Population-level impacts of reducing artificial light at night across an entire migratory corridor

Artificial light at night (ALAN) is a widespread anthropogenic pollution that poses challenges for wildlife in nearly every ecosystem on planet Earth. Although there has been substantial progress on understanding the effects of ALAN on animal physiology and behaviour, few studies have estimated its population-level impacts in the wild, leaving the potential conservation benefits of ALAN reduction unresolved. Here, we evaluated the population-level effect of ALAN reduction through its impact on juvenile Chinook salmon (Oncorhynchus tshawytscha) vulnerability to predation during oceanward migration through California's Sacramento-San Joaquin Delta (Delta). Removal of all ALAN sources was conservatively predicted to allow up to 1123 additional successful migrants per million migration attempts, depending on the baseline rate of through-Delta migration success and phototaxic behaviour. Reducing the impact of ALAN in this system could have small but worthwhile benefits for salmon at a cost and degree of contention that is trivial compared with many other habitat restoration actions.