Artificial light is altering the behaviour of coral reef communities by waking up sleeping fish and attracting predators.
A study by scientists from the UK, France, French Polynesia and Chile investigated how light pollution from humans affects the nightlife of coral reefs. They published their paper in Global Change Biology,
“The night usually brings a veil of darkness that allows marine life to hide from nocturnal predators,” says Professor Andy Radford, author of the study from Bristol’s School of Biological Sciences.
“But artificial light generated by human activities exposes coral reef inhabitants to unexpected danger.”
Marine animals up to 200 meters deep use natural light from the sun, moon and stars as cues to help inform behaviours such as rest, migration, reproduction and spatial orientation.
However, the study found that prolonged exposure to artificial light interferes with these cycles and disrupts the biology and behaviour of reef animals.
In many reefs nocturnal predators sleep and hide away during the daylight period.
As the sun sets, the lack of light means prey-fish can hide away from predators and avoid being hunted down.
But the researchers found that reefs with artificial lighting had more predatory fish than those without man-made light.
“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,” says Radford.
Currently, artificial light can be detected along 22% of the world’s shorelines. With coastal populations expected to double by 2060, this figure is likely to increase dramatically.
The researchers filmed the reef at night using infrared night-vision cameras attempting not to interfere with any fish. Infrared light was used because fish cannot see infrared.
In previous work, scientists have feared that increasing levels of artificial light may also impact coral reproduction.
Many corals will release their eggs and sperm at particular times of the year, sometimes even only on a specific night. This release is thought to be triggered by light from the moon, therefore more artificial light may mix up these signals and override traditional timings.
Scientists warn this could lead to less efficient coral reproduction at a time when coral production is desperately needed.
The team found that changes to coral nightlife could be observed after an average of 25 consecutive nights of exposure to artificial light.
“Many of the species detected on artificially lit reefs were not nocturnal, but those that are only usually active during the day,” says Dr Emma Weschke, lead author of the study.
“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.”
However, short-term exposure to artificial light at night did not result in any distinguishable changes.
“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.
“Reducing the impacts of artificial light could help build resilience on valuable coral reefs.”
The team suggest the solution may be as simple as occasionally turning off lights at night to avoid over-stimulating the reef communities.
“Unlike greenhouse gasses and plastics, artificial light is a pollutant that doesn’t leave a residue when switched off,” says Weschke.
“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.”
Searching for ways for reefs to survive global warming
The Ultramarine project – focussing on research and innovation in our marine environments – is supported by Minderoo Foundation.