Outmigration survival of a threatened steelhead population through a tidal estuary

Juvenile steelhead (Oncorhynchus mykiss) 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 mark–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.