State of Our Watersheds: Light pollution could be hurting salmon

Intense artificial lighting could be changing salmon behavior in urban Puget Sound, making them more likely to be eaten by predators.

That’s one of the many specific findings in the recently released State of Our Watersheds Report. The report from the 20 treaty Indian tribes in western Washington documents the ongoing loss and degradation of salmon habitat.

The Muckleshoot Indian Tribe calls out intense man-made light as one of the likely reasons for the decline in salmon stocks in the Lake Washington, Green-Duwamish and White-Puyallup River basins.

From the State of Our Watersheds Report:

Artificial nighttime lighting can modify the behavior of various aquatic organisms, including salmonids. Affected behaviors may include foraging, predator avoidance, reproduction and migration. Often fish are attracted to artificial light and their behavior may more resemble daytime behavior than nighttime behavior. In urban areas, high-intensity artificial lights are common near rivers, lakes and streams.



Artificial lighting studies and experiments led by the U.S. Fish and Wildlife Service were conducted in Lake Washington (2014) and Lake Sammamish (2015), in the Lake Washington Ship Canal (2007 and 2008) in the Cedar River (2004). Fish usage “hot spots” were found in brightly lit areas and along shadow lines created by artificial lighting. Chinook salmon were generally attracted to artificially lit areas. Artificial lighting may attract juvenile salmonids and expose them to increased rates of predation from visual predators such as cutthroat trout, smallmouth bass, and northern pikeminnow. Birds such as mergansers and herons are also present, and have been observed anecdotally foraging in artificially lit areas.

So-called “dark sky” ordinances on the local level could lessen the problem. Dark sky ordinances regulate the amount of light pollution allowed in any community. But the ordinances are not enough. “It is critically important that local, state and federal governments ensure that environmental assessments and permit reviews include the effects of artificial lighting on aquatic habitat, and that initiatives to retrofit and reduce artificial night lighting are undertaken especially along urban lakes and streams,” the tribe points out in the State of Our Watersheds Report.