Gannets prove to be discard specialists

By Paul Robinson - 04 Jun 2014 11:32:0 GMT
Gannets prove to be discard specialists

The unique gannet is also of aesthetic interest, with a body built for speeds up to 100kph (62mph.) Personally, I like their striking facial characteristics, like the related booby, a pelican or a heron; gannet image; Credit: © Shutterstock

This big seabird nests on isolated rocks in the ocean, free from predation, but it is one of the great divers, predating fish several metres beneath the surface, further than any non-penguin. This study takes the discard fishing of the northern gannet, Morus bassensis (named after the Bass Rock in Scotland) and investigates the ecology of large trawlers' gannet followers. The research was undertaken by Dr Thomas W. Bodey, now of the University of Glasgow and his colleagues from University College, Cork and the University of Exeter. They publish in Current Biology with the title: - "Seabird movement reveals the ecological footprint of fishing vessels.".

The gannet forages far and wide and has increased in population for decades, presumably by using the discarded fish as a dietary supplement. Anthropogenic effects such as these have been obvious since fishermen took to the sea, but this population increase is probably related more to regulations about the take of fish that vessels are allowed to land. With continued unsustainable usage of the sea's resources, there is intense pressure on many species.

This positive effect of discards is the opposite of many negative results. Those animals could have been left, living, in the marine ecosystems, instead of increasing the scavengers' roles. The proposed ban on discards will obviously tend to decrease the population of gannets. Then we will be able to assess this ecological footprint that fishing vessels have left for many generations. The extent of the footprint was gauged to some extent by the effect of the distance that a trawler was from a gannet. Even at 11km away, the gannets would respond, giving a real footprint of 22km diameter, but the deeper effects could stretch to the gannet colonies and far beyond.

"Fine tuning," by these birds also distinguished whether the boats were using certain fishing gear and when they began fishing. These interactions give some idea of how the marine environment is affected by broad-scale behavioural change within birds, fish and marine mammals. While gannets forage using their visual senses for 500km, purely in daylight, other animals use varied sensory abilities to detect boats, prey or each other. The possibility of change as discards are banned will be much more than interesting!

The data was gathered from 6 breeding colonies around Ireland using GPS on 74 gannets and similar tracking data from trawlers. And how did the researchers know that the gannets were eating discards. The fish they were eating were caught by trawl nets well below the extreme limit of a gannet's dive. So the scientists were quite clever, too.