There have been drastic increases in artificial light at night over the last two decades throughout the world, especially in urban areas like Seattle, Washington. Lights have been added in previously dark areas and existing lighting has been retrofitted to include LEDs. LED lights are brighter than previous bulbs and emit a different color of light (light spectra) which change the type and increase the level of light in the environment.  

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Artificial light at night (ALAN) can be a threat to aquatic ecosystems.  Light in the sky penetrates through the water and brightens the water column. Even small increases in light at night can dramatically improve some predators’ ability to hunt and kill their prey. This prey includes threatened juvenile salmon in Lake Washington, a large waterbody in the middle of the Seattle metropolitan area.  

USGS Western Fisheries Research Center scientists are investigating the relationship between ALAN, predators’ hunting ability, and juvenile salmon survival in Lake Washington. This includes using a new instrument that overcomes significant issues experienced when attempting to measure light at night underwater. Most light sensors are not sensitive enough to measure ALAN (they are made for daytime use), or they are made for other purposes. The new can measure light at the surface and through the water column by wavelength (color). This technology is used by NASA and NOAA to measure ocean water quality and it is the first time the instrument has been used in freshwater to study artificial light levels. 

The results of this work will help resource managers understand the impact of artificial light relative to other factors affecting juvenile salmon survival. It will also inform urban management, describing specifically where and what types of light may be the greatest issue so they can plan accordingly and reduce the impact of artificial light.