Reef fish are more placid around islands infested with invasive rats

Around rat-infested tropical islands in the Indian Ocean, damselfish eat algae that are nutrient-poor and alter their behavior.

Along madeleine cuff

Invasive rodent populations on tropical islands have long been known to pose a serious threat, as seabirds voraciously eat their eggs and chicks. We now know that they change the behavior of fish around the island.

In the Indian Ocean’s Chagos Islands, invasive black rats arrived with western settlers in the 1700s and are now found on about 34 of the archipelago’s 55 islands. Seabird populations such as gannets, frigatebirds, puffins and petrels have plummeted on infected islands.

Fewer birds means less bird droppings. This means a decrease in nitrogen and phosphorus in the waters around the islands, which support algal growth around the reefs.

Previous studies have revealed that algal nutrient density is low around rat-infested islands in the Chagos Islands. Now, Rachel Gunn, then at Lancaster University in England, and her colleagues have shown that changes in this nutrient flow directly affect the behavior of reef fish.

Jewel damselfish (plectroglyphidodon lacrymatus) are herbivorous fish that actively guard patches of turf algae growing on coral reefs and “farm” them by removing the algae they do not eat.

Gunn and her colleagues set up Go-Pro cameras in 60 underwater locations around the Chagos Archipelago to record and analyze footage of damselfish defending their territory to determine their level of aggression. A camera placed above the water was used to map the area size of the damselfish.

Fish farmed larger plots around rat-infested islands because the algae were less nutritious and less aggressive, Gunn says.

“Aggression and territorial defense are very high-energy behaviors, so they require high levels of nutrients to perform their behaviors,” says Gunn. It makes no sense for fish to aggressively defend larger territories with lower values. “It’s not worth investing the extra energy to be aggressive,” she says.

Jewel damselfish was the easiest species to study given its abundance and daring behavior in water. But nutrient deficiencies may also be affecting behavior in other species, says Gunn.

Pushing to eradicate invasive rats from tropical islands is another reason, Gunn says. Eradication of rats can revive seabirds, restore nutrient flow to reefs, and give resident fish an opportunity to return to their natural aggressive behavior.

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