With the lights on, it’s lights out for sockeye salmon

Artificial light at night provides a nonstop buffet for cutthroat trout in Lake Washington

Imagine a massive buffet of your favorite foods. What does it include? Mine would have heaps of pastries, fresh fruit, carbonara pasta, corndogs, and ice cream. Now, imagine that someone has assembled your buffet, but the trays of food are placed throughout an 8-story building. You can eat all the food you find! Here’s the catch—the building lights are only on for 30 minutes. You can keep looking after the time is up, but you will be searching in the dark. With such a short window of time to effectively search, you might only find one plate of food, or you might strike out altogether. The duration of time the lights are on restricts your access to your tasty buffet.

In this scenario, you are a cutthroat trout and the buffet is juvenile sockeye salmon. In a natural ecosystem, cutthroat trout have the predatory advantage during periods of low light—dawn and dusk. Just like at your buffet, these intervals limit the amount of time where cutthroat trout can effectively hunt, resulting in a balance between predator and prey. In most lakes it’s typical for 25% of young sockeye salmon to survive and embark on their journey to the ocean, where they will grow for a few years before returning to the same lake.

Now, imagine you have 8 hours to search the building for your buffet. You will find a ton of food! This is the life of a cutthroat trout in Lake Washington, a lake bathed in artificial light from nearby cities. Artificial light doesn’t stop at the surface of the water, it penetrates through the water column, and extends the time when cutthroat trout can efficiently hunt juvenile sockeye. This has a big impact on the juvenile sockeye population. In Lake Washington, only 3% survive their lake phase—most of them are gobbled up by cutthroat trout.

Artificial light from nearby skyscrapers, industrial buildings, and streetlights has increased as the areas around Lake Washington continue to develop. The area is now brighter than it’s ever been—approximately 100 times brighter than a starlit sky—due to an increase in population and recent lighting retrofits. Artificial light increases the time that cutthroat trout can effectively hunt each night which decreases juvenile sockeye survivorship in the lake, impacting sockeye populations in the future.

Lake Washington used to have one of the most prolific sockeye salmon runs in the lower 48 states. Due to the huge declines in juvenile survivorship and adult returners, Lake Washington has been closed to sockeye fishing since 2006. Unfortunately, taking Lake Washington sockeye off the menu hasn’t caused the population to rebound. The time has come to look at additional ways to support the salmon population in the lake.

We need to increase the amount of juvenile sockeye salmon that survive in Lake Washington. My research aims to reduce the effectiveness of cutthroat trout by decreasing artificial light at night. To do this, I measure the light level throughout the lake, compare fish behavior in bright and dark areas, and record where fish hang out seasonally. This data will be used to set adequate light reduction goals in nearby neighborhoods and cities. Thoughtful lighting changes like focusing light to where it’s needed, decreasing bulb brightness, and changing the color of light we use, can minimize the impacts to salmon without sacrificing safety or convenience.

We need artificial light, but we can be smarter about how we use it. Together, we can thoughtfully rebalance the scale between cutthroat trout and juvenile sockeye in Lake Washington to ensure that we have a robust sockeye population for future generations.

Tessa Code is a graduate student at the University of Washington and she works as a technician for the US Geological Survey Western Fisheries Research Center. Her research uses hydro-acoustics and light sensors to study the effect of artificial light on fish predator-prey dynamics in waterbodies around Seattle.