Seabird colony creates 'halo' of depleted fish stocks

Ascension frigate. Credit: Sam Weber New research has shown that a large seabird colony on Ascension Island creates an "halo" where fewer fish can live. Ascension is a UK Overseas Territory that houses tens to thousands of seabirds of different species. New research by the University of Exeter, Ascension Island Government and the University of Exeter has found that flying fish numbers have dropped to 150km (more about 90 miles) off the island. This could only be explained by seabirds foraging. These findings, which provide rare evidence of a long-standing theory that was first proposed at Ascension, show how food-limited seabird communities naturally are and why they are so sensitive to human fishing. Dr. Sam Weber of the Centre for Ecology and Conservation at Exeter's Penryn Campus in Cornwall said, "This study tells me a lot about large colonies of animals and their numbers are limited." "During the breeding season, these birds flock to Ascension Island. The intensity of their foraging is naturally highest around the island. "As they consume the most readily available prey near the island, they must travel longer distances to eat, which causes the 'halo to grow outwards. A chick is fed by a masked booby. Credit: Sam Weber Masked booby. Credit: Sam Weber "Once people can't find enough food for their needs, they stop growing the colony." "Human impact such as fishing can disrupt this natural balance and have adverse effects on the populations of top predators marine like seabirds. "What surprised me the most was the size of the footprint that we found. It shows that Marine Protected Areas might need to be large, as some predators depend on prey stocks spread across large areas. After British ornithologist Philip Ashmole who proposed the idea after visiting Ascension Island 60 years ago, the pattern of prey loss revealed by the study is called "Ashmole’s halo". The researchers counted and tracked the foraging trips of seabirds, as well as examined the regurgitated food. Ascension's nesting seabird species include frigatebirds, brown boobies, and masked boobies that feed on flying fish. Continue reading Poor diet could have contributed to the decline of major Atlantic seabird nesting colonies Additional information: Sam B. Weber, et al., "Direct Evidence of a Prey Depletion "Halo" around a Pelagic Prey Colony," PNAS (2021). Information from the Journal: Proceedings of National Academy of Sciences Sam B. Weber and colleagues, "Direct evidence of prey depletion halo surrounding a predator colony," (2021). www.pnas.org/cgi/doi/10.1073/pnas.2101325118

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