A huge colony of seabirds on Ascension Island creates a “halo” in which fewer fish live, new research shows.

Ascension, a UK overseas territory, is home to tens of thousands of seabirds of various species that prey on flying fish.

The new study by the University of Exeter and the Island Government of Ascension finds reduced numbers of flying fish up to 150 km (more than 90 miles) from the island – which could only be explained by seabirds foraging for food.

The results – which provide rare evidence for a longstanding theory first proposed at Ascension – show how food limited seabird populations naturally are and why they are often so sensitive to competition with human fishermen.

“This study tells us a lot about large animal colonies and how their numbers are limited,” said Dr. Sam Weber from the Center for Ecology and Conservation on the Penryn campus in Exeter in Cornwall.

“These birds are concentrated on Ascension Island during the breeding season, and the intensity of their foraging is naturally highest near the island.

“As they consume the most accessible prey near the island, they have to travel longer and longer distances to feed, which causes the ‘halo’ to expand outward.

“As soon as individuals cannot find enough food to meet the energy they spend searching, the colony stops growing.

“Human impacts such as fishing can upset this natural balance and negatively impact populations of marine top predators such as seabirds, even if they do not harm the birds directly.

“What was particularly surprising was the size of the footprint we found.

“It shows that marine protected areas may need to be very large as some predators rely on prey in a huge area.”

The pattern of prey depletion revealed by the study is known as “Ashmoles Halo,” after British ornithologist Philip Ashmole, who first suggested it about 60 years ago after visiting Ascension Island.

For the study, the researchers counted flying fish, tracked seabirds foraging, and examined their vomited food.

Nesting seabird species on Ascension that hunt flying fish include frigate birds, masked boobies, and brown boobies.

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The research team included the RSPB and the Royal Dutch Institute for Marine Research.

The study was funded by the UK Government’s Conflict, Security and Stability Fund and a grant from the Darwin Initiative.

The paper published in the magazine Proceedings of the National Academy of Sciences, is entitled: “Direct evidence of a prey depletion ‘Halo’ surrounding a pelagic predator colony.”

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