Effects of Artificial Lighting at Night on Predator Density and Salmonid Predation

Predation of juvenile salmonids within California’s Sacramento–San Joaquin Delta (the Delta) has been identified as a contributing factor to low survival during out-migration through the system. Artificial lighting at night (ALAN) may contribute increased levels salmonid predation by attracting predators and prey, increasing predator reaction distance, boosting foraging success. To assess ALAN effects on (piscivorous fishes) density relative risk Chinook Salmon Oncorhynchus tshawytscha smolts in Delta, we preformed field-based experiments with introduced ALAN. We used adaptive resolution imaging sonar cameras generate estimates light dark treatments throughout nightly 30-min intervals. simultaneously deployed event recorders estimate impact intensity (lux) smolts. Early (1–3 h past sunset), were unrelated However, late (3–5 presence density, lux. was also positively related late-night under ALAN, coupled benefits likely contributed lux–risk relationship. The exact mechanism behind this discrepancy between early- trends is unknown could be result our experimental design or community sampled here. if these temporal prove robust future investigations, reduction campaigns maximize human while minimizing negative impacts salmonids. Overall, findings align others suggest that increases predation. Although many questions remain unanswered, it appears reducing artificial illumination practical management strategy reduce