Date Published: 6 June 2013

Gannet colonies have their own distinctive fishing areas

Recent research into the behaviour of British seabirds has shown that colonies of gannets maintain vast exclusive fishing ranges despite doing nothing to defend their territory from rival colonies.

A team of researchers observed that northern gannets, which can fly hundreds of miles on a single fishing trip, avoided visiting the fishing grounds of gannets from neighbouring colonies.

This information could change understanding of animals' foraging patterns because individual gannets do nothing to enforce this territory or communicate its boundaries when out at sea. A bird entering from a neighbouring colony would be free to fly and fish at will.

Dr Ewan Wakefield of Leeds University Faculty of Biological Sciences, said:

" The accepted view is that exclusive foraging territories are associated with species such as ants, which aggressively defend the feeding areas around their colonies, but this opens the door to a completely new way of thinking about territory. We found the gannet colonies also had neatly abutting and clearly defined feeding areas. Gannets may be a byword for gluttony but, clearly, they don't eat off each other's plates."

Researchers from more than 14 institutions based in the UK, Ireland and France tracked the flights of almost 200 northern gannets flying from 12 colonies around the British Isles.

Rather than seeing criss-crossing flight paths as the birds headed out from their colonies, they found themselves plotting a strictly segregated map. The most striking example was seen off the west coast of Ireland where gannets from two colonies, Bull Rock and Little Skellig, are within sight of each other yet head off in different directions.
The explanation has nothing to do with territorial behaviour, but instead seems to be a matter of mathematics reinforced by the culture of colonies.

Dr Thomas Bodey, researcher at the Exeter University, explained:

" Gannets get their chance when shoals of fish are forced to the surface, often by predatory whales and dolphins, but when the gannets start plunging into the water and feeding on a shoal, the fish start diving. You have to be one of the first gannets to get there and that is where the maths comes in: if you go into an area that is being used by birds from a closer colony, there is a higher chance that individuals from that colony will be there first."

The same applies when waste is being thrown off the backs off trawlers, another key feeding opportunity for gannets. Cultural transmission within the colonies then seems to reinforce geographical considerations.

" Gannets readily follow each other when at sea. Finding such separation between colonies, even when visible from each other, indicates that competition for food cannot be the only explanation and suggests cultural differences between colonies may be important. As with humans, birds have favoured routes to travel, and if new arrivals at a colony follow experienced old hands then these patterns can quickly become fixed, even if other opportunities potentially exist," Dr Bodey said.

The UK supports 60-70% of the world's northern gannets and the discovery that colonies depend on particular sea areas has implications for the location of marine protected areas and offshore energy development. The northern gannet is Europe's largest seabird, with a wingspan of around 2m. It nests on steep cliffs and rocky islands. Attaching the matchbox-sized satellite transmitters and GPS loggers used to track the birds was sometimes a major challenge. At the biggest mainland UK colony at Bempton Cliffs in East Yorkshire, a military abseiling team from the Joint Services Mountain Training Wing was called in to help.

This research has wide ranging implications for our understanding of animal behaviour. Co-author Dr Keith Hamer, Reader in Animal Ecology at Leeds University Faculty of Biological Sciences, said:

" We knew that species like ants that forage over short distances show segregation, but now we find that segregation not only occurs in gannets but that it is there just as strongly as in ants. That immediately opens the door to asking how many other species that we assumed would not show segregation actually do. There is no reason to believe that gannets are unique."

Professor Stuart Bearhop, Professor of Animal Ecology at Exeter University, said:

" We understand an awful lot about what these seabirds do on land, but until recently we knew shockingly little about what they do at sea. The technology is now allowing us to leave the coast with them and we are discovering more and more of these amazing and unexpected patterns."

Ref. to Paper:
Ewan D. Wakefield, Thomas W. Bodey ,et al.. "Space partitioning without territoriality in gannets," is published in Science (DOI: 10.1126/science.1236077; URL: http://bit.ly/12293hd) on 6 Jun.

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