Reef fish are more placid around islands infested with invasive rats

Around rat-infested tropical islands within the Indian Ocean, jewel damselfish change their behaviour as a result of the algae they eat is much less nutritious

By Madeleine Cuff

Invasive rat populations on tropical islands have lengthy been identified to pose a severe risk to seabirds, as a result of their voracious urge for food for eggs and chicks. Now we’re studying that they alter the behaviour of fish across the islands too.

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

Fewer birds imply much less fowl droppings. This in flip means a fall in nitrogen and phosphorus within the waters across the islands, which help the expansion of algae round coral reefs.

Previous analysis has revealed that round rat-infested islands within the Chagos archipelago, the algae are much less nutrient-dense. Now, Rachel Gunn, then at Lancaster University, UK, and her colleagues have proven that this transformation in nutrient flows immediately impacts the behaviour of reef fish.

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

Gunn and her colleagues positioned Go-Pro cameras underwater in 60 completely different places across the Chagos archipelago, recording and analysing footage of the damselfish defending their territories to evaluate their ranges of aggression. A digital camera arrange above the water was used to map the scale of damselfish territories.

Around rat-infested islands, the fish farmed bigger patches as a result of the algae have been much less nutritious, they usually have been additionally much 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 bigger territory with decrease worth. “It just isn’t worth investing that extra energy in being aggressive,” she says.

Jewel damselfish have been the simplest species to review, given their abundance within the water and daring behaviours. But the dearth of vitamins may be affecting the behaviour of different species, says Gunn.

It is but another excuse to press forward with the eradication of invasive rats on tropical islands, says Gunn. Removing rats would convey again seabirds, restoring nutrient flows into the reefs and giving the resident fish an opportunity to revert to their naturally aggressive behaviours.

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