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

Abstract

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