Coupling habitat, hydrological and population models to evaluate strategies to recover an endangered salmonid population

Introduction Habitat degradation, changes to hydrology, over-harvest, and hatchery practices have contributed to the decline of salmon populations in California over the past two centuries. The accumulation of these stressors led to such low salmon population numbers in California’s Central Valley that Sacramento River winter-run Chinook salmon (SRWRC; Oncorhynchus tshawytscha ) were federally listed as endangered in 1994. Since then, significant effort and resources have gone towards supporting the recovery of these populations, including habitat restoration, hydrology modifications, harvest restrictions, and revisions to hatchery management practices. Despite these efforts, the SRWRC population remains at risk. There is an urgent need to explore what combination of management actions are needed to substantially improve their status. Methods In this study, we use a long-term population dynamics model, specifically an updated version of the Winter-run Chinook salmon life cycle model (WRLCM), to evaluate a large suite of potential recovery scenarios that include both singular actions and multiple actions to identify which scenarios best support population recovery of endangered SRWRC. The WRLCM couples water-planning models, hydrodynamic models, habitat capacity models, and life stage-specific survival models within an encompassing stage-structured salmon life cycle model, to predict how SRWRC will respond to changes in suites of management actions. We evaluated scenarios that included increased quantity and quality of rearing, outmigration, and spawning habitat, modified reservoir operations to increase egg survival, reduced incidental harvest in ocean fisheries, and modified hatchery practices. Results Our results demonstrate that no singular recovery action was sufficient to achieve recovery objectives over a 26-year simulation period (1995–2020), but scenarios that included multiple actions could increase total population size by over 7 times the historical population size, without the need for hatchery supplementation. These results illustrate the synergistic effects that habitat, flow and other actions can have when taken in concert. Discussion Our results indicate that a holistic, multi-faceted set of recovery actions will likely be required to recover endangered winter-run Chinook salmon in the Sacramento River.