Comparison of Length-at-Date Criteria and Genetic Run Assignments for Juvenile Chinook Salmon Caught at Sacramento and Chipps Island in the Sacramento–San Joaquin Delta of California

There are four distinct runs of Chinook Salmon (Oncorhynchus tshawytscha) in the Central Valley, named after their primary adult return times: fall, late-fall, winter, and spring run. Estimating run-specific composition juveniles entering leaving Sacramento–San Joaquin Delta is crucial for assessing population status processes that affect juvenile survival through Delta. Historically, run captured field has been determined using a length-at-date criteria (LDC); however, LDC assignments may be inaccurate if there high overlap timing size In this study, we use genetic to assess accuracy at two trawl locations Sacramento River (Delta entry) Chipps Island exit). Fin tissues were collected from approximately 7,500 samples between 2007 2011. Tissues analyzed 21 microsatellites determine individuals, which compared with assignments. Across years, was extensive among distributions fork lengths genetically identified juveniles, indicating compositions based on would tend underestimate fall-run especially late-fall-run both locations, greatly overestimate spring-run (both locations) winter-run (Chipps Island). We therefore strongly support ongoing efforts include tissue sampling identification key monitoring system.