Richard Collins: New evidence on seabirds seems to confirm Ashmole's Halo

The size of a seabird breeding colony can be limited by the amount of food available in its vicinity
Richard Collins: New evidence on seabirds seems to confirm Ashmole's Halo

A beautiful sunrise at Skellig Rock.

Ocean wanderers from all over the North Atlantic converge on Irish sea-cliffs and islands each summer. The seabird breeding colonies of the Skelligs Saltees and Blaskets are among Ireland’s greatest wildlife attractions. Visiting these avian cities is an exhilarating experience, but spare a thought for the local fish; what is it like for them?

In 1963, ornithologist Phillip Ashmole suggested that the size of a seabird breeding colony can be limited by the amount of food available in its vicinity. As bird numbers rise, he argued, the local fish stocks become exhausted and a ‘halo’ forms, inside which fish numbers decline rapidly.

Over the course of the breeding season the ‘halo’ extends outwards; birds have to fly farther and farther to get food for their chicks. Indeed, constraints on the food supply force seabirds to postpone nesting until they are older, more streetwise, and capable of breeding successfully. Even then, strict family planning rules apply; not enough food is available for parents of most species to raise more than one chick each year.

Richard Collins: 'The seabird colonies on Ascension were huge, until domestic cats, introduced in the 19th Century, decimated them.'
Richard Collins: 'The seabird colonies on Ascension were huge, until domestic cats, introduced in the 19th Century, decimated them.'

What became known as ‘Ashmole’s halo’ seems a plausible hypothesis but its existence had not been verified experimentally. Now scientists from Exeter University and the Ascension Island government have managed to gather evidence supporting it.

Ashmole came up with the halo idea following a visit to Ascension, a volcanic outcrop in the South Atlantic between Africa and Brazil, which was discovered by the Portuguese navigator Joäo da Nova on Ascension Day 1501.

The seabird colonies on Ascension were huge, until domestic cats, introduced in the 19th Century, decimated them. The cats were eradicated in 2004 following a two-year campaign. The bird populations recovered and Ascension is now an internationally important seabird breeding haunt. The waters around the island have been designated a marine reserve.

The Exeter research team studied the birds’ feeding strategies and tracked them on foraging trips. Fish can’t be seen easily, so it is much more difficult to estimate their numbers. Fortunately there’s an exception in tropical waters. The pectoral fins of flying fish have developed into wings, enabling them to make high-speed leaps into the air and fly low over the surface before plunging back into the sea. It’s a strategy the fish use to escape from predators. Aerial sightings enable the numbers present to be estimated. Examinations of regurgitated food show that flying fish are a primary prey item of seabirds.

Few flying fish were recorded close to the colonies at Ascension but the extent of the halo was surprising. Reduced numbers were detected up to 150km away. Seabird ‘aggregations’, the researchers conclude, ‘profoundly influence the oceans that surround them’.

“Ashmole’s halo constitutes rare visible evidence of the food limitation that naturally limits many predator populations and can be exacerbated by human impacts such as fisheries,” they warn.

A 2007 study found that gannets breeding on Scotland’s Bass Rock forage up to 100km and more from the colony. If seabirds are to avoid the slashing cuts of windmill blades, Ashmole’s halo will have to be taken into account when locating off-shore wind-farms.

Sam Weber et al. Direct evidence of a prey depletion ‘halo’ surrounding a pelagic predator colony. PNAS. 2021.

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