Reef fish are more placid around islands infested with invasive rats - New Scientist

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Around rat-infested tropical islands in the Indian Ocean, jewel damselfish change their behaviour because the algae they eat is less nutritious

Life 5 January 2023

By Madeleine Cuff

A jewel damselfish — Jewel damselfish around rat-infested islands defend territory less aggressively

Dr Rachel Gunn/Lancaster University

Invasive rat populations on tropical islands have long been known to pose a serious threat to seabirds, due to their voracious appetite for eggs and chicks. Now we are learning that they change the behaviour of fish around the islands too.

In the Chagos Islands in the Indian Ocean, invasive black rats arrived with Western settlers in the 1700s and can now be found on approximately 34 of the 55 islands of the archipelago. On infested islands, populations of seabirds such as boobies, frigatebirds, noddies and shearwaters have been decimated.

Fewer birds mean less bird droppings. This in turn means a fall in nitrogen and phosphorus in the waters around the islands, which support the growth of algae around coral reefs.

Previous research has revealed that around rat-infested islands in the Chagos archipelago, the algae are less nutrient-dense. Rachel Gunn, then at Lancaster University, UK, and her colleagues have now shown that this change in nutrient flows directly affects the behaviour of reef fish.

Jewel damselfish (Plectroglyphidodon lacrymatus) are herbivorous fish that aggressively guard patches of turf algae that grow on coral reefs and “farm” them by weeding out the algae they don’t eat.

Gunn and her colleagues placed Go-Pro cameras underwater in 60 different locations around the Chagos archipelago, recording and analysing footage of the damselfish defending their territories to judge their levels of aggression. A camera set up above the water was used to map the size of damselfish territories.

Around rat-infested islands, the fish farmed larger patches because the algae were less nutritious, and they were also less aggressive, says Gunn.

“Aggression and territory defence is a really high-energy behaviour, so it requires a high level of nutrients in order for that behaviour to be carried out,” says Gunn. It doesn’t make sense for the fish to aggressively defend a larger territory with lower value. “It just isn’t worth investing that extra energy in being aggressive,” she says.

Jewel damselfish were the easiest species to study, given their abundance in the water and bold behaviours. But the lack of nutrients could also be affecting the behaviour of other species, says Gunn.

It is yet another reason to press ahead with the eradication of invasive rats on tropical islands, says Gunn. Removing rats would bring back seabirds, restoring nutrient flows into the reefs and giving the resident fish a chance to revert to their naturally aggressive behaviours.

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