine an optimum size range at release to maximize adult returns to the Tucannon River and determine whether the program target SAR of 0.87% was achieved by any of the length‐classes. Return of hatchery adults (age 4 and older) for both SAS and SAR peaked for the 140–159‐mm length‐class. Smaller size at release resulted in lower survival, whereas fish larger than this size range matured prematurely either as minijacks or jacks and the majority never made it back to the Tucannon River. Based on this study, to maximize adult returns, hatchery smolts from the Tucannon River spring Chinook Salmon hatchery program should be released in the 140–159‐mm range. None of the length‐classes came close to reaching the adult SAR target of 0.87% (SAR for fish that were 140–159 mm at release was 0.15%). The expectation that changes to smolt size will lead to reaching the 0.87% SAR target is unrealistic for this population under current hatchery rearing and environmental conditions.