Riverine fish density, predator–prey interactions, and their relationships with artificial light at night

Artificial light at night (ALAN) has emerged as a prevalent anthropogenic stressor in many aquatic ecosystems impacting a wide range of taxa and ecological processes. In fishes, ALAN attracts both predators and prey, potentially resulting in increased predation mortality and sublethal impacts. Increased predation mortality is especially problematic in anadromous fishes, given that many populations are diminished and out-migrating juveniles must transit illuminated waterways. Additionally, ALAN management is complex, because lighting benefits human safety, economies, and recreational pursuits. The Sundial Bridge (Redding, CA) is an iconic illuminated structure that spans a section of the little remaining spawning habitat of the endangered winter-run Chinook Salmon. It was hypothesized that bridge ALAN increased Rainbow Trout predation of winter-run fry, and a lighting management plan was implemented. However, ALAN impacts on this predator-prey interaction and species-specific responses were unknown. Therefore, we used tethered salmonid fry and ARIS sonar cameras to determine whether variable ALAN treatments (0%, 25%, 50%, and 100% intensity) altered Rainbow Trout density (RTD) and fry predation risk, while investigating the temporal relationships of RTD with ALAN. ALAN significantly increased RTD on river right when any amount of ALAN was present, potentially leading to competition and fitness consequences; RTD did not change significantly in response to ALAN on river left. Although RTD generally increased in response to ALAN, salmonid fry predation was almost nonexistent. Therefore, while ALAN may decrease out-migrant survival in other waterways, there was no evidence at our study site. Furthermore, the discrepancies between riverbanks demonstrate the complexity of ALAN and how it interacts with other environmental parameters potentially providing optimal foraging habitat. The Rainbow Trout attraction to ALAN, which occurred immediately with diminishing ambient light, indicates that complete ALAN removal may be necessary to mitigate ecological consequences of ALAN. When removal is unrealistic, ALAN should be diminished as early in the night as possible to minimize aquatic ecosystem impacts, while maintaining benefits for human populations.