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 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 earlyand 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. The diel light cycle is a driving force behind animal behavior and ecological interactions (Navara and Nelson 2007; Hölker et al. 2010a, 2010b). However, abundant and increasing artificial lighting at night (ALAN) disrupts ecological processes across a wide range of taxa (Gaston et al. 2013, 2014a; Zapata et al. 2019). Artificial lighting at night affects animal migration and orientation, foraging and predation, reproduction, and even human health (Navara and Nelson 2007). Although ALAN impacts in terrestrial ecosystems are apparent, ALAN also affects *Corresponding author: thomas.nelson@noaa.gov Corrections added on 27 February 2021, after first online publication Received June 23, 2020; accepted November 23, 2020 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Transactions of the American Fisheries Society 150:147–159, 2021 © 2020 The Authors. Transactions of the American Fisheries Society published by Wiley Periodicals LLC on behalf of American Fisheries Society ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1002/tafs.10286