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Juvenile steelhead are exposed to numerous threats in heterogeneous, estuarine environments, yet understanding of survival patterns and processes during this migratory stage is often limited by studies that use surrogate species or are restricted in duration and spatial specificity. Lack of detailed survival information in this critical migratory stage limits the effectiveness of management to maintain juvenile life history diversity in threatened populations. We used acoustic telemetry with multistate release-recapture models to investigate survival patterns during a key stage of the juvenile emigration of anadromous steelhead through the Sacramento-San Joaquin River Delta of California, United States, over multiple years, including three drought years. Survival was highly variable both within and among the six years of the study; estimated total survival through the Delta ranged from 0.06 (May 2014) to 0.69 (March 2011). Survival in the upstream reaches was associated with river discharge into the Delta, while survival through the lower reaches was associated with migration route. The lack of a single factor associated with survival in all reaches counteracts preconceived ideas of survival processes. Hydrodynamic manipulation and habitat improvements are recommended to support this anadromous population in a changing climate.

ably provided field assistance under challenging field conditions. Betty Black, Eric Freeburg, Taylor Jackson, Philip Lung, and Amanda Sceusa provided lab assistance. Otolith microchemistry and water chemistry analyses were completed in the GeoMed Analytical Laboratory, directed by Robyn Hannigan at the University of Massachusetts, Boston, U.S.A., and Robin Hannigan and Eric Freeburg provided valuable guidance on analytical procedures. Cheryl Crowder at Louisiana State University prepared gonadal tissues for histological examination. Richard Doucett and Melanie Caron's contributed practical discussion and analyses of stable isotopes. Chuck Smith of ENCO assisted with laboratory analyses for metals. All organic laboratory analyses were conducted at the North