Scientists have demonstrated that light pollution, particularly blue light, can change the behavior of fish after just a few nights and can have lasting effects on their offspring.
Researchers studied the reactions of female zebrafish exposed to artificial light at night (ALAN), which is the predominant source of light pollution worldwide. They observed that after nine nights of exposure to different wavelengths of ALAN, the fish exhibited reduced swimming activity, increased social clustering, and spent more time near the aquarium walls. These anxiety-related behaviors were evident across all light wavelengths, but blue light triggered the most rapid and significant changes. Notably, the findings indicate that light pollution can have enduring consequences: fish born to light-exposed mothers swam less, even if they had not been directly exposed to the light themselves. This research was conducted by teams from the Institute of Hydrobiology at the Chinese Academy of Sciences and the Max Planck Institute of Animal Behavior (MPI-AB).
Artificial light at night (ALAN) disrupts natural darkness, impacting ecosystems. ALAN is prevalent both outdoors, through streetlights and illuminated buildings, and indoors via electronic devices that captivate our attention in the evenings. This light pollution is known to affect countless organisms by interrupting their biological rhythms, which are governed by the cycles of light and darkness.
“Sleep is a fundamental process in animals that is affected by ALAN, prompting us to explore its implications for their daily activities. Essentially, we wanted to understand how it influences their behavior,” explained Wei Wei Li, the primary author of the study and a doctoral student at MPI-AB.
“The light levels in our research reflected what animals are already experiencing from various outdoor sources at night. We discovered very significant negative impacts on both fish behavior and their progeny after just a few nights of intense light exposure.”
The risks of blue light
Given that blue spectrum light exposure is known to negatively affect humans, the researchers aimed to see if different wavelengths would have varying effects on fish behavior. They exposed female zebrafish to all-night light for various light conditions: nine distinct wavelengths in the visible spectrum and white light. The luminosity was set at 20 lux, resembling streetlight intensity at a distance that animals would encounter outdoors.
After eight nights of exposure, they found that all light wavelengths caused the fish to swim less, remain close together, and stay near the aquarium walls, indicating increased anxiety—a behavior known as “thigmotaxis” or wall-hugging. However, blue light’s effects were apparent earlier, after just five days of exposure, with the wavelength of 470 nm having the most pronounced impact. “This aligns with findings in humans, where blue light from screens significantly disrupts sleep and other biological cycles,” added co-author Aneesh Bose, who conducted this work while at MPI-AB.
The study was not aimed at identifying a specific mechanism, but the authors theorize that sleep deprivation may underlie their observations. The behavioral changes emerging after five or eight days of ALAN exposure, rather than immediately, could be attributed to a lack of sleep. “Fish might manage a few sleepless nights, but after prolonged disrupted sleep, the consequences catch up with them,” explained Bose, now a researcher at the Swedish University of Agricultural Sciences.
Enduring effects
The study also uncovered that the effects of light pollution extend beyond the individual fish and are transmitted to their offspring. After the female zebrafish were exposed to ALAN, they were allowed to breed, and their young were raised in natural light conditions. After 15 days, researchers assessed the swimming behavior of the larvae with automated tracking software designed to measure the tiny fish’s activity levels. Offspring from light-exposed mothers showed less movement during the day, despite having no exposure to nighttime lighting themselves.
“Our research indicates that light pollution disrupts the natural behaviors of fish, which could have implications for their fitness and overall performance,” noted Ming Duan, the last author from the Institute of Hydrobiology at the Chinese Academy of Sciences.
To address these impacts of ALAN on wildlife, the authors stress the importance of examining the types of light emitted by human activities. Duan remarked, “Many areas we illuminate at night are situated close to animal habitats. The most effective action we can take is to reduce the use of blue wavelength light in areas where animals are trying to rest.”
