Light Pollution Is Making Fish Anxious

The fish that Weiwei Li checked on every morning had spent an unusual night. Their small aquaria were illuminated by LED lights of a single wavelength—iridescent blue, rusty red, or a tangerine orange. This artificial light replaced what would have been darkness had the fish lived in the wild, in rivers or lakes, someplace away from human settlements.

A behavioral ecologist based at the Max Planck Institute of Animal Behavior, Li is interested in how certain kinds of artificial light, which mimic light pollution in bodies of water near urban settlements, affects the health of fish. Her results were concerning. At the conclusion of the study, published this year, Li and colleagues found that not only do artificial lights at night cause what they consider “anxiety-like” behavior in zebrafish, this behavior is passed down to their offspring, even when the offspring are born into an environment with natural light regimes

“The light levels that we used in our study matched what is already shining into the homes of animals at night through the many sources we place outdoors,” says Li. “And we found extremely strong and clear negative effects on the behavior of fish and their offspring after only a few bright nights.”

The study focused on zebrafish, tiny slick minnows, because they have highly developed photoreceptors and a well-studied neurobiology. Their photoreceptors number seven in total and capture a wide swathe of the light spectrum, from ultraviolet light all the way to infrared frequencies. But Li and colleagues found that the fish were particularly disturbed by artificial blue light. After four nights spent under the lightbulbs the zebrafish began to hug the walls of their observation tank. They were lethargic, swimming less and covering only short distances. “Even their body shape changed,” Li says.

Fish, like other animals, use natural day-night cycles to establish an internal body clock, which orchestrates their development, physiology, and behavior. Artificial light in the night throws off this rhythm. “This is consistent with what is known in humans, that exposure to the blue light of our electronic displays has the biggest effect on our sleep and possibly other physiological cycles,” says co-author Aneesh Bose.

The fish were particularly disturbed by artificial blue light.

Light pollution from urban developments has been a major problem for organisms living in exposed lakes, rivers, and estuaries for more than a century. Even some marine life has been affected. Turtle hatchlings making their way to the ocean are disoriented by city lights, having evolved to navigate by moonlight. Artificial lights reduce and delay feeding in Japanese eels, while white light increases the metabolic rates of gastropod mollusks, both examples of digestion cycles gone awry.

“I was surprised the anxious behavior was inherited,” Li says of the zebrafish. “I didn’t think the offspring would change” after their mothers had gone through the experiment, “but they did,” she says. Offspring from mothers who had participated in the experiment and were subject to blue and red light were affected, but not those born to mothers who had experienced only the artificial orange light at night.

In the original experiment, which lasted for 10 days and nights, only female fish were studied. At the end of the test, a few of the fish from each treatment were encouraged to mate, and their offspring hatched into an ordinary aquarium, where no artificial lights were used at night. 

One solution to the light pollution problem is to use “spectral tuning”—the cutting out of some of the more damaging wavelengths, in favor of softer illumination—when designing lighting for urban areas. “There are some test beds in Europe that now have yellow-red light, for example,” says Franz Hölker, head of the Light Pollution and Ecophysiology group at the Leibniz Institute of Freshwater Ecology and Inland Fisheries. This helps to prevent melatonin suppression in humans and many other mammals.

But it’s only a partial solution. “When you look at the big picture, you see that different organisms are often affected by different parts of the spectrum,” says Hölker. “In the end, it’s always a question of weighing up the winners and losers—a challenge for sustainable lighting,” he says. Ultimately, Hölker advocates the use of tailored and shielded light and the reduction of light intensity and duration. Less light could help fish have a better future, for generations to come.

Lead image: Rogerio Paulo Silva / Shutterstock

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