Artificial light on coastlines lures small fish to their doom, coral reef study finds

Artificial light shining from coastlines around the world is acting like “a midnight fridge” full of tasty snacks, threatening young fish who can be drawn to it and who are then eaten by predators also attracted by the brightness, according to a study.

It has long been established that light pollution hampers people’s ability to see the night sky and harms migrating birds, insects and other animals. But its impact on marine ecosystems has rarely been taken into account, said Jules Schligler, the lead author of the study at the international coral ecosystem research centre in Mo’orea, French Polynesia.

Nearly a quarter of the world’s coastline, excluding Antarctica, was artificially lit, according to a satellite study carried out a decade ago, and it was probably more than this by now, Schligler said.

His study, which involved creating 12 coral test sites in the waters off Mo’orea and shining an underwater light on half of them, found the artificially lit corals first attracted fish larvae and then predators that ate them.

“We found that the coral with the light attracted two to three times more fish compared to the naturally lit control site,” Schligler told the Guardian. “The coral with the [artificial] light is a bad environment for the larval fish because there are more predators, opportunistic fish passing by, that ate them.”

The findings, he said, meant artificial light should be seen as “another threat to marine animal populations and coastal ecosystems”.

And while artificial light might appear to benefit predator fish, Schligler said more research was needed. “It could be bad for their sleep, or they could eat too much, we don’t yet know.”

The study did not look at why the larval fish were drawn to the artificial light, but there were two possibilities, he said. “The artificially lit coral could be like a midnight fridge full of tasty plankton that are drawn to the light too. The plankton attract the larvae, and then the larvae are followed by their predators.

“Or it might be both the light itself and the prospect of food that attract them. Either way it makes them all behave unnaturally.”

The findings, presented at the Society for Experimental Biology conference in Prague, focused on two species – yellowtail dascyllus ( Dascyllus flavicaudus ) and blue-green chromis ( Chromis viridis ) – but could be applied more broadly, Schligler said.

“We can only extrapolate to a certain point but our findings, and other tests we did on crab and shrimp, generally indicate that marine animals are attracted to artificial light,” he said.

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Oren Levy, head of the laboratory for molecular marine ecology at Israel’s Bar-Ilan University who was not part of the Schligler-led study, welcomed the Mo’orea research findings, saying they were in line with his own work.

Beyond the risk of being eaten, Levy said the Mo’orea study indicated artificial light was negatively affecting how fish aged as well as their health. “It harms the reefs too,” he added, pointing to a previous study that showed artificial light interfered with reproduction and caused corals to collapse.

More positively, Levy and Schligler said that preventing light pollution, using timers and shades for example, was not difficult. “And we can start to take light into account for things like marine protected areas,” said Schligler.