It’s well understood that with the setting sun comes a transformation of the coral reef; a veil of darkness under which the bustle of the day time marine life retreats for rest, hidden in relative safety while its nocturnal predators comes out to play. 

But, as a new study to emerge from the University of Bristol suggests, the natural order of things is being turned on its head, thanks to light pollution from artificial light illuminating those areas of refuge, exposing the once safe marine life to new levels of predation.

It’s the perfect recipe for a feeding frenzy; that artificial light can both wake sleeping fish and attract predators, changing the natural dynamics of coral reef communities. 

The study, published earlier this month in Global Change Biology, was conducted by an international team of scientists from the UK, France, French Polynesia, and Chile in what has been recognised as the ‘first large-scale experiment’ to investigate the impact of light pollution on the nightlife of coral reefs.

Leading the study, which measured the nighttime activity on coral reefs exposed to artificial light using infrared cameras to document its impact, 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 crevices in pursuit of prey.”

While sleeping fish are usually protected by the darkness, artificial light generated by humans is exposes such inhabitants to “unexpected danger”, said the study.

Using purpose built underwater infrared night vision cameras – a form of light unobserved by fish – the study was able to film the reef at night without interfering with fish behaviour. It found that a large number of fish were present on artificially illuminated reefs at night compared to control sites with no artificial light. Further investigation revealed that these active fish were predators – feeding on the zooplankton, small fish, and invertebrates.

“Many of the species detected on artificially lit reefs were not nocturnal fish, but those that are only usually active during the day,” said Dr Weschke. “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.”

It’s perhaps an unsurprising conclusion to be drawn, yet until not it has been a relatively under-studied area of human impact upon marine life. It’s according to the study’s senior author, Professor Andy Radford, also from the University of Bristol, that “artificial light makes it much easier for predators to locate and capture prey, reducing their foraging effort.”

It’s why, he said, the teams believes artificially illuminated coral reefs are attracting greater numbers of predatory fish.

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. 

It’s been suggested by co-author, Professor Steve Simpson “that this is positive news” indicating that there could be “low-cost solutions that are quick to implement.”

“Reducing the impacts of artificial light could help build resilience on valuable coral reefs,” he said. 

And, unlike greenhouse gasses and plastics, artificial light is a pollutant that “doesn’t leave a residue when switched off”. 

“Limiting artificial light in both intensity and duration, prioritising it for essential needs and reducing aesthetic use, will help re-establish naturally dark nights that marine ecosystems evolved with,” said Dr Weschke. 

The work was conducted by researchers from the University of Bristol in the UK, the Centre de Recherches Insulaires et Observatoire de l’Environment, French Polynesia and Pontificia Universidad Católica e Chile. 

The paper Artificial light increases nighttime prevalence of predatory fishes, altering community composition on coral reefs was authored by Emma Weschke and published in Global Change Biology.

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