Seabirds create a "halo" of depleted fish

In a new study from the University of Exeter, researchers have discovered that a vast seabird colony on Ascension Island creates a “halo” in which fewer fish live.

Located far from the mainland, the UK Overseas Territory of Ascension Island is home to tens of thousands of seabirds of various species. Some of these species, including frigatebirds, masked boobies and brown boobies, prey on flying fish. 

The experts have found that the numbers of flying fish are greatly reduced within a 90-mile radius of the island, which is a phenomenon that can only be explained by foraging seabirds.

Dr. Sam Weber is an expert in the Centre for Ecology and Conservation on Exeter’s Penryn Campus in Cornwall.

“This study tells us a lot about large colonies of animals and how their numbers are limited,” said Dr. Weber.

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

“As they use up the most accessible prey located near to the island, they have to travel increasingly long distances to feed, causing the ‘halo’ to expand outwards.”

“Once individuals can’t find enough food to break even with the energy they expend finding it, the colony stops growing.

Dr. Weber noted that human activities such as fisheries can interfere with this natural balance and have negative effects on populations of marine top predators like seabirds, even if they don’t directly harm the birds.

“What was particularly surprising is the large scale of the footprint we found. It shows that Marine Protected Areas may need to be very large because some predators rely on prey stocks across a huge area.”

The pattern of fish declines revealed by the study is known as “Ashmole’s halo,” named after British ornithologist Philip Ashmole, who first proposed a theory about this type of prey depletion about 60 years ago after visiting Ascension Island.

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Chrissy Sexton, Earth.com Staff Writer