The impact of trawlers and their waste on seabird populations appear to be much larger than previously thought. A study by the University of Exeter highlights that the ‘footprint’ of a trawler is as large as 11km radius. This footprint influences the foraging behaviour of seabirds as they now associate trawlers with easy-pickings.
Using GPS the marine researchers from Exeter discovered that the northern gannet changed their behaviour in the presence of large boats and particularly trawlers.
Scientists at the Centre for Ecology and Conservation at the University of Exeter’s Penryn Campus in Cornwall and the Coastal & Marine Centre at University College Cork analysed GPS tracking data from 74 gannets from six breeding colonies around Ireland, and combined these with similar GPS tracking data from fishing vessels.
Using these GPS tracks the scientists were able to put a distance on the influence of the trawlers. Incredibly gannets from as far away as 11 km were attracted to the trawlers. Large seabirds such as gannets are known to feed on fish discards from trawlers as they was eating fish that live at depths far below the depths that the birds can dive to.
Dr Thomas Bodey of the University of Exeter, who led the study, said: “Our work suggests each fishing vessel has a substantial footprint, with the behaviour of seabirds affected within a 22km diameter circle surrounding it, much larger than we expected.”
Not only do the gannets travel distances to find the trawlers the birds are also able to determine if the journey is worthwhile. By looking closely at the way that birds and boats interacted the scientists concluded that the birds could tell whether the trawler was actively fishing and could even adjust their behaviour based on what type of fishing gear the trawler was using at the time.
Dr Mark Jessopp of the Coastal & Marine Centre at University College Cork, a co-author of the work, added: “The fact that birds responded differently to boats depending on whether they were fishing or not, and the type of gear they were carrying, indicates just how finely attuned these animals are to the opportunities humans can provide“.
Co-author Professor Stuart Bearhop, also of the University of Exeter’s Centre for Ecology and Conservation, said: “We know that seabirds are facing many impacts within the marine environment, and we have tended to think that interactions with fishing boats were a localised phenomenon. Our work indicates that the scale of impact on these top predator’s behaviour is much broader.”
Gannets are the UK’s largest seabird, foraging up to 500 km from their colonies. They forage almost exclusively during daylight hours, with birds resting on the sea surface at night. They are visual foragers with no external nostrils and relatively small olfactory bulbs.
All fishing boats greater than 15 m in length must carry a GPS transmitter as part of the European Union Vessel Monitoring System.
Current Biology: Seabird movement reveals the ecological footprint of fishing vessels
Photo credit: An adult gannet is shown with its chick. The researchers also discovered that individual gannets can adjust their behavior depending on whether the vessel is actively fishing or not, and also based on the type of fishing gear carried. image: Alyn Walsh