Artificial light can wake sleeping fish and attract predators, changing nighttime coral reef communities, according to new research using novel underwater infrared cameras.

The study, published today in Global Change Biology, was conducted by an international team of scientists from the UK, France, French Polynesia and Chile and the first large-scale experiment to investigate the impact of light pollution on the nightlife of coral reefs.

Lead author, Dr Emma Weschke, from the University of Bristol’s School of Biological Sciences, said: “When the sun sets, coral reefs undergo a dramatic transformation. The vibrant fish we see in the day retreat to sleep among the corals and elusive nocturnal species emerge from caves and cervices in pursuit of prey.”

Senior author, Professor Andy Radford, also from Bristol, added: “The night usually brings a veil of darkness that allows marine life to hide from nocturnal predators. But artificial light generated by human activities exposes coral reef inhabitants to unexpected danger.”

Using purpose built underwater infrared night-vision cameras, the scientists could film the reef at night without interfering with fish behaviour. This is because fish cannot see in infrared.

A larger number of fish species were present on artificially illuminated reefs at night compared to control sites with no artificial light. Further investigation revealed these species were predatory fish—feeding on the zooplankton, small fish and invertebrates.

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Dr Weschke said: “Many of the species detected on artificially lit reefs were not nocturnal fish, but those that are only usually active during the day. Finding that light pollution can cause fish to stay awake later than usual is concerning because sleep—like for us—is likely essential for regenerating energy and maintaining fitness.”

Professor Radford explained: “Artificial light makes it much easier for predators to locate and capture prey, reducing their foraging effort. Which is why we think that greater numbers are being attracted to artificially illuminated coral reefs at night.”

The changes observed in the nightlife on the reef were observed after an average of 25 consecutive nights exposure to artificial light. Only a few nights of exposure were not long enough to elicit any noticeable changes in fish communities compared to controls.

“This is positive news as it suggests that there could be low-cost solutions that are quick to implement,” said co-author Professor Steve Simpson, also of Bristol’s School of Biological Sciences. “Reducing the impacts of artificial light could help build resilience on valuable coral reefs.”

Dr Weschke added: “Unlike greenhouse gasses and plastics, artificial light is a pollutant that doesn’t leave a residue when switched off.

“Limiting artificial light in both its intensity and duration, prioritising it for essential needs and reducing aesthetic use, will help reestablish naturally dark nights that marine ecosystems evolved with.”